Electricity for Glassblowers

Rev. 2002-09-01,  2003-08-30, -11-14, 2004-03-03, 2007-02-17, -04-02, -06-11, -11-04
2009-12-04, 2010-10-23
[Search on date pattern to find latest changes, more than one may be found.]

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Reversing and Repair of Electric Motors

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The purpose of this page is provide reasonably accurate information to glassblowers about understanding and doing their own electrical work.
 If you are not comfortable dealing with electrical wiring, don't do it!
But you may still be able to learn from this page information that may prevent a fire or help you understand the inspectors, contractors, and workers that you have to deal with.

As far as possible, information provided here complies with the NEC 1999 (National Electric Code for the United States created and maintained to guide cities, who mostly use the code intact, in maintaining standards that avoid fires from electrical failure.) [There was an update in 2002, which I have not reviewed as of 2003-11. A copy costs about $60 and lots and lots of people will sell you the book, or a CD-ROM version, etc.  I have not yet found a web site with a summary of changes.]

SAFETY - The electricity used in glassblowing can kill or seriously injure people in the shop, just as it can in the home, but often home equipment comes with safety features that have to be designed into glass shop equipment. Features designed to increase safety should not be defeated or bypassed and rules for safe wiring should not be ignored. All cases should be grounded and all circuits with the remotest chance of human contact should be protected with a GFCI. When working with power connections, the electricity should be cut off, yet the worker should still behave as though it might be electrically live (as it may be if a fault exists) and use gloves and insulated tools while testing for voltage on connections.


Certain features of electricity require almost no knowledge of theory. It is possible to function safely knowing only practical things.

  • While USA 60 cycle power is very accurate, the voltage delivered is approximate and can range from 110 to 125.  Below 110 is considered a brownout. Most of the time the voltage is between 115 and 120.  For this reason, you will see reference to house power being 110, 115, or 120 (and rarely 125). Double voltage for driers and ranges is exactly twice and thus may be 220, 230, 240 (or 250).  Strictly speaking, one should always pair the low and high accurately, 110/220, 115/230, or 120/240, but casual usage will often say 120/220, etc.  There are other voltage levels associated with 3 phase, such as 208, that will be ignored here.  I will use 110/220.

  • All wiring connections must have a solid mechanical connection - just twisting wires and taping them is not acceptable. The mechanical connection may be a wire nut, a split bolt, or a terminal connection. In the old days, wires were twisted and soldered before wrapping in tape, which is not acceptable now.

  • All electrical connections must be protected from outside electrical contact - they must be insulated. In the case of a wire nut, the plastic cone provides the insulation if the wire ends were not stripped off too long - no copper should be showing. Split bolts are normally taped over. Terminal connections must be contained in a box that prevents physical damage and contact.

  • All electrical wiring must be protected from physical damage and all connections must be enclosed or out of reach. In the most practical terms, this means that wiring must be enclosed in conduit or if Romex, placed so that it can not be crushed or stepped on.  Wire running over joists, for example, must have wood blocks so stepping in the area does not crush the insulation.  Ends of wiring runs must be placed in junction boxes so there are no visible wire nuts or terminals.

  • Wiring must conform to standard rules as to size of wires for current being carried. Current is the casual name for the "stuff" [electrons] that moves in wires to do work. The measure of current is amps and certain size wires are required to carry each level of amps so the wire will not overheat. The most common amp levels and their matching wires are given just below. The theory further down discusses why.
    15 amps 14 gauge
    20 amps 12 gauge
    30 amps 10 gauge
    50 amps   6 gauge

  • Each size of wire must be protected with a matching circuit breaker (or fuse.) These devices are carefully designed to carry the current specified and to trip (or blow) rather quickly if the current goes higher. The list above is the most common breaker sizes also. The purpose is to protect from fire. If wire is too small for the breaker used, the wire will get hot and may start a fire. It is always wrong to replace a breaker that is constantly tripping due to overload with a larger one. [Rarely it may be useful to reduce the size of a breaker to protect the equipment at the far end of a larger wire.] With age, a breaker may start tripping at lower than its rated amperage and should be replaced with the same size.

  • There are rules about how many connections can be made inside a junction box and how many wires can pass through. The bigger the box, the more wires and connections. Again, the goal is to avoid overheating. The exact numbers are in the Code.  Most boxes have the number of cubic inches stamped on them and that can be used with a general rule of wires and connections. "1. Count the number of wires for the box. Don't count outlet/switch pigtails and count all ground wires as one. 2. Take that number, add one for each cable clamp, and two for each device (like a switch or outlet). 3. If the box contains only 14-gauge wires, multiply the total by 2 cubic inches. Or, for 12-gauge wires, multiply the total by 2.25 cubic inches." Hometime - How-To - Project Help - Electrical

  • When a device is to be unplugged instead of hard wired, there are standard connectors matched to the amps and volts of the connection. These are called plugs and sockets/outlets or male and female connectors. The shape of the plug on the removable part declares "This device needs no more than this amount of power" while the matching outlet declares "I can safely deliver up to this amount of power." The common household outlet with two parallel prongs and U shaped ground prong is rated at 15 amps and 120 volts.
    Most people have seen the 30 amp and 50 amp 240 volt plugs used on dryers and ranges. There are many other pairs of plugs and sockets matched to levels of amps and volts. The different arrangements are intended to prevent plugging into the wrong power.
    [A major change occurred with the 1999 NEC - before this version, it was allowed that a drier or a range needing 240 volts might use a three prong plug - two current carrying lines and a combined neutral and safety ground. These plugs had two flat blades arranged in a V with the ground prong between the legs of the V. All new appliances and all new construction under the 1999 NEC must use a 4 prong plug, separating the white neutral wire from the green or bare safety ground. All recently built appliances are easy to convert to the new plug; older housing may require serious rewiring to bring in the fourth wire.]

  • A junction box must never have power from two different breakers going though it - that is when a breaker is tripped off a worker should be able to open all the connections in a box connected to that breaker and be sure that no power is coming from another breaker that is still on. Similarly, if two outlets side-by-side are served by different breakers, they must be in separate boxes, not paired in a double wide box.

  • European 220 volt power is not the same wiring as American 220.  In American 220, the two hot wires exchange phase 60 times a second and are 110 volts above and below ground at maximum.  In European, the hot wire is 220 volts above ground at maximum and the neutral wire is at or near ground.


Electricity travels through conductive materials in the form of moving electrons. Depending on the physical structure of the material, the electrons may move freely or meet with more resistance. When resistance occurs, energy is lost as heat. This may be a good thing, if a heating element is desired, or a bad thing if the goal getting as many electrons as possible out the far end of the wire with as much energy as possible..

The common measurement of electron flow is done in units of AMPS (which in electronic circuits may be scaled down to milliamps 1/1000 Amp, abbreviated ma.) The force driving the electrons is defined in VOLTS (or millivolts or kilovolts). Power is measured in WATTS, defined as Volts times Amps and power does work. The shorthand formula for power is W=EI, where E is the symbol for voltage no matter what the units, and I is current. There are very specific relationships of the electron to the volt and the amp, but such details can be researched elsewhere.

Resistance is measured in OHMS. There is a very specific relationship of Ohms to Volts and Amps, E=IR. That is, voltage equals current times resistance. The formula can be rearranged using the rules of algebra - I=E/R or R=E/I. To give specific examples, a 120 volt light bulb that draws 1 amp will have a resistance of 1/120 ohm. Since watts are related to volts and amps, it is possible to determine wattage from volts and resistance.

All normally used materials have some resistance, therefore there is some heating and some voltage loss. Silver, aluminum, and copper have fairly low resistance and copper is used because silver costs too much and aluminum has other problems. If wire is small diameter it has higher resistance and loses more voltage over a given length. Therefore, standards set the size of wire to be used for normal runs so that voltage does not drop below an arbitrary level.

Standard wire ratings for reasonable runs in a house
15 amp 14 gauge 0.0641" diameter
20 amp 12 gauge 0.0808"
30 amp 10 gauge 0.1019"
50 amp 6 gauge (actually 55 amp)
Voltage does not matter here, 12 volt, 120 or 240 (although a volt of loss in 12 volt is a lot more important than at 240)

If you go and look at extension power cords you will see good examples of the trade off of length and power. A common cheaply sold 16 gauge 100 foot cord will only deliver 10 amps out the end - pulling more will drop the voltage too far to run the tools properly. A 50 foot cord will deliver 13 amps and a 25 foot cord 15 amps. To deliver 15 amps at the end of a 100 foot cord requires a 14 gauge cord. [On the other hand, a 12 gauge cord should deliver 20 amps, but it is commonly equipped with a 15 amp parallel blade plug and therefore can only be reported as being rated at 15 amps, but it will have less voltage drop at the end of the cord than a 14 gauge.

Two goals are in mind here: avoid fires from heating the wire by overloading it and avoid voltage loss over a long wire with a load at the end. Long runs of wire should be sized larger. A light weight 16 gauge extension cord 100 feet long (under $10) will only deliver 10 amps out the end (even though the plugs are rated 15 amp) because there is too much voltage drop at higher amps. A circular saw run on this cord will burn out very quickly because as the saw bogs down, its back voltage falls and more amps try to get through, increasing heating. To get a full 15 amps out of a 100 foot cord, a 12 gauge (about $45) must be used.

[This site Copper wire has a lot of info, with a lot of typos also.]

AWG  dia    circ  open   cable  ft/lb   ohms/
     mils   mils  air A  Amp    bare    1000'

10   101.9 10380    55    33    31.82   1.018
12    80.8  6530    41    23    50.59   1.619
14    64.1  4107    32    17    80.44   2.575
Mils are .001".  "open air A" is a continuous
rating for a single conductor with insulation in open air.
amp" is for in multiple conductor cables.  Disregard the
amperage ratings for household use. 

AMPS - A measure of how many electrons are being pushed through the wire. An amp is to electricity like a gallon is to water. Modern house service is commonly 200 amps. Circuit breakers commonly limit service to 15, 20, 30 or 50 amps on each circuit.

OHMS - A measure of resistance of wire; resistance being the blockage of electron movement.  Resistance is measured by applying a voltage and measuring the current flow - 1 ohm lets 1 amp flow under pressure of 1 volt. Some people have a problem with resistance because we are so used to increasing the pressure to change flow - opening a valve to force more water through a fixed resistance. With electricity, the voltage is normally fixed, commonly at about 115 volts, so we change the resistance to get more current. If the material (say copper) is not changed, then more area (bigger diameter) decreases resistance. A longer wire increases resistance. Changing material can also change resistance - iron has more resistance than copper which has more resistance than silver.  Heating elements are made of alloys of iron, among other things.

VOLTS - A measure of the pressure available to push electrons through a wire. A volt is to electricity like pounds per square inch (psi) is to water. American house power is specified to be 110-125 volts and appliances must work over that range. At my house, about 117 volts is common most of the time (I just went and measured it and found it to be range from 118.7 to 119.2 at noon on 2000-8-9. I expect it to go down as the temp goes above the 94 it is right now). Below 110 is considered a brown out condition, lights are visibly dimmer.

WATTS - A measure of the power being used or required, the product of amps and volts. For a light bulb, 120 volts times 0.833 amps yields 100 watts. From a modern 15 amp outlet running a heater, 1875 watts is commonly specified; this is 15 amps times 125 volts and will be lower if the voltage is the more common 115-120.

KILOWATT-HOURS - Billing in the U.S. is commonly done in kilowatt-hours where kilo means 1000. (KWH*3.6=MegaJoule in metric.) It is the product of watts and time. A thousand watt heater left on for an hour uses a kilowatt hour; a 100 watt light bulb left on for 10 hours also uses a kilowatt hour. In the United States, the cost of a kilowatt-hour varies around the country depending on how electricity is generated and within a city depending on rules of the service contract (big businesses who agree to have their power cut off in an emergency get a lower rate as reward.) My electric bill last month was based on 7.78 cents ($0.0778) per kilowatt hour and my average usage was $5.98 per day. I did no glassblowing last month. [In 2007-06, Texans are paying 13.5 cents per kwh.]

Down below is a table showing rates in various states (selected as having glassblowers mostly) and regions. Note how low the rate is in Washington state with hydroelectric power. The electric cost of running an operation in California or Massachusetts would be more than twice as high as Washington state. [these figures before the deregulation disaster in CA]
"On a national level, the price of electricity sold by utilities in 1998 averaged 6.75 cents per kilowatt-hour.... In the residential sector, on a cents-per-kilowatt-hour basis, the price fell to 8.27 cents during the year from 8.43 cents in 1997. Both the commercial and the industrial reported lower electricity prices in 1998 at 7.43 and 4.50 cents per kilowatt-hour sold, respectively.  This decline in the price of electricity was a result of the lower cost for fossil fuels and rate reductions in response to the emerging environment of competition in the industry. "
DOE report at http://www.eia.doe.gov/cneaf/electricity/epav1/epav1_sum.html
A serious glassblower should negotiate for lower rates.

SERVICE - House and business wiring begins at the service entrance, but only strange people like me do it themselves so you may wish to skip through this section.
Virtually all homes and small business sites are supplied with power in the form of three wires that deliver 220-240 volts. Business sites may have higher voltages (440 or more) and three-phase that let more power be delivered over the same size wires. This power is delivered to a service entrance, usually a head raised above the roof on high on the side of the building (the height is set by the Code.) Wires bring the power down to a meter base where usage is measured for billing and then wires take the power through a main breaker, which may inside or outside the breaker panel. High voltage appliances, like air conditioning and kilns, remote from the breaker box, must have a cutoff switch at the unit for safety of workers, so there is no risk from someone turning the breaker back on while they are working.

BREAKERS - The purpose of a breaker or fuse is to keep the wiring from overheating and causing a fire. Therefore replacing a fuse or breaker with a higher rating just because it keeps blowing is a no-no. Circuit breakers have replaced twist in fuses as the primary protection in all new construction. Cartridge fuses are still used in inline switching panels. A circuit breaker is a heat activated switch: a tiny amount of the current flowing through the breaker is used to generate heat and when the heat gets high enough, a metal latch trips and the breaker trips. (For this reason, a breaker can "just wear out", although normally this takes decades.) Breakers are designed to hold a load at their rated amperage forever and to trip faster and faster as the amp goes over the limit. It may take a minute or more for a 20 amp breaker to trip at 21 amps and a fraction of a second at 30 amps (the exact timing is set by standards like UL (Underwriters Laboratory.))

Under the Code, if a breaker box has more than six breakers, it must have a master breaker to cut off all power to the box with one switch. This breaker can be inside the box or separate. A box is rated at the size of the master breaker, 125 amp, 200 amp or more. The number of breakers that can be put in the box is actually determined, according to the code, by the number of amps and the rating of the breakers. The code recognizes that not all equipment will be used at the same time and not every circuit will be used at full load, so it allows a certain amount of overload - more apparent capacity than the main breaker will supply, but it places a limit on this. When I moved in this house it had 2 breakers in a 6 breaker box with no master and a 60 amp meter base. Now it has a box with 20 slots for breakers, 18 of which are filled, 2 with 15 amp (1 a GFCI), 4 with 220 volt 40 amp double wide breakers for the garage and to glass center in the yard, 2 with 220v 30 amp double wide for the air conditioner, about 10 with double 20's and the balance with single 20's. No matter how you add that up, there is a lot more than 200 amps in the breakers. I have over-wired this small (1100 sq.ft.) house. There are, for example, 5 distinct 20 amp circuits available around the edges of a 30 by 30 foot area in the back yard and 5 separate circuits in the kitchen. (The dishwasher circuit has an outlet on it, as does the gas oven electrical. The first does not usually run if we need power for a griddle, the latter draws little power any time.)

Normally, wiring design is done from the other end, because it is recommended that most large appliances have a circuit and a breaker to themselves. The amperage of the appliance is stated (20 amps for disposal, 30 amps 240 volts for a drier, 50 amps 240 volts for a range) and the length of the run determines the wire size, usually the numbers given above. Then a breaker is installed to protect the wire and the unit. Because of the cost of copper wire, over wiring is rarely done. However, 15 amp outlets are commonly wired with 12 gauge wire and 20 amp breakers because the code says you can put up to 6 15 amp outlets on such a circuit. If a circuit were run with 6 gauge wire (50 amp service) but a dryer or kiln only calling for 30 amp service were installed, then installing a 30 amp breaker would be appropriate to protect the innards and connection wiring of the appliance. A note of the higher rated wire should be placed on or near the breaker for future use.

OUTLETS - Outlets are the devices mounted to the wall (or floor) to plug in devices. Often units with heavy current usage, such as kilns or air conditioners, are direct wired (directly to terminals in a box), but code requires that a cutoff be easily at hand. Outlets are coded by the size and shape of the slots according to a published standard. Usually higher amperage connectors are heavier metal or have more contact area (as in twist lock connectors.)

An outlet announces by its shape that it can supply a given amperage and voltage. The matching connector on equipment declares its need for that voltage and less than that amperage. It is always best to use the proper connectors in glass work to avoid confusion and possible destruction of equipment. A lower voltage outlet (120) should never be used on a higher voltage circuit (240) As long as the breaker protects the wire, there is somewhat more flexibility with higher or lower amp outlets.

PLUGS - Plugs and outlets are designed to work together. As it says under outlets above, there is a standard that specifies the shape of a plug to carry a specific current and voltage. Physical requirements, such as a twist lock to hold the connection may also affect the choice. In the past, dryers and ranges operated under an exception to the NEC that permitted two hots and a wire that served as both neutral and safety ground. This was eliminated in the 1999 revision of the code, and now all dryers and ranges come with four prong plugs (two hots, neutral and ground) and all new construction must use this pattern. I mention this because people often wire their 220 annealers using these common and less costly dryer/range connectors.
  There is a product that conducts heat and keeps oxygen in the air away from connections to reduce corrosion and increases in resistance which result in the plug getting hot to the touch and aging it.  Sold as anti-seize compound or conductive paste or silicone grease is commonly sold for coating aluminum terminals, spark plugs, and aluminum light bulb bases. Permatex is one brand.  It is an messy product when used on plugs when these are pulled. It needs to be applied to the prongs before insertion into the socket. 2007-11-04

GFCI -The purpose of a GFCI is to protect people from a small current leakage (4-6 ma.) across the heart that is large enough to set it into fibrillation. It is not a circuit breaker, which deals in amps and protects the wiring from overheating. A circuit in the GFCI detects that the current flowing out is different from that flowing back, with the assumption being that the missing current may be flowing through a body, across the heart, to another ground. Mine tripped when a heating coil grounded part way around. Installing a GFCI at the circuit breaker box can be very expensive (i.e. a plain breaker costing $4.99 may be matched with a $45.99 price for the GFCI.)  Protection for up to 20 amps can be had much cheaper by installing a GFCI outlet (normally 15 amps at the outlet for about $10) most of which protect and feed through 20 amps to other much less expensive outlets, 15 or 20 amps. 2/27/96
For higher current one choice is Leviton High Current GFCI, part #8895 link, up to 80 amp/ 240 volts 1 or 3 phase with contactor, 2009-12-04
The sensitivity of a GFCI is based on tests which demonstrated a healthy adult could stand that current across the heart without the heart stopping. Today I was setting up the heating element used for preheating my furnace and touched the tip of the thermocouple to check its location. Something was at 120 volts and I got a brief shock. The GFCI tripped. I don't know what might have happened if I did not have the GFCI, anything from a sharp jerk of muscles to severe damage and burns. Several years ago, I got a fairly full shock when I touched the contacts on a wall switch because the plate was off. I barely recall the muscle pain and spasm, but it was considerable. I don't expect to give up using GFCI's to find out how it felt.. 11/10/96


THREE-PHASE - If the voltage of regular house power is examined on an oscilloscope, it looks like a simple sine wave, the smooth curve going up and down. It is off (0 volts) 120 times a second and goes from +120 to -120 on each swing. This is technically called single phase power, a term that is almost never used. Single phase power takes two wires (often a third for safety ground.) Single phase electricity sine plot
When 240 power is required, a second sine wave is introduced exactly the opposite of this one so at the peak, there is 240 volts from peak to peak and zero at the crossing.  Good overview (new window).
Three phase power has at least three wires (usually 4 for a safety ground) and when the voltage on the three wires is examined over time it is discovered that each of the wires has the voltage going through zero at a different time, evenly dividing the interval graphed like this: because of the timing (and the fact that power comes out of the generator this way) it is possible to deliver power more smoothly and to deliver more power over the same size wires. For this reason, most big power users receive their power as three phase and either divide it up among different single phase uses on the property (carefully balancing the loads between phase to make the power company happy) or using the three phase directly. Most really serious glass fusers and saggers (with kilns measured in feet, not inches) use 3 phase feeds to the kiln. Besides, it is cheaper. Three phase electricity sine plot


De Clarke wrote:
What is hazardous, and you should not do it, is to upgrade the breaker and outlet without upgrading the wiring. Circuits and outlets are rated for their ability to carry sustained current at X amps. If you shove 30 amp through wiring sized for 15 amp, the wiring will get warm. Quite warm. This is what's known as "risk of electrical fire" :-) If I were living in a house where creative things had been done to the breaker panel by the previous owner, I would examine the gauge (thickness) of wire used at the outlet, and make sure (a quick chat with the local lighting or electrical parts store) that it is appropriate for 20 amp. Someone on this list may be able to advise on the table of wire gauges to amperages... it is not the kind of thing that I have memorized.

"Connie should carefully check your "new" house, because it seems like an older house, and see whether the wire matches the breakers. This can be done by squinting at the tiny print on the wires or by going to a hardware store, smiling nicely and getting 1 inch samples of solid wire in these gauges.
Please note that the standard outlet - two parallel prongs - is announcing to equipment that this outlet can deliver 15 amps. If it connected with 12 gauge wire, which almost all modern homes are, then it can be protected with a 20 amp breaker.
The code even specifies how many 15 amp outlets may be put on a single 20 amp circuit - 6 as I recall.
For this reason, it is often possible to pull 20 amps out of a single outlet, with everything else on the circuit off. The metal in a 20 amp plug is the same as a 15 amp plug. However, I recommend and use a product like Ox-Gard, which is a conductive paste that keeps connections (battery or aluminum) from oxidizing. I find that my plugs run a lot cooler under heavy loads when I clean them to a polished look and then use a dab of Ox-Gard on each prong before plugging them in. (Caution, Ox-Gard is black and goopy, don't get it on clothes, etc., that you like.)


RATE TABLE 2007, 2006 from source Electric Power Monthly - Average Retail Price of Electricity to Ultimate Customers by End-Use Sector, by State http://www.eia.doe.gov/cneaf/electricity/epm/table5_6_a.html  Other data available. 2007-06-11
Table 5.6.A. Average Retail Price of Electricity to Ultimate Customers by End-Use Sector, by State, February 2007 and 2006
(Cents per kilowatthour)  MF-Selected states
Census Division
and State
All Sectors
Feb-07 Feb-06 Feb-07 Feb-06 Feb-07 Feb-06 Feb-07 Feb-06 Feb-07 Feb-06
New England 16.55 16.53 15.21 15.2 12.84 10.98 9.58 7.06 15.33 14.9
Massachusetts 16.87 18.3 16 17.39 13.45 11.31 6.07 5.52 15.82 16.56
Vermont 14 13.28 12.03 11.42 8.87 8.44 -- -- 11.95 11.28
Middle Atlantic 12.82 12.74 12.31 11.17 7.65 7.22 10.74 9.8 11.59 10.9
New Jersey 12.84 11.36 11.61 10.24 11.09 8.71 10.96 6.42 12.01 10.41
New York 15.93 16.61 14.65 12.94 8.56 8.16 10.98 10.7 14.19 13.49
East North Central 9.15 8.74 8.45 7.98 5.89 5.1 6.19 6.18 7.82 7.17
Illinois 9.96 8.05 8.69 7.52 6.63 4.47 5.82 5.69 8.51 6.73
Indiana 7.25 7.78 6.95 7.06 4.87 4.72 10.13 9.26 6.18 6.19
Michigan 10.38 9.47 9.05 8.36 6.54 5.47 9.29 9.8 8.67 7.71
West North Central 7.3 7.43 6.2 6.24 4.82 4.64 6.29 6.42 6.27 6.19
Iowa 8.65 9.37 6.72 7.19 4.67 4.97 -- -- 6.62 6.89
Minnesota 8.58 8.37 7.03 7.03 5.64 5.1 7.88 8.37 7.16 6.82
South Dakota 7.18 7.3 6.22 6.21 4.9 4.72 -- -- 6.44 6.4
South Atlantic 9.2 9.2 8.47 8.18 5.45 5.36 9.05 7.11 8.26 8.02
Delaware 11.96 8.7 10.86 7.43 8.62 5.47 -- -- 10.81 7.42
Florida 10.88 11.13 9.68 9.99 7.67 7.45 10.26 10.28 10.12 10.32
North Carolina 8.79 8.83 7.14 7.06 4.92 4.97 -- -- 7.49 7.32
East South Central 7.71 7.68 7.81 7.67 4.93 4.4 9.8 10.49 6.73 6.38
Kentucky 6.85 6.69 6.56 6.25 4.27 3.48 -- -- 5.65 5.04
Tennessee 7.42 7.31 7.84 7.63 5.4 4.97 9.8 10.49 6.95 6.68
West South Central 10.66 10.74 9.15 9.24 7.01 7.41 8.79 8.67 9.13 9.16
Oklahoma 7.58 8.35 6.74 7.39 4.95 5.88 -- -- 6.63 7.28
Texas 11.98 11.91 9.69 9.72 7.72 7.99 8.55 8.43 10.08 9.94
Mountain 8.5 8.36 7.39 7.32 5.33 5.28 5.06 5.96 7.19 7.07
Colorado 9.15 9.2 7.37 7.97 5.91 6.09 2.82 3.73 7.66 7.95
Idaho 5.76 6.14 4.83 5.38 3.16 3.55 -- -- 4.74 5.17
Nevada 11.36 10.88 10.24 9.87 7.53 7.01 9.15 9.46 9.47 8.95
New Mexico 8.71 8.99 7.87 7.67 5.4 6.17 -- -- 7.39 7.58
Pacific Contiguous 10.91 10.45 9.85 10.18 7.47 6.84 8.46 6.38 9.82 9.57
California 14 13.46 11.31 11.75 9.24 8.65 8.51 6.38 11.88 11.69
Oregon 7.64 7.46 7.03 7.01 4.99 4.45 6.74 6.46 6.9 6.6
Washington 6.99 6.67 6.49 6.51 4.78 4.37 5.71 5.75 6.39 6.1
Pacific Noncontiguous 18.71 18.48 15.95 16.57 15.26 15.55 -- -- 16.64 16.9
Alaska 14.46 13.63 11.41 11.63 11.2 9.44 -- -- 12.46 11.96
Hawaii 22.41 22.94 20.42 21.25 16.82 17.87 -- -- 19.69 20.51
U.S. Total 9.88 9.8 9.28 9.04 6.2 5.87 9.65 8.57 8.74 8.43
  [1] See Technical notes for additional information on the Commercial, Industrial and Transportation sectors.
  Notes: See Glossary for definitions. Values for 2005 are final. Values for 2006 and 2007 are preliminary estimates based on a cutoff model sample. See Technical Notes for a discussion of the sample design for the Form EIA-826. Utilities and energy service providers may classify commercial and industrial customers based on either NAICS codes or demands or usage falling within specified limits by rate schedule. Changes from year to year in consumer counts, sales and revenues, particularly involving the commercial and industrial consumer sectors, may result from respondent implementation of changes in the definitions of consumers and reclassifications. Retail sales and net generation may not correspond exactly for a particular month for a variety of reasons (i.e., sales data may include imported electricity). Net generation is for the calendar month while retail sales and associated revenue accumulate from bills collected for periods of time (28 to 35 days) that vary dependent upon customer class and consumption occurring in and outside the calendar month. Totals may not equal sum of components because of independent rounding.
  Sources: 2006 and 2007: Energy Information Administration, Form EIA-826, "Monthly Electric Sales and Revenue Report with State Distributions Report;" 1992-2005: Form EIA-861, "Annual Electric Power Industry Report."



Wire gauge, tensile-strength, DC-resistance calculation

size (input size value)
unit (select AWG, mil, mm, CM or sq-mm)
construction (select solid or stranded)

Input size value and set unit and construction, then press "compute" to get converted values. For large size over 0 AWG, use 2/0, 3/0, 4/0 .. etc.

Ordinarily, the next two gauging systems are used with electrical wire and cables.

AWG (American Wire Gauge)
It is known as BS (Brown & Sharp wire gauge). The AWG guging system is defined as geometrical progression because of drawing mechanism nature. The 36 AWG is defined as 5 mil diameter, and the 4/0 AWG is defined as 460 mil diameter. (See ASTM B 258)

represents diameter of solid wire in mil (1/1000 inch) or cross-sectional area in circular mil.

millimeter wire gaging system
represents diameter of solid wire in mm or cross-sectional area of stranded wire in mm^2 (square mm).

Most of conductors is composed with copper. Most important parameter of conductors are cross section which determines DC-resistance and most of current capacity.

Conductor size 16 AWG
area 1.309 mm^2 (square-mm)
area 2583 CM (circular mil)
diameter 58.7 mil (1)
diameter 1.491 mm (1)
DC-resistance 0.0134 Ohm/m
tensile strength 39.26 kgf (2)
weight 11.634 kg/km (Cu)
weight 3.533 kg/km (Al)
construction: stranded
Note: lay factor of stranded conductor is assumed 2 %.
conductor size 16 AWG
area 1.309 mm^2 (square-mm)
area 2583 CM (circular mil)
diameter 50.8 mil (1) diameter 1.291 mm (1)
DC-resistance 0.0132 Ohm/m
tensile strength 39.26 kgf (2)
weight 11.634 kg/km (Cu)
weight 3.533 kg/km (Al)
construction: solid


"If you are more interested in current carrying ability than physical size, then also remember that a change of 3 AWG numbers equals a doubling or halving of the circular mills (the cross sectional area). Thus, if 10 AWG is safe for 30 amps, then 13 AWG (yeah, hard to find) is ok for 15 amps and 16 AWG is good for 7.5 amps.

The wire gauge is a logarithmic scale base on the cross sectional area of the wire. Each 3-gauge step in size corresponds to a doubling or halving of the cross sectional area. For example, going from 20 gauge to 17 gauge doubles the cross sectional area (which, by the way, halves the DC resistance).

So, one simple result of this is that if you take two strands the same gauge, it's the equivalent of a single wire that's 3 gauges lower. So two 20 gauge strands is equivalent to one 17 gauge. "

Appliances in the United States are required to work over a range of standard voltages such as 110-125 volts. Sometimes this is stated as 115 volts, a mid point.  According to TXU Electric which bills me and Oncor which maintains the wires, voltage at my house is to be 120 volts plus or minus 5% or 6 volts, making the acceptable range 114-126 volts. 2009-05-15
The power company does not deliver a constant voltage; it couldn't over varying lengths of delivery line and power load. It does deliver a very precise frequency so that clocks run accurately. When voltage gets below 110, we talk about brown out conditions. When I read 108 at one of my outlets, I call the power company and they come out at reset one of the two transformers in the alley. In our old neighborhood, the load is high enough that if one of the transformers trips off, we still have power, but the equipment can not deliver enough current to keep the voltage up. The normal measured voltage at my house has been 117-118.
Since the power delivered to non-3 phase customers is delivered on 2 hot wires and a neutral wire, the voltage is across the hots is [almost*] always twice the voltage between each hot and neutral. When I measure 117 volts at my kitchen outlet, my air conditioner is running off 234 volts.

[* it is possible to draw a lot more current off one leg of the 220-230 pair so the voltage is lower on one leg, perhaps measuring 113/118. The power company hates this. Circuit breaker boxes are designed to avoid it. It is not a good idea of have all your 110-120 load on one side of the box, because all the appliances on that side, when on, will have lower voltage to work with.] 2002-08-29

>I have to work this out because there is no clear answer (maybe) but here goes... my problem is to estimate the max current or better yet wattage to allow for a piece of wire in the furnace. In normal electrical wiring the manufacturers would say "a 2mm cable
has a current rating of say 10 amps" <

You are asking the wrong question.
When you cite the manufacturers, you are leaving out all the assumptions.
The proper statement might be "An insulated 2mm cable will carry 10 amps with less than
a 2 volt drop over 100 feet when the cable is part of a cord with no more
than 3 wires and is not enclosed in conduit."
Resistance wire is specified by the way the wire behaves when arranged horizontally as a straight wire in still air. For example, Omega's Temperature Handbook, page H-14, tells us that if 11.96 amps is forced through 18 ga. Kanthal type wire, the wire will reach 1200F. The voltage needed to do this depends on the length of the wire. When the wire is coiled and/or arranged against a wall of insulation, it will get hotter. How hot depends on how tightly coiled it is.
Since we can't choose (normally) our voltage, we work out the total wattage and discover how many watts we can get out of an element at that voltage. Too short an element, too many amps and it melts. Too long an element, too much resistance, too low amps, not enough wattage. Then the total wattage is divided by the element wattage gives number of elements.
In a sense, you are right, you have to work it out because there is more than one answer - you could use the next gauge larger or smaller wire and recalculate to a new answer.
As Henry says, normally you work it out so that the element is not working at 100%, but will deliver enough power at 80% to avoid running at the limit.

Contact Mike Firth


Common metric prefixes for size
femto one trillionth (10^-12) femtosecond
pico one billionth (10^-9) picofarad
micro one millionth of the unit microsecond
milli one thousandth of the unit milligram
centi one hundredth of the unit centimeter
deci one tenth of the unit deciliter
deca ten times the unit xx
hecta one hundred times the unit hectare, hectometer
kilo one thousand times the unit kilometer
mega one million times the unit megawatt
gigi one billion (1,000 million) gigabyte
tera one trillion (1,000,000 million) terabyte.