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I'm wiring a 100 amp subpanel in a shed that is 100ft. from the main service panel. I have 3 - #2 AL wires, 2 hots and 1 neutral. What size ground wire is recommended?

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  • Not an expert, but usually they call for 2-2-2-4. The feeder breaker is usually limited up to 90 amps. You should do a load calculation to see how much extra power you spare from your panel. A 100 amp panel can be fed by 100 amp breaker or less, even 30 or 40 amps.
    – crip659
    Commented Mar 11 at 17:27

2 Answers 2

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You can use a #8 copper or insulated #6 aluminum wire.

You must use a 90A breaker, not 100A, because #2 is only 90A wire. See Table 310.16, 75C column, since you cannot use the 90C column with consumer tier electrical panels.

If you have a critical need for 100A, use 1 AWG aluminum. If your task is EV charging, you don't have a critical need for 100A, that's a common (but understandable) mistake made by EV novices. Ask about that and we'll put you right.

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Harper's answer is correct, as usual. A few more things to consider:

Subpanel Size

Your subpanel can be a 100A panel even with a 90A feed. In fact, it can be a 200A panel with a 90A feed. This is important for 3 reasons:

  • Breaker Spaces

Larger (total feed capacity) panels tend to have more breaker spaces. Often a shed will have a 12 space panel, but it doesn't take much to fill that up. You generally (there are exceptions) need AFCI or GFCI on almost everything and those generally (there are some GFCI exceptions with some panel types) need full-size breakers. So a 20 or even 30 or 40 space breaker is not unreasonable. Nearly all panels are the same width (designed to fit between 2 studs on 16" spacing) and they all need the same space in front of them (30" x 36") so the only real difference is vertical space and you can't put much else on the same wall section anyway.

  • Main Breaker

The general differences between a subpanel and a main panel are whether there is a main breaker or not and whether the neutral is bonded to ground or not. Neutral always needs to be separate from ground in a subpanel. But a main breaker is OK! In fact, a main breaker can function as the required disconnect for a panel that is in a different building from the main panel.

  • Price

A bundle of "mid to large size main panel" + "main breaker" + "a few branch breakers" is often almost the same as "small subpanel" + "disconnect switch" + "a few branch breakers". And when it is all done you have lots of spaces left over for future expansion if needed - at very little, if any, extra initial cost.

Ground

You generally need two ground rods (or one Ufer ground if you are pouring concrete) connected to a ground wire connected to the subpanel. This is in addition to the ground wire (sized as Harper answered) that goes back to the main panel.

Load Calculation

Unless you are installing tankless hot water (don't) or EV charging (can be done, but needs to be done right), you are unlikely to need more than 90A in the shed. But you need to figure out how much capacity your main panel can spare. To do that, you do an NEC Load Calculation on the current main panel. Subtract from the service size and see what's left. If that's 90A, great! If it is less, that's OK but you need to make sure you don't exceed the available amount with everything you put in your shed.

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