Online, most household wiring guides suggest 14 gauge wire for 15 amp circuits, 12 for 20 amps, and 10 for 30 amps. Do these assume the voltage is 120, or does voltage not come into play when choosing wire gauge?
For example, I have an appliance that requires a 240 volt 15 amp power source. (Its plug is NEMA 6-15.) If I dedicate a circuit to this appliance, which is about 25' from the panel, will 14 AWG be sufficient? Why or why not?
When 14 gauge wire is appropriate, I've seen 12 used instead. (My home has several circuits like this.) I realize 12 AWG wire may be more difficult to work with, and it costs more than 14 AWG. Beyond these, are there reasons not to do so? Are there any advantages?
Best Answer
The simple answer is you use the wire that your local code and the device manufacturer specifies for the situation.
But I think you are asking about how wire sizes are chosen for a particular application/current/voltage combination:
When choosing wire, current dictates the size of the conductor and voltage dictates the insulation.
Current causes the wire to heat up due to resistance. Metal expands and contracts when heated and cooled. This expansion and contraction, if too large, can loosen connections. Loose connections increase resistance, cause more heating, and will eventually allow a gap large enough to cause an arc or a high enough temperature to ignite surrounding materials or melt the insulation and cause an arc. A larger conductor reduces the resistance, which reduces these temperature changes. Therefore, using a large enough conductor keeps the expansion and contraction under the level that electrical fittings can tolerate without failing.
Similarly, voltage causes arcing so the covering (insulation) has to be designed to prevent arcing at the rated voltage. Usually you'll see common electrical wiring rated for 600 volts.
Another reason to use larger gauge wiring is to prevent voltage drop on long runs. This is generally not a problem when running wire in a house, but detached structures are a common place to see larger wire used.