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Researched, Written, HTML'd and Maintained by Mike Morris WA6ILQ
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A Brief Overview of AC Power: Color Codes, And How To Avoid Killing Yourself and Others Researched, Written, HTML'd and Maintained by Mike Morris WA6ILQ |
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Notes:
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I live in the southern California area, and a while back I made several business trips to do software upgrades at customer sites - thirty sites in six states in less than twenty days. Most were in the five western states so it wasn't as bad as it could have been... I scheduled one 6-day trip across a 4-day weekend and used the days off to help out a group of teenage hams that were setting up a new amateur radio repeater site. They had built an 8 foot by 8 foot (2.5m by 2.5m) concrete block building and installed an adjacent 30 foot (9m) tower - a completely new site on some hilltop private property that initially had a UHF autopatch repeater and later on the tower grew to 60 feet and supported an Icom IC-706 HF / 6m / 2m remote base radio. The slab was poured and the building was built from the ground up, using volunteer labor (the kids called it "slave labor"). Construction supervision was by one of the kids uncles, he was a retired mason (bricklayer) who also knew how to pour concrete and he insisted that all permits and inspections be done properly. They had trenched to the nearest power panel and laid a 2 inch diameter conduit for 240vAC mains power, an additional 1 inch conduit for telephone (autopatch) and ethernet wiring, plus a extra 3/4 inch conduit for future use or in case they forgot something. I was drafted by one of the local hams I know for my knowledge of repeaters, but I ended up probably saving someone's life when I took over supervising the AC power wiring installation, and in the process teaching a few young hams and their folks how not to commit suicide by electricity. I received a phone call a few weeks later from the mason thanking me and saying that the electrical inspection had passed "with flying colors".
In the electronics world, color codes rule - everyone knows the resistor color code (the cleanest version I've heard is "Big boys race our young girls but Violet generally wins gold or silver"), and back in the tube era there were standard color codes for transformer windings and even for molded capacitors (see the Allied Electronics Data Handbooks on the Tech-Index page at this web site).
Well, integrated circuits and transistors have replaced tubes as the predominant technology, and the "red is always +12 volts DC and black is always ground" mentality has (unfortunately) taken over.
Well, it's not that way in the electrical power world, where the black wire will kill you. This fact was brought home to me in a painful way in 1970 when I discovered two adjacent metal workbenches in a college school electronics lab were 120 volts AC hot to each other, and again later on when I was working at NASA / Jet Propulsion Laboratory in Pasadena CA... JPL handles all the unmanned space missions, NASA / Houston handles all the manned ones.
USA AC POWER WIRING FACTS:
1) In the USA the mains power is fed from the utility power pole to the building power entrance on three wires (mentally picture the secondary of a center-tapped transformer, with the center tap being ground / neutral). This method allows full voltage from one end wire (black) to the other end wire (also black, but supposed to be marked with red phase tape, more on that later) and half voltage from either end to the center tap / neutral (supposed to be white or marked with white phase tape). Other countries do it differently, and you will find the local color code is different as well - for example common european colors are brown for hot, blue for neutral, and green / yellow for "earth" (safety ground). And some countries use black for neutral! (See the Wikipedia color code page linked below)
Note that there is no such thing anymore as "110 volts" or "220 volts" although those numbers are still commonly used. In 1967 the USA changed its power grid from 110 volts or 115 volts or 117 volts or 120 volts (local power company choice) to a standard 120 volts / 240 volts. The official allowed tolerance is ±10%, so the official acceptable range is 108 volts to 132 volts. However I have found 120 / 208 volts (from two phases of a three-phase distribution system) in light commercial, some apartment complexes, and some condominium buildings. This means that many standard 240 volt appliances (like electric stoves, clothes dryers, etc.) are going to be underperforming when run on 208 volts. You will find that some manufacturers include a 208 / 240 wiring option, others can be ordered specifically for 208 volts, others are available for only 240 / 220 volts.
Back in the 1990s I replaced an open heater element in a friend's electric clothes dryer - and we chose to replace it with a 208v element as that was the power he had in the building. The 208v element was an available standard part (but a special order) from the distributor's warehouse. Later I found that an aftermarket manufacturer made a "universal" element with three connections... neutral, 208 and 240 and it was available at half the price! A few years earlier I helped replace a stove burner in a different friend's cooktop. They were available only in one type... labeled as 220/240 volts.
2) Once you get past the residential main entry panel (usually where the power meter is located) the ground wire is not the same as the neutral / zero volts wire. The solid green wire (or green with a yellow stripe or stripes), green screws, and round hole in common power outlets is the safety ground (also known as the "Bonding Conductor" or the "Equipment Grounding Conductor", or internationally the "protective earth" conductor) and connects the housings of all equipment to ground at the electrical service entrance point completely independently of any other wiring. The water lines, natural gas lines, metallic air conditioning lines, metallic air conditioning ducts, etc. are also supposed to be connected to the safety ground. Internationally the standard for the "protective earth" conductor uses green insulation with a yellow stripe or stripes where it used to be solid green. In the USA the solid green insulation is still very common, or it can be a bare uninsulated copper wire. Even when it is a bare copper wire (usually AWG 12, 10 or 8) you will hear electricians refer to it as the "green wire" - don't let that bother you. Whatever form it takes it can never ever be interrupted by any type of switching, be it a circuit breaker, a switch, a fuse or anything else. This "green wire" is intended to permanently "bond" all of the normally unenergized parts of the equipment / appliance / fixture to the safety ground bus back at the power service entrance. Under normal circumstances there is no current in the green wire - in other words this wire is strictly for handling fault current, and never, ever carries load current.
This article was proofread by two friends before posting... one is a retired electrician with 20+ years of experience in both residential and commercial environments, the other is a retired county building inspector (25+ years), now deceased (RIP Allan Ogden W6SPK 1942-2017).Personally, I don't trust the conduit / flexible conduit to be the bonding ground - in addition to making sure that the conduit / flex is grounded, I always pull a green wire along with whatever is going inside the conduit and treat the conduit / flex only as a construct for the physical protection of the enclosed wiring. Several times that required me to go to the next larger diameter flex. In fact, I've been called anal when folks see me use nothing but EMT (solid metal tubular conduit) or flex, metal outlet boxes, and lots of green pigtails to ground every outlet box, metal pull box and metal switch box (in addition to the outlet or switch itself) to the green wire. My usual response is: "I am doing this job as if MY mother or daughter will be living here". And I have NEVER had a problem with the building inspector (except for the fact that he thinks I overdo things).
A comment from Allan:
"In older wiring systems (where 'old' is defined as the mid-nineteen-seventies and before), the conduit or flex may be ASSumed to be the bonding system. Some of the time, this is an un-founded ASSumption".
In case you are curious, the international standard green-yellow marking of the bonding ground conductors was introduced to reduce the risk of confusion by colorblind installers. Between 7% and 10% of men cannot clearly distinguish between red and green, which is a particular concern in older schemes were red marks a live conductor and green marks safety ground. If you are purchasing wire and you have a choice between green / yellow and solid green, please pick the green / yellow.
3) The "zero volts" or the return wire is white (which frequently fades to grey) and is referred to as the "neutral" wire or the neutral connection. The white / grey wire, the silver screws (commonly called the white screws), the wide slot on power outlets and the threaded sleeve of the light bulb sockets are all connected to neutral. Neutral is insulated from ground everywhere EXCEPT at the service entrance where it is ALSO tied to the ground buss. Yes, green and white / grey are tied together at the service entrance, and only at the service entrance. The hot wire is the power source, the neutral white / grey is the power return. As with the green wire safety ground above, the white / grey neutral wire may never be interrupted by any disconnecting mechanism. The switch should ALWAYS be in the hot lead, so that when it is off the fixture or outlet is electrically DEAD... but I've seen too many residential ceiling light circuits in the bedrooms and bathrooms wired with the switch in the neutral lead... after all, it's a series circuit, and it doesn't matter, right? Note that while the white / grey wire is at zero volts, it is never considered as or ever referred to as "ground"! It is always referred to as "neutral" or "the neutral wire".
One personal pet peeve: Over the years I've seen several situations where a residential bathroom light fixture body is hot to the faucet because:
(a) the fixture body and mounting box wasn't grounded...
(b) the switch was in the neutral lead and...
(c) the hot wire insulation inside the box had deteriorated and the copper wire was touching the fixture.
Repeating what I said above: the switch should ALWAYS be in the hot lead, so that when it is off the fixture is electrically DEAD.
Another comment from my retired building inspector friend: "Do not trust any piece of plumbing to be at ground. Plastic fittings and PVC, PEX or ABS pipe are too prevalent for this ASSumption to be valid".
ALWAYS make sure that the electrical fixture or outlet is green-wire-grounded, with a pigtail if need be, and always make sure the shell of the light bulb socket(s) i.e. the threaded base(s) are at neutral potential. Yes, this last point is impossible on table lamps and other household equipment that uses simple two-wire AC cords unless you deliberately use a polarized plug, (which you should). In the 40+ years since the mandatory electrical overview class by a NASA / JPL electrician and a NASA / JPL safety supervisor (discussed later) I've not purchased or installed a non-polarized 2-wire plug. And I've cut several non-polarized plugs off the ends of various cords (like on kitchen appliances) and installed polarized ones. Replacement polarized plugs are widely available, even at Home Depot.
4) Any color except green or white / grey is a HOT wire I will repeat myself in fewer words: ALL OTHER COLORS ARE HOT. Engrave that in your brain. Deeply. It may save your life or that of a family member, friend or co-worker.
5) Brass screws, the narrow slot on 2-prong or 3-prong power outlets, and the center button / center contact of the light bulb sockets are always the hot wire (but I've seen too many incorrectly rewired table lamps that have the shell hot and the center button as neutral, even with a polarized plug). This is why some quality "zip" cord has a brass colored conductor and a silver colored conductor – the polarized plug goes on one end with the wide blade on the silver color, and the lamp socket goes on the other with the shell on the silver. Other types / manufacturers of zip cord have a "rib" molded into the plastic insulation over one wire so you can tell the two conductors apart. I've heard conflicting comments from knowledgeable people - some say that the ribbed conductor should be the neutral; some say it should be the hot. Pick one and stick with it.
My inspector friend told me that construction electricians are supposed to use one color, usually black to signify an unswitched circuit, and another hot color to signify a switched circuit. As an example of this usage, the hot wire from the power disconnect breaker to a wall-mounted light switch should be black, and the switched hot wire (from the switch to the outlet or light fixture) should be another color: blue, brown, yellow, orange, red, etc. Unfortunately when you use "NM" or Romex (discussed later) this won't happen. When I help homeowners do AC wiring, I prefer flex with multiple wires and it's not "should be" but "is". In situations where you have multiple circuits fed to somewhere else through a single conduit or flex I've seen blue, brown, yellow, orange and red frequently used as unswitched feeders downstream from a breaker panel (and don't forget to follow the NEC and bump up the diameter of the neutral appropriately if you run multiple hot circuits with a shared neutral(s) in one run of flex or of conduit).
My electrician friend told me a story that he had read in an electrical trade magazine many years ago: a child (in bare feet) was electrocuted when he contacted a metal portion of an older illuminated business sign. The sign was at the child's shoulder level and mounted on a wooden wall and surrounded by a flower bed. As part of the coroner's investigation the sign was dismantled to see what had happened. It turned out that:
a) sprinkler water and rain water had migrated inside the sign (which was over 20 years old),
b) the sign was constructed in such a way that water pooled in the bottom,
c) the green wire was fastened to the bottom sheet metal of the sign with a single common sheet metal screw,
d) the submerged end of the green wire had corroded to the point it had broken,
e) the hot wire had been routed inside the top of the sign and the insulation became brittle due to a combination of age and the rising heat from the light bulbs,
f) the overheated baked / brittle insulation had flaked off the wire in chunks and those pieces were found in the bottom of the sign,
g) the now exposed hot lead had come into contact with the metal housing of the sign.
None of the above had been noticed since the sign still worked, but the combination resulted in 120 volts AC on the shell of the sign. A child in bare feet standing in a damp flowerbed coming in contact with the energized housing resulted in a fatality.
The article suggested extending the green wire to a position that is on the side or top of the housing to prevent pooled water from causing a disconnected green wire. It also suggested bonding together all sections of a multi-section metal housing sign with green jumpers.
In short, the green wire has one very important purpose - to trip the breaker or blow the fuse by being a fault current pathway BEFORE a person's body touching a charged surface becomes the path of least resistance to ground.
6) The most common mains power usage of the wire color red is to signify the other hot leg (i.e. non-black) of a 208 / 240 volt circuit. On the other side of the circuit breakers you may find it used to signify switched circuits as mentioned above. I have, and frequently. Personally, I always use black on the source side of a switch, and a non-black color (anything but red or orange is my first choice) on the load side.
7) The USA National Electrical Code (NEC) allows for a situation where an electrician runs out of a wire color while on the job and you can find the answer at any electrical supply store, and at most construction supply stores (even Home Depot) - electrical tape in various colors (sometimes called "phase tape"). A band of colored tape wrapped around a wire signifies a "wrong color" - for example, white tape wrapped around black (actually any color) insulated wire marks that wire as being a "white" (i.e. neutral) wire. Red tape wrapped around a green wire says that the green wire is a red (hot) wire. Colored tape wrapped around the end of a wire - even black tape - indicates that the tape color takes precedence over the wire insulation color. If you ever need to use color / phase tape, use it liberally: a couple of 50% overlapping layers for at least 2 inches, at both ends of the wire run, and in every splice box and / or pull box in-between (a lot of electricians skip the intermediate splice and pull boxes, and the inspectors let them get away with it). My local electrical contractors supply house, the local Home Depot, and even the local ACE Hardware store stocks colored tapes in red, orange, yellow, green, blue, violet, brown and white. And note that larger wire sizes are available only with black insulation, and that phase tape is mandatory in that situation. I've been told to always use a phase tape marker on all the black conductors - even to the point of using a black piece of tape on a black wire. Others have said that black on black is overkill. One friend puts a piece of clear heat shrink over the phase tape. That's also been called overkill.
I don't have to go far to see an example of phase tape: I lived in a house that was built in 1939 from 1964 to 2007. Notes that were left in an envelope in the house breaker panel indicated that the house was completely rewired somewhere between mid-1958 and late 1959. The feed from the power pole terminated at a breaker panel on the side of the garage. That panel had the meter, the main disconnect breaker, and 4 breakers just for the garage and workshop. That panel also fed a 2 inch underground conduit (to the house) containing three large black feeders for the power to the house. One of the black wires had no tape (not a good idea). The second had several turns of red tape around each end. The third had several turns of white that had faded to grey. There was no green - in the late 1950s the code allowed the conduit to be the safety ground. In the house every bedroom ceiling light switch had a black wire coming into it, and a black wire with yellow tape going out of it. The kitchen and dining room were remodeled between 1962 and 1964 and the switched outlets and ceiling lights in those two rooms had no identifying tape - interesting, no? The two bathrooms were rewired around 1982-1983 (under my supervision) as part of a total rebuild and they have proper colored wire to start with.
While we are mentioning tape, don't use the cheap chinese bargain-bin no-name black stuff on any AC power wiring that you are responsible for (or anywhere else, for that matter). It loses its "stickiness" with age and unravels, or it melts and turns into a gooey mess. Use the real Scotch "Super 33" or "Scotch 33+" tape. Scotch "Super 88" is even better as it's thicker, has a better adhesive, and it has better IR / UV resistance (which only counts if it's outdoors). The price difference between the cheap stuff and the Scotch 33+ / Super 33 is not worth having a splice "unwind" itself and possibly cause a short or worse, an electrical fire. Have a local union electrician show you how to tape up a wirenut. There is a non-obvious trick involving stretching the tape in the middle and not stretching the tape at the ends that you need to know in order to have your work pass inspection. While you are talking to him, have him show you how to mark a wire with phase tape. He will probably use more tape that you think is necessary, but he knows what the local building inspector(s) want to see, and a few inches more tape on each phase marker, on each wirenut and / or splice is cheap compared to being forced to go back and do it over.
8) The NEC is fluid - it is revised frequently, especially after major incidents. Thomas Edison built the first central generating station in 1882 to power a string of streetlights in New York City. Eleven years later, in 1893, the Chicago World’s Fair had the first public demonstration of public building interior illumination. After a large number of these early electrical systems caught fire, several independent code groups got together and published the first National Electric Code (NEC) in 1897. Since then the NEC gets revised every three years to reflect any problems that have been identified and to reflect any new electrical materials being introduced. One such code-revising incident was the First Interstate Bank fire on May 4, 1988 in downtown Los Angeles that started on the 12th floor (of a 62 floor building). The fire was caused by an overloaded AC power outlet, and a lack of inspection after wiring changes were made by the electricians (employed by the building) to support a new minicomputer installation two years earlier. At the time it was the worst high-rise fire in history and was the incident that "wrote the book" on fighting high-rise fires. The 1974 movie "Towering Inferno" was strangely prophetic - in the movie the fire is caused by electrical overloads...
Another incident that resulted in a change in the NEC was the 1980 Las Vegas MGM Grand Hotel fire that killed 85 people. The after-incident investigation found that the cause was a piece of liquid-tight flex conduit that fed the lights in a display case in a restaurant. Somehow a hot-to-conduit short developed. But one of the clamps in the flex conduit path had worked loose, resulting in a non-zero resistance between the flex and "real ground". The short was low enough resistance to draw substantial current but not enough to trip the circuit breaker. So the current kept flowing, and the heat kept building until it started the carpet on fire. I'd conjecture that THIS was the event that prompted the NEC to ban the reliance on grounds through flex or solid conduit and require that green wires be pulled on every circuit.
The organization behind the NEC does excellent work, but in my personal opinion, the NEC's reliance on the green wire to trip a circuit breakers is too focused on just one method. If you want to protect against hot-wire shorts to ground, then use the device that's specifically designed to detect them and open the circuit -- the ground fault circuit interrupter! (Abbreviated GFCI, frequently shortened to GFI) If a GFI breaker had been used on that MGM Grand circuit, it would have immediately tripped even without a green wire in the conduit. But having a green wire AND a GFI is better. Since that incident there are also Arc Fault Circuit Interrupters (AFCIs) (commonly called arc fault breakers) and explaining the difference between GFCI and AFCI is beyond this document, but look here. (Off-site pointer, opens in a new browser tab)
And there are also Dual Function Circuit Interrupter (DFCI) receptacles and breakers that combine both Ground Fault and Combination Arc Fault (GFCI & AFCI) protection in one unit. (Off-site pointer, opens in a new browser tab)
Another personal opinion: the minimum electrical wire sizes specified in the NEC for amperage are inadequate as a wire size guideline for proper operation of equipment. The NEC considers the purpose of the circuit breaking mechanism (a breaker or a fuse) to be protecting the wiring (to prevent fires), not to protect the load from undervoltage. In other words, they don't care about proper performance of the load (your equipment) with minimal voltage drop, just the safety and integrity of the power panel, wiring and receptacle feeding it. Unfortunately too many architects, builders and remodeling project managers allow the construction electricians to bid a job only based on local code requirements. And the electricians that do the work don't care because (a) the smaller diameter wire is easier to run, (b) the smaller diameter wire costs them less (i.e. lower material costs raise their profits), (c) they get paid the same hourly rate for running thin wire as thick wire, and (d) they won't have to live there, nor will they have to do the future maintenance (they are the construction electricians, not the repair or maintenance electricians).
The older Radio Amateur's Handbook published by the ARRL (older being defined here as late 60s / mid 70s) had a VERY nice one-page American Wire Gauge (AWG) table in them that included a column that listed the current carrying ability of the wire and another column listed the resistance of the wire per one hundred feet. It's worth hunting down a copy and xeroxing that page for your personal file cabinet. You will discover that doubling the current rating is NOT just one step up the AWG table. And remember, a wiring resistance of 10 ohms per 100 feet and two amps of load current is equal to a forty volt drop at the load 100 feet away - twenty volts outbound, and twenty volts on the return. This means that your 200 watt soldering iron is running on 80 volts instead of 120 volts and just might not get hot enough to solder that connection. And that's just a 200 watt load - how about that RV or 5th wheel trailer that has one, two, or even three 1200 watt air conditioners in it? And don't get me started on the stupidity of running solid wire in an RV... also known as a rolling, vibrating continuous earthquake. If stranded wire is legal for 120 volt and 240 volt AC circuits in boats then why isn't it legal in RVs? Using any solid conductor wiring in a moving and vibrating vehicle (RVs, boats, etc) can lead to stress fractures in the solid conductors, especially if the wiring is not fully and firmly anchored down. Every marine supplier sells stranded wire for the 120 volt and 240 volt circuits in a boat, why not in an RV? Stranded Romex also exists, it is pricier, but does last much longer than solid wire in any application which has vibration or requires movement and / or flexibility. It's available in both 2-conductor and 2-conductor with ground.
Back to Low Voltage issues... Low voltage causes even more problems on electric motors - you can actually kill a motor with low voltage / too much voltage drop. Using conductors that are just two AWG sizes larger can cut that voltage drop in half or even 2/3. (This is why the kids mentioned in the story at the top of the page used a two-inch conduit from their new building to the nearest panel - they ran a ground, a neutral, and two hot wires. And all were "fat" wires (deliberately oversize in the conductor diameter).
The NEC specifies that residential electrical must be a 14 gauge wire or larger (remember that smaller AWG numbers indicate a larger wire size) for use on a 15 amp circuit, but only up to about 50 feet of wire length (and outlets are completely forbidden on 14 gage circuits). The NEC specifies 12 gauge for a residential 20 amp circuit, but only up to about 50 feet. For a 30 amp circuit the NEC wants you to use 10 gauge - but again, only up to about 50 feet.
The NEC handles commercial buildings differently from residential - for example, in commercial environments the smallest wire size allowable is 12 gauge.
Regular circuit breakers are current and thermal trip devices (GFCI and AFCI breakers have additional trip mechanisms). The current detector is a coil - i.e. magnetic. Common breakers have a short time magnetic trip and a longer period overcurrent thermal trip. Every breaker design has a trip curve that shows exactly when they will trip based on the current. Also the ambient temperature of the breaker will affect its trip point. Remember that common breakers are rated for peak load at 100% of nameplate rating. The sustained load test specification is 80% of that, so a 15 amp breaker can trip at sustained loads of 12 amps, a 30 amp breaker can trip at sustained loads of 24 amps and a 50 amp breaker can trip at sustained loads at or above 40 amps - all will still meet specification. So for a 15A breaker the safe limit is 80% of 15 amps which is 12 amps (which is why the vacuum cleaner "amp" wars from the 1970’s to the 1990’s stopped at 12 amps of "cleaning power").
You can verify the trip point with a clamp-on ammeter - you'll be able to tell pretty quickly if the current draw is exceeding the breaker rating.
Any time you remove a breaker from the panel take a close look at the buss bar in the back of the panel and the mating clips on the back of the breaker for any signs of overheating like a discoloration in the metal. Good breaker panels have copper buss bars. Cheap panels have aluminum buss bars that corrode (especailly in coastal or other high humidity areas). Most of the less expensive panels are made to a price and have aluminum buss assemblies. If the connection is bad at the buss bar it may leave pits in the bar. If the pitting or corrosion is excessive then REPLACE it with a copper bar. If possible clean the bar carefully with emery cloth and coat it with an aluminum inhibitor like Penatrox before snapping in the new breaker. Another point: always check the main breaker size. I once found a residential breaker panel with a "100 amp" sticker on the inside of the panel... but the panel had a 250 amp main breaker between the meter and the buss bars. That panel had a 50 amp breaker pair (i.e. 240 volts) labeled "air conditioner", a second 50 amp pair labeled "oven/microwave", a 50 amp pair labeled "stove", a 30 amp pair labeled "dryer" and a assortment of 15 amp and 20 amp breakers for the bedrooms and other rooms. Do you see a potential problem there? I was surprised that the breakers were even labeled. I showed this to the homeowner, and within a month the entire panel had been replaced - with a brand new 250 amp panel.
Another 80%-of-peak-load issue is found on older motor homes or RVs that use a "TT30" connector. The connector was dubbed the "travel trailer" connector but was adopted by the entire RV industry as the standard AC power connector until the so-called "50 amp" connector (the NEMA 14-50R) was introduced (which is misnamed as it's actually two independent 50 amp circuits on a 4-conductor connector). The TT30 is rated at 30 amps and requires 10 AWG solid or stranded wires. What many people don't realize is that the 30 amp outlet / inlet is rated for 30 amps but only for up to 3 hours. If the load continues for over 3 hours it should be limited to 80% or 24 amps per the NEC. As I said above, the NEC is designed to protect the power supplying system, not the load! It is not uncommon for the common TT30 connector prongs (actually, any connector) to corrode and develop contact resistance resulting in an overheat condition. I've seen the TT30 plastic around the hot pin soften and in one case melt from the heat. And it's not just corrosion that can generate the overheat situation, I've seen a loose connection on one of the screws in the plug do it and melt the housing of the plug. Another problem with the TT30 is that the pins are sized for AWG 10 conductors. If you want to reduce the voltage drop with AWG 8 conductors you are out of luck.
Aluminum wiring was introduced to homes in the USA in the mid-1960s. At that time the price of copper was very high, and aluminum was a cost-effective alternative. Many houses were wired with aluminum wire through the mid 1970s. Aluminum wire requires outlets, switches and breakers designed for (and labeled as rated for) aluminum wire. Aluminum wire connections can overheat enough to start a fire without ever drawing enough current to trip a circuit breaker or blow a fuse. If you ever work on an aluminum wired location make sure the breakers, switches, outlets and wire nuts are stamped "CO/ALR" (for "copper-aluminum-revised"). The early devices marked "AL-CU" or "CU-AL" are no longer approved and should be replaced with the later CO/ALR units. Fires start at connections and terminations, because that's where the oxidation occurs, which causes loose connections with high resistance and overheating. One note - aluminum is not as good a conductor as copper, a circuit that would normally be wired with 12 gauge copper required 10 gauge aluminum. Many jurisdictions no longer allow aluminum wire, when found the aluminum must be replaced with copper. This can be a nasty surprise when a house is put up for sale and the pre-sale inspection drops the aluminum bombshell... the house has to be completely rewired before it can even be listed for sale.
If you have any questions about what wire sizes or what types of insulation are legal for your conditions and building environment you need to check with the local AHJ (Authority Having Jurisdiction, normally the building code inspector)) before you get started. It's way cheaper to ask first rather than to be forced to rip everything out and start over. And don't trust the sales guy at Home Depot to know the local codes (he may have been selling grass seed the week before). If the locals have different rules than the NEC, they are usually tougher and they are the ones that you have to follow to get the local inspector to sign off on the work... (as long as you use the proper size and type of wire - i.e. copper, THHN insulation, or whatever they want to see - you will never have a problem if you go lower in AWG / larger in wire size (conductor diameter), or better (in insulations specifications).
The actual NEC is big (over 900 pages), expensive and pretty dry reading, but overview books are available at most good bookstores and at web-based booksellers like Amazon. And there are the specialty technical-only booksellers.
A more accessible source of information is the overview books written for do-it-yourselfers that available at Home Depot and similar places. My favorite is "Ugly's Electrical Reference" by George Hart, it's about 200 pages and you can buy it at Home Depot. And don't forget your local public library - they sometimes have copies of the current or a relatively recent NEC in the reference section, and some bigger libraries have overview books and older issues of the NEC in the circulation section. And you occasionally find the $25 and $35 Home Depot overview books for $1 or so at church rummage sales. Used bookstores are interesting places as well.
I am not a professional electrical, nor do I play one on TV or on this web page. But I do know how to read, and I have read sections of the NEC, and several other books. I've also had long discussions with union journeyman electricians, and both city and county building inspectors. Every bit of AC power wiring that I personally do uses wire sizes that are at least NEC commercial - and in many cases I go even further (I've been known to use #6 wire (that's what is called for in a 70 amp circuit) for a 75-foot run to a 20-amp air conditioner since low voltage does nasty things to AC motors). Personally, in the last 10 years I've never installed any branch circuit smaller than #12 (even for a two-amp bathroom ceiling fan).
Most circuits in the modern home in the United States are wired with non-metallic sheathed (i.e. "NM") 14‑2 or NM 12‑2 cable (often referred to by the trade name "Romex" - another name that has become a generic). This type of cable is the least expensive for a given size and is appropriate for dry indoor applications. The designation NM XX‑Y indicates, respectively, the type of sheathing (in this case, non-metallic), the size of the main conductors, and the total number of circuit conductors not including the green safety grounding conductor. For example, NM 14‑2 cable contains three 14 gauge conductors, a black wire, a white wire and a bare wire (for ground). NM 12‑3 cable contains four 12 gauge conductors, a black, a white and a red plus the bare wire ground. When NM XX‑2 cable is used on 220v circuits (like to a window air conditioner), the one black, one white, and a bare grounding conductor conflicts with the NECs rule about the white conductor identifying the neutral conductor (a 240 volt 2-wire circuit does not have a neutral). The NEC allows the white wire to be used as the second hot conductor, but mandates the white be marked with phase tape on each end (preferably red).
Update 2010:
Since this article was written a number of manufacturers have started color coding the outer sheath of their NM‑cables. The color indicates the gauge of the wire inside the sheath. The color coding started about 2001 and as of this update is still voluntary. If you have older wiring with color sheaths, don’t assume it complies with the current color coding. Check sheath labeling for wire gauge and wire count. Most manufacturers that do color the sheaths now follow this color code:
BLACK = 8-gauge or 6-gauge wire, 45- or 70-amp maximum circuits.
ORANGE = 10-gauge wire, 30-amp maximum circuit.
YELLOW = 12-gauge wire, 20-amp maximum circuit.
WHITE = 14-gauge wire, 15-amp maximum circuit.
GRAY = UF (underground feeder). Since all UF cable is gray, check the printing on the sheath labeling for gauge and conductor count.
UF is rated for direct-buried or run in conduit and is used primarily to bring power to detached garages, outbuildings or outdoor lighting. It must be protected from physical damage by conduit where it is exposed (enters or exits the ground).
(end of 2010 update)
By the way, electrical color codes are more extensive than anything I've listed above. See the web page at:
www.allaboutcircuits.com/textbook/reference/chpt-2/wiring-color-codes/ for an excellent overview.
Wikipedia has an excellent writeup that goes into a lot more detail than this article at: http://en.wikipedia.org/wiki/Electrical_wiring. It also includes a color code chart for other countries.
There's an article on three-phase power that includes color codes at http://en.wikipedia.org/wiki/Three-phase_electric_power.
The mathematics of three phase power is covered in a different article at http://en.wikipedia.org/wiki/Three-phase.
There is also some information on single phase power at http://en.wikipedia.org/wiki/Single_phase_electric_power.
At the new radio site mentioned above the project supervisor (the mason) listened when I pointed out my concerns (mentioned above) about undersized wire and high power loads in both sides of a duplex outlet so instead of using #14 they used #10 on every outlet. The kids wired every branch outlet with a #10 black, a second #10 (red, blue, brown or yellow) and a #8 white (neutral), to each outlet box (plus the green/yellow safety ground, also #10), so that in the future 220vAC can be easily obtained at any location in the room if needed. A few folks thought that the #10 was serious overkill for the load involved. The project supervisor (the mason) ignored them.
The two overhead lighting circuits were wired from two different breakers (one on each phase) with #12 wire - a green/yellow, a white and two hot wires (red and blue). They mounted two light switches by the door, feeding two four‑foot single tube fluorescent fixtures. The two phases, two breakers, two switches and two fixtures on the lighting insured that there would be light even if one of the lighting switches or breakers failed, or if they lost a phase from the upstream breaker panel. Doing things that way is just good planning and design... I've seen too much new construction that has only one circuit per room. When I help wire a house I extend three circuits to each room: the first powers the ceiling light, the second for the outlets on two adjacent walls and the third for the outlets on the other two adjacent walls. No matter what breaker you have to switch off to repair something, if you need light you just turn on the ceiling light, or plug your work light into an outlet on the opposite wall - you won't need an extension cord that runs to the next room or to the far side of the house.
Tools that you might not think of: A 100 foot Greenlee brand nylon fish tape is rather expensive, but worth it as it will last you for many, many years (like my dad used to say, "Buying quality tools only hurts once"). Add to your shopping list a squeeze bottle of Scotch #77 wire lube to help the wire slide through the conduit or flex and a black Sharpie marking pen (to label the circuits - just use the Sharpie to write on the inside of the outlet boxes as to where the wire is from, i.e. "FRM CB13" ("From circuit breaker #13"), on the inside of the ceiling light fixture you could write "FROM CB #4 VIA LEFT SWITCH BY DOOR". The next gentleman that has to work on your wiring will bless your thoughtfulness. Some people write the panel and breaker info on the back side of the outlet and switch plates, but plates can be replaced or swapped around when the house or a room gets repainted. I've seen industrial locations where the outlet places were engraved (and paint filled) with the breaker panel number and the breaker number... the two outlets on my workbench at NASA-JPL were labeled "264/2LA-13" and "264/2LA-15"... building 264, panel 2LA, breaker 13 and 15.
The school electronics lab workbenches I mentioned above? They were sitting on a linoleum tile floor, which was on top of a second, older, asbestos tile floor, each of which acted as an insulator. One bench had been added to the rear of the room a number of years after the lab was built, and placed in a back corner of the room to be used as an equipment repair bench (electronics students are rough on equipment...). The story was that the back bench AC outlets had never worked because they had never been hooked up; hence it was used as a storage bench and library table. I accidentally discovered its exterior was hot one day when I leaned on it and touched an adjacent bench. Thirty seconds (with a VOM) later I determined that the shell of the bench was 120vAC hot to either of the nearest two benches, and 5 minutes later I knew that it wasn't just high resistance leakage current - you could light a 120v 100 watt incandescent light bulb to full brilliance by connecting one side to the "dead" workbenches AC outlet ground screw and the other side to the adjacent benches ground screw. Upon dismantling the bench, we discovered that it was connected with the black wire to the metal framework (the bench "ground"), and green to the hot wire of the outlets, after all black is ground, right? The breaker feeding the "hot" bench was quickly located and switched off, and locked out until the wiring was corrected by and signed off by the school physical plant department. Yes, we could have swapped green and black ourselves, but to quote our instructor "Never get a union electrician angry at you - and especially when your paycheck is signed by the same person as signs his paycheck". Besides, I think he wanted the campus electricians to see the reversed wiring - it had been that way for at least the eight years he had been there (can you say "liability"? ... I knew you could).
About five or six years later I was working at NASA / JPL in Pasadena and was suddenly dropped into a similar situation. Our group was charged with the regular maintenance of all of the data processing equipment used in two adjacent buildings dedicated to the ongoing operation and support of spacecraft missions (buildings 230 and 264). As such, in many cases, JPL designed and built its own equipment. One multi-cabinet rack mounted system used 36 power supply shelves of four separate power supplies each (+12v, -12v, +5v and -5v DC). The tech that had wired them had done so in the "electronics" fashion. He had treated the black wire of the AC power cord as "zero volts", the white as "hot", and the green as chassis ground. Both the AC fuse and the AC power switch of each power supply shelf were wired in the white lead, which left exposed terminals in the power supply hot to the chassis when the power was switched off - not a good thing when you have your hand inside the equipment to change one of the eight hard-to-get-at fuses (the first for the AC side and the second for the DC side of each power supply), or to press a reset button on one of the circuit boards.
Note: There are two schools of thought on the sequence of switch and fuseholder. When I was there the in-house manufactured equipment I worked on was wired with the hot wire going first to the switch, then to the fuse holder, and then the load. The idea was that when the switch was off then everything inside the box - even the fuse holder - was dead. The other school of thought says that the fuse is first so that if it blows everything is dead.
Unfortunately the equipment was installed in a long row of 20 six-foot-tall equipment cabinet racks in the main computer room of the Space Flight Operations Facility (commonly called the SFOF, pronounced "ess-fof"). This was the main scientific center for all of JPL, and was used to support multiple simultaneous spacecraft missions. The biggest problem was that the SFOF had several missions in progress, the most visible of which at that time was Viking... and on a absolutely unchangeable deadline: Viking One was over half way to Mars, Viking Two was 18 days behind, and simple physics tells you that neither were going to wait for the folks back home to get their equipment rewired! Remember folks, physics makes the universe go 'round. Fortunately the wiring error was discovered (by one of my co-workers) before someone was hurt, and after several discussions between us, our group manager, the system designer, the SFOF electricians, their supervisor and the Lab safety supervisor it was decided to simply cut the 36 AC power plugs off the equipment power cords (this was in the days of wired-in power cords with molded-on plugs) and to mount 36 brand new plugs with the black and white wires deliberately reversed on the screws of the plugs. Established electrical safety protocol(s) of the time required a 3 inch by 5 inch red cardboard trouble tag to be taped to the front and rear outer doors of each rack, 36 more were taped to the front of each power supply tray, and 36 more were tied to the plug end of each of the 36 power cords. Yes, a lot of overkill, but when two facility electricians, the electricians supervisor, the lab electrical safety specialist, his supervisor, his department manager, the equipment designer, his supervisor and the local federal OSHA representative are breathing down the back of your bosses neck pointing out his workers lives are involved, the cardboard tags are cheap... Each of the 144 tags identically described the same identical deliberate wiring error in the plug, the same identical reason for it, provided the same three contact names and phone numbers, and listed the same will-be-repaired-before date... we should have had the print shop make 144 of them or used a rubber stamp kit... (yes, this was the redundancy proliferation and reduction group in the department of redundancy department in the redundancy control section of the redundancy management division - typical NASA). Plus we were instructed to lock the rack cabinets until the problem was repaired - and locked equipment cabinets was something that was never, ever done in the SFOF equipment rooms. There was a reason that there were CO2 extinguishers hung on wall brackets every 20 feet on every SFOF equipment room wall... So we cheated (a little)... we locked the cabinets but every tech and every lead tech was issued a key the next day... (all 26 of us). The supervising tech and the department manager were offered one, but both declined.
At some point in the above discussions one of the federal OSHA representatives casually suggested that the JPL Safety folks have a staff electrician walk the electronic technicians (especially those in the design / construction and maintenance groups) through an AC Power Orientation class. A mix of "you're absolutely right", "good idea", some bobble-head-style head nodding and other similar affirmations were heard and seen. By the time the OSHA representative's casual suggestion filtered up the three or four management layers, across the bureaucratic morass, and back down again to our group it had turned into two (identical) formal training classes (complete with a 3-ring binder workbook), one in the morning for 1/2 of the day shift plus the entire night shift, and a second one in the late afternoon for the other 1/2 of the day shift plus the entire evening shift. Attendance at one of them was mandatory which meant that the evening shift and night shift folks got overtime pay (our group provided 24x7x366 support). Plus it would be videotaped for new employees, i.e. due to employee turnover - the newbie could watch the tape and get the full benefit of the class (or so they say...) The first version of this article was written from those 1976 class notes... (To this day I'm still surprised that the class wasn't made mandatory lab-wide).
Months later, after the two Viking landers were successfully down on Mars (touchdown of Viking 1 was on July 20, 1976 at 0512 Pacific Time, and I was in the SFOF watching the video monitors, along with what seemed like every lab employee whose badge gave them access to the building). Viking 2 landed on August 7th and after the background noise level around the SFOF had quieted back down to something approaching "normal", I scheduled progressive equipment outages with the appropriate folks and I budgeted one shift per day across three weeks to remove, rebuild and reinstall the power supply shelves... with the black wire as hot, and the white wire as neutral and the green wire as chassis ground. As long as we were doing rebuild work on the system we also decided (with management approval) to zero out the maintenance technicians "wish list" for that system... We relocated all eight power supply fuses on the shelf to indicating fuseholders mounted in the front panel of that shelf, plus we added a guarded reset button to the front of the logic shelf... you had to lift a cover before you could press the button (it was in parallel with the one on the main logic card... we were tired of removing six screws on a cover panel and then twisting our hands into a contortionists form of the Vulcan Nerve Pinch to press the secret magic reset button, and then replacing the panel and the six screws). Another senior tech and I modified two power supply shelves at a time, six shelves per rack, for all six racks of equipment. Plus, as good hams always do, we added a few other improvements that we deemed necessary, then red-lined a new set of as-built drawings and submitted them back to Documentation Control. As we progressed down the line of cabinets we stripped the red trouble tags off the equipment and gladly tossed them in the trash (I should have kept one of them).
As a final note, improper AC wiring is far too common, especially in residential areas, and if you don't have to pull new wire it is usually easy to fix. Recently a friend was transferred by his employer to an area near me. He and his wife had been shown over thirty houses by various agents, and together they trimmed the final list to five. My friend asked for my help in making the final selection - a fresh set of eyes, so to speak. As we walked through the five houses I took about 20-30 digital pictures per house plus written notes on the visible problems / defects in each house, and I also bent over and plugged an outlet wiring tester into every AC outlet that was easy to get to (the tester is a rubber plug with three indicating lamps under a clear cap - it is available from most electrical supply houses, Home Depot and even Ace Hardware). Over 40% of the outlets in the five houses we looked at had either open grounds or reversed hot and neutral wires... this is not a good percentage! And in cases like this an open ground usually indicates that the selling homeowner has simply replaced the old 2-wire outlets with new 3-wire outlets to make the house look like it's been rewired...
Click here for photos of an outlet tester and of an outlet and GFCI tester (sometimes called a GFI tester). If you are going to buy one new, get the GFI tester version. And note that the three indicators only need a few milliamps to light up - an outlet can test good but be functionally dead (i.e. not delivering usable power due to a high resistance connection somewhere upstream).
Permission to any group to use this writeup is available for inclusion in any newsletter or bulletin for noncommercial, nonprofit, educational and public safety use within the scope of the U.S. Copyright Law. Just drop me an emailed note before you go to press.
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Copyright © Michael R. Morris WA6ILQ July 2003 and date of last revision.
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The author, Michael Morris WA6ILQ, can be contacted here.
This web page, this web site, the information presented in and on its pages and in these modifications and conversions is © Copyrighted 1995 and (date of last update) by Kevin Custer W3KKC and multiple originating authors. All Rights Reserved, including that of paper and web publication elsewhere.
