Wire Gauges and Cord Sizes
There is a complete Wire Gauge Capacities table on this site that shows detailed information regarding wire gauge sizes and amp ratings. However, the majority of 120 volt RV questions regarding wire sizes can be answered quite quickly by the following table:
Note that the above chart gives you the amp ratings for each conductor. This should be identical to the circuit breaker rating. However, these ratings are based upon continuous usage. If a brief intermittent current surge occurs your wiring won't instantly smoke. If the proper circuit breaker is in place it will trip before causing irreparable damage to the cord set.
Shore power cords generally come in two sizes - 30 amp and 50 amp. A 30 amp cord will have # 10 wiring in it. The 50 amp cord will have three # 6 wires in it to carry the current but the fourth wire is the ground wire and it will be of a smaller #8 gauge, which is adequate because it will only be used in the event of a short circuit and the breaker will trip soon thereafter.
These ratings are based upon a standard acceptable voltage drop. As you pass more amps through a cord the voltage will drop due to the resistance in the cord. Longer lengths will increase the resistance and the voltage will drop. This isn't an issue with normal shore power cords but if you do increase the length of a cord substantially and then run it at it's maximum current rating you will have a severe voltage drop. An example would be if you wanted to run a 20 amp portable air compressor off a 150' long extension cord. In this case the #12 cord would be hard pressed to pass 20 amps that far and the voltage would drop, possibly damaging your air compressor. In this case I'd jump up to a #10 cord to minimize that voltage drop.
Submitted by Mark Quasius - 2/24/06
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