This is not the complete list of abbreviations for wire types, but covers the most commonly available extension cord types.
S = Service
J = Junior
O = Oil Resistant
T = Thermoplastic (Vinyl)
P = Parallel (flat cord)
W = Weather approved
The abbreviations are combined to describe the characteristics of the wire. The wire itself usually has a higher voltage rating than the overall cord because the plug ends are rated seperately, and may not have as high a rating.
Type "S" - Heavy duty, rubber insulated, portable cord. Stranded copper conductors with individual rubber insulation. 600 volt rating.
Type "SJ" - Same as "S" but rated at 300 volts because of less insulation.
Type "SJ" - Same as "S" but rated at 300 volts because of less insulation.
Type "SJO" - Same as SJ but with an oil resistant outer jacket instead of the rubber.
Type "SJT" - Same as SJ but with a thermoplastic (vinyl) outer jacket. Generally not as flexible.
Type "SJTOW" - Same as SJT but with an oil resistant outer jacket and rated for oudoor use.
Type "SPT-2" - A parallel or "flat" cord, all vinyl insulation, 300 volt rating. May or may not include a third grounding conductor. (See below.)
Type "SPT-3" - Same as SPT-2 but heavier construction.
Often the important criteria for a cord is the amount of amps it can carry. This is dependent on the length of the wire and the size of the conductors (or gauge). The following chart shows a rough guide of cord capacities:
| Conductor size (AWG) | 10 ft. | 25 ft. | 50 ft. | 100 ft. |
| 16 gauge | 20 amps | 15 amps | 13 amps | 10 amps |
| 14 gauge | 20 amps | 15 amps | 13 amps | 10 amps |
| 12 gauge | 20 amps | 15 amps | 15 amps | 13 amps |
Again, this is what the wire itself would be rated, but the plug ends must have an equal or better rating as well. Most consumer cords will not be rated for more than 15 amps.This is because of the rating of the plug ends. (If you think there is a missing line in the chart above, you are right! Apparently, UL stopped approving 18 gauge extension cords a few years ago, so we removed it from this chart.)
As you may have figured out, a lower gauge number indicates a bigger wire. Wikipedia says that this is because the gauge number system is related to how many passes through the manufacturing machinery a wire must go through to get to the desired size. A smaller wire takes more passes through the die, so it gets a higher number. The gauge is usually specified as "AWG", which stands for "American Wire Gauge". This is an accepted standard that establishes ways to measure wire sizes.
There is a formula which is very useful, which is W = V x A
This stands for Watts = Volts x Amps.
Here is an example of how you can use this formula. Let's say that you have 6 stem lights with 200 watt bulbs in each. They are plugged into a multi- outlet strip, and the strip runs to an extension cord.
So, you have 200 x 6 = 1200 total watts.
With a few exceptions, lighting in the USA is plugged into a 120 volt circuit.
So now the formula looks like this; 1200watts = 120volts x Amps
Which can be re-written to look like this Amps = 1200 ÷ 120
Therefore, A = 10 amps
So you would have to make sure your outlet strip and extension cord were rated for at least 10 amps. You can add up the watts for all electrical devices on a circuit (computers, stereos, air conditioners, etc) and get the amps needed with this formula. Big motors used to operate machinery have different requirements, which will not be covered here.
Please note that using a multi-outlet strip or a cube tap does not increase the capacity of the wire!
For the purposes of determining overall electrical requirements for a job, this formula also applies to low voltage lighting. If you had 20 low voltage lights on a circuit, with 50 watt bulbs in each, you would have 20 x 50 = 1000 total watts. You would still use 120 volts for the "V" in the formula, since this is the input voltage into the transformer.
So; 1000 = 120 x A
or A = 1000 ÷ 120
A = 8.33 amps.
I think of voltage as the pressure pushing the electricity through the wire. Low voltage is less pressure. Thicker wire therefore is almost always better, and this is especially true for low voltage lighting. It is important to make sure that all wire connections in a low voltage lighting circuit are tight because the low "pressure" will have trouble pushing the electicity across a loose connection. This will soon cause the loose connection to heat up and damage the transformer. Some low voltage lighting manufacturers recommend that you let your newly installed lights burn for about a half hour and then feel the connections you have made. If any of them are warm, you should tighten those connections.
Some manufacturers also suggest that low voltage lighting runs longer than about 20 feet might experience "voltage drop", which means that the light bulb at the farthest point will be dimmer than the closest bulb. Some transformers for low voltage lighting contain a "boost tap" which is designed to overcome this by jacking up the output to 13 or 14 volts.
Finally, for some trade show specific information;
While most convention centers allow the flat extension cords (SPT) to be run under carpets, Chicago does NOT. If you want to run cords under the carpet in Chicago, get the "S" type cords. I believe they now usually want you to use the 12 gauge wire as well.