what is the required ground size relative to the associated hot and neutral? For example: if I am connecting a sub-panel with two #4 hot leads and one #4 common lead, what size wire is required to extend the earth ground?
Electrical – ground wire sizing
electrical distributionelectrical-panel
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Best Answer
It's more subtle than you'd think, but not too complicated. And the breaker size is essential to an NEC-compliant answer.
Baseline is NEC table 250.122 - Minimum Size Equipment Grounding Conductors for Grounding Raceway and Equipment.
However, if you are using larger than minimum conductors for the overcurrent device feeding them (to limit voltage drop, for instance on a long run) then the grounding conductor size also needs to be increased by a proportional amount.
So if you are using 4 AWG copper at 70 amps, 8 AWG copper meets code.
4 AWG aluminum at 50 amps, 8 AWG aluminum meets code.
[should have an example of 4 AWG at lower current to demonstrate proportional upsizing here, but not happening right now]
The simple, safe, but not cheapest answer is the same size as the phase conductors. That will meet code, by exceeding it most (nearly all) of the time. Code does not mind if you exceed the minimums, only if you fail to meet them.