What size equipment ground does a 100 amp feeder need?

8 AWG copper or 6 AWG aluminum, per NEC Table 250.122, sized to the 100 amp overcurrent device protecting the feeder. Upsize proportionally under 250.122(B) if the feeder conductors were enlarged for voltage drop.

Does a detached building subpanel still need ground rods?

Yes. NEC 250.32(A) requires a grounding electrode system at a detached structure with a feeder, in addition to the equipment grounding conductor run with the feeder itself.

I Am Adding a 100 Amp Subpanel. What Size Copper Ground Wire Do I Need?

A user asked the app what copper ground a 100 amp subpanel feeder needs. NEC 2023 Table 250.122 sizes the equipment grounding conductor by the rating of the overcurrent device protecting the feeder. For a 100 amp breaker, the table calls for 8 AWG copper or 6 AWG aluminum. That is the wire that bonds the subpanel enclosure and rides with the feeder conductors back to the source.

Sized to the breaker, not the wire

The EGC follows the breaker ahead of the feeder, not the feeder conductor size. A 100 amp breaker feeding 3 AWG copper, or 1 AWG aluminum SER, still needs only the 8 AWG copper ground. If the subpanel is actually protected at 90 amps, the table row for 100 still applies because the rows read up to and including the listed rating; 60 amps would drop you to 10 AWG copper. There is one big adjustment: 250.122(B) says if you upsize the ungrounded conductors for voltage drop, the EGC must be increased proportionally by circular mil area. A detached shop fed with conductors bumped from 3 AWG to 1 AWG copper needs the ground bumped by the same ratio, which typically lands on 6 AWG.

Detached buildings add a second wire

For a subpanel in the same building, the feeder EGC is the whole grounding story. A subpanel in a detached garage or shop also needs its own grounding electrode system under 250.32(A), usually two ground rods, connected with a grounding electrode conductor sized from 250.66, which for this feeder is also 8 AWG copper but protected from damage per 250.64(B). The rods do not replace the feeder EGC; since the 2008 cycle you must run the equipment ground with the feeder regardless.

Keep neutrals and grounds apart

At every subpanel the neutral bar floats and grounds land on a separate bonded bar per 250.32(B) and 250.142(B). The 8 AWG EGC is what makes that separation safe, giving fault current a low impedance path back to trip the 100 amp breaker.

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