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If a 12 gauge, 50 foot long copper wire can support 20 amps, why is the amount of wire in 6 gauge cable so much thicker and only rated to support 55 amps if your looking at 50 feet? Wouldn’t a stranded or solid copper conductor, only need to be as thick as 3 X 12 gauge wires to support 60 amps?

JACK
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Fernando S
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  • Would think that makes sense. There is more to it than just size/area of the wire. Amount of heat generated by the electricity flowing in the wire is one the main things. If I remember high school enough(asking a lot) most if not all of electricity only flows on the outside surface of the wire. – crip659 Oct 15 '22 at 12:24
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    60 Hz skin depth for copper is ~8.5mm, so not much of a factor below 250MCM, unless operating at much higher frequencies (which is not normal for power distribution.) – Ecnerwal Oct 15 '22 at 13:04

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Amp limitations are based on temperature rise.

Temperature rise is a function of resistance (causing heating as amperage passes through) AND surface area (to radiate heat from.)

12 gauge wire has a cross sectional area of 3.31 mm2

6 gauge wire has a cross sectional area of 13.3 mm2

So, 6 AWG is 4 times larger (more copper) than 12AWG, and has about 1/4 the resistance as a result.

However, 6AWG has only twice the surface area of 12 AWG. So it has a harder time getting rid of the heat.

...and no, you can't run 3 12AWG in parallel instead of one 6 AWG, at least under NEC rules. You're better off budget-wise running 4 AWG aluminum anyway.

Incidentally, for both 6AWG copper or 4 AWG Aluminum if you use 60°C cable (NM/B or "Romex") it's 55A, but if you use wires (typically rated 90°C or better these days, but 75°C will do) in conduit on 75°C rated terminations it's 65A. Can't use the 90°C column other than for derating, as the terminations are not rated that high, normally.

Ecnerwal
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  • Think OP was asking more about just using 3 12g bare wires(in a single insulated cable) instead of needing 6g. – crip659 Oct 15 '22 at 13:18
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    Can't do that where NEC applies, period. Suspect most other areas have similar rules, but given the question is about AWG-sized wires, NEC likely applies. – Ecnerwal Oct 15 '22 at 13:34