How do I properly size the ground wire for a subpanel?

Question

What is the general rule for sizing ground wire relative to the conductors?  I know I saw this somewhere on here, but I cannot find it.
I'm running aluminum (#1 AWG aluminum) to my subpanel and I'd rather not try to tie a #1 wire into my ground bar.  Can I use #4 Al for ground?

mark f · Accepted Answer

YES you can use #4 Al as your ground.  Your 1AWG Al is good for a 100 AMP feeder (actually 110).  From NEC 250.122 this requires an #8 CU or #6 Al ground wire as a minimum (or 6CU and 4 Al if you are really running a 110Amp Feeder).
As a reminder, your feeder will be 4 wires, the two hots and the neutral will be #1AL (unless you've de-rated the neutral to 70% or from specific load balancing calculations if that is allowed in your area).

NEC 250.122 table:

mark f · Answer

If your question is not related to Feeders, but instead about what size wire to run from your ground bus-bar to your earth ground (copper rod or old plumbing tie in) than the answer is NO;   AL is not to be used when within 18" of the ground/earth.

NEC250.92: Where used outside, aluminum or copper-clad aluminum grounding conductors shall not be installed within 18 in. (457 mm) of the earth.

(Thanks @Ecnerwal for the corrections!)

How do I properly size the ground wire for a subpanel?

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