What size wire for a 100 amp sub panel 20 feet away

Question

What type and size wire do I need to run to a 100 amp sub panel about 20 feet away I know it will be a 4 wire but that’s it.

Answer 1

The basic answer is to go to an ampacity chart. There are typically three relevant numbers:

• 60 C for copper NM cable (indoors only) or UF cable: not even an option according to the linked chart
• 75 C for copper SE cable or wires in conduit: 3 AWG hots & neutral, 8 AWG ground
• 75 C for aluminum SE cable or wires in conduit: 1 AWG hots & neutral, 6 AWG ground

Note that the ground sizes are based on my understanding of NEC 250.122. But common 4-wire SE cables include a larger 4th wire. My understanding is that is because SE is used as a feeder where the wires are hot/hot/neutral (3 wire) or hot/hot/hot/neutral (4 wire), so if you get a 4 wire to use as hot/hot/neutral/ground you end up with a larger ground than you really need, and possibly a larger neutral as well, but that's OK.

Remember that your feed breaker size and wire size do not have to match the capacity of the subpanel. If the subpanel is rated at 100A, or even 200A, that's OK with a 60A feed (or whatever). A subpanel in another building needs a disconnect, in the same building it does not. But a subpanel anywhere can use a main breaker (i.e., really be a "main" panel with a main breaker, as long as neutral and ground are kept separate) as a disconnect.

A Load Calculation is critical to determine the amount of power available for your subpanel. With a 200A feed, if your existing loads on your main panel are > 100A then you can't set up your subpanel to use 100A. You really need two load calculations - existing load without the subpanel and load for just the subpanel. There are a lot of things that can be done to solve the overall load problem, if it turns out to be a problem.