I would like to install a 120/240 volt 100 amp sub panel on a detached garage that is about 200 ft away. I have read that one should strive for a voltage drop of less than 3% and from what I can tell this means I need #2 cu or 1/0 al for the two hot and neutral. What I'm having a hard time determining is if the ground wire to the sub panel needs to be the same size or if it can be smaller. I've read that the ground for a 100 amp sub panel needs to be #8 cu or #6 al but don't I need to take the long distance into account?

I really appreciate any help on this!

Edit: Thank you Harper for pointing out that the 3% rule applies to expected loads, not the supply breaker size. Given my expected use I can probably go with slightly smaller wires for the two hot and nuetral. However I still have the same question about how to size the ground wire.

  • The 3% target applies to the loads you actually expect, not the breaker trip value on the supply breaker. Right off the bat you should not plan to load the circuit beyond 80%. – Harper - Reinstate Monica Jan 21 '19 at 5:03
  • Thank you Harper for pointing this out, I've edited my question to include this point. – eNaught Jan 21 '19 at 16:56

You start with an 8AWG copper wire...

You are correct that for a 100A feeder, the associated ground wire needs to be an 8AWG copper or 6AWG aluminum wire. If you are using a four-wire direct burial cable, then just use the ground wire in the cable as it will be quite amply sized for the job it is doing; if you are using PVC conduit, then bare 8AWG copper would be my choice to keep conduit fill down.

...and then upsize from there based on how much upsizing of the other wires you do

When you start upsizing wires for voltage drop control, the size of the ground wire is governed by NEC 250.122(B):

(B) Increased in Size. Where ungrounded conductors are increased in size from the minimum size that has sufficient ampacity for the intended installation, wire-type equipment grounding conductors, where installed, shall be increased in size proportionately, according to the circular mil area of the ungrounded conductors.

For instance, using 1/0 aluminum wire for the hots and neutral in your example vs. the 1AWG that is required to meet Code ampacity rules gives us a size ratio of 53.49mm2/42.41mm2 based on the NEC Chapter 9, Table 8 conductor sizes (we can do this math in mm2 since the conversion factors cancel out here) or about 1.26. Multiplying that by the 8.367mm2 area of an 8AWG copper conductor gives us about 10.55mm2 for the resulting wire. This isn't a standard conductor size though, so we should use a 6AWG copper conductor (which has 13.30mm2 of area) or a 4AWG aluminum conductor for this ground wire.

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