What size wire for a 100A sub-panel at 275 feet?

Question

I need to run power from my house out to my 35X70 workshop out back. What I have; a dedicated panel with 150 amp breaker on the side of my house that is currently only powering my well. My workshop is 275 ft. away. I intend on putting a 100 amp sub-panel in the shop. I have most of the 2” conduit ran in a trench from my house to the workshop. I intend on having a welder, compressor, wall unit a/c in a small office, lights and maybe an outside outlet to hook up for a travel trailer. I want to use copper wiring. What main wiring is recommended to run inside the conduit from the 150 amp/panel on the house to the 100 amp sub-panel in the workshop?

Answer 1

I use Southwire's wire size calculator.

At that length aluminum feeder would be much cheaper, selective boxes for; feet, single (phase), aluminum, direct burial/conduit/overhead, minimum conductor size, input 275 (for the length), input 240v for the voltage, max voltage drop leave at 3 (but we will play with this), current at end (since you don’t know guess at 80), input 1 for parallel sets.

This came up for 2/0 with a standard 3%. Input in 5 in the % and the wire size drops to 1 awg with a 4.44% voltage drop

In the US the National Electric Code there are suggestions of 3% and 5% for voltage drop but these are only suggestions.

If you actually used 80 amps of 240v then the drop would be 10.6v with 1awg or #1 wire not a problem.

if you are drawing 50 amps 240v on the #1 wire the voltage drop would be only 6.6volts (got that by playing with the amperage and % voltage drop).

Only during really heavy loading of your circuit would #1 aluminum have a voltage drop that was at the max recommended by the NEC and it never exceeds 5% up to 80 amps.

Going to a larger feeder breaker 90 or 100 requires a larger wire size but I doubt you will need that much power.

I would get the 150 amp panel with more circuits you can feed it with anything below 150 as long as the wire size matches the feeder wire requirements.

Answer 2

See what Ed Beal says about getting a larger panel with more spaces. I understand gravitating toward the smallest panel that will do the job today, that's your "thrifty gene" doing its thing. But a) panel spaces are cheap, and b)having to replace a panel because you ran out of spaces is a nightmare, and c) a 100A panel can support a lot more stuff than you think, and you'll want to do that later. So wildly over-sizing the panel (in terms of number of spaces) is always a good idea. This sometimes means getting a higher bus ampacity because that's what's cheapest at 24 spaces, say.

That's especially true when you install without a permit, and then the inspector comes 'round later and holds you to NEC 2020 standards, and you discover you need GFCI breakers in places you did not expect... and those take more room in the panel.

Ed's advice is also spot-on about 3% not being mandatory and #1 aluminum being a viable choice.

To be clear, aluminum is the right stuff for feeder. Copper is practically a waste of money, the wires will be landing on aluminum lugs in any case.

But here's an alternate way of looking at things. Certain sizes of wire are extremely popular and have "price point sweet spots" (or availability: that they are made at all in certain cable types). #2, #2/0 and #4/0 are popular because of the "whole service derate rule" (310.15B7) which allows those sizes for 100A, 150A and 200A whole services.
You CAN'T use this rule to use #2 for 100A, since it isn't your whole service.
But you CAN use this rule to use 2/0 for 150A, since it is your whole service.

So, since Ed has made a good case for 2/0... I say go ahead and use that to bring 150A to the subpanel. That will need 150A breaker protection, but your main panel's main breaker already is 150A. So, you won't need another breaker. I say that because I have a hunch it might be one of several panel types which make this easy.