If a 12 gauge, 50 foot long copper wire can support 20 amps why is 6 gauge cable so much thicker

Question

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?

Answer 1

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.