What Size Wire For 40 Foot Run To Shop From Main Panel To Add 220 To My New Shop?

[John Hasler]

Allow for the largest machine you'll ever run + lighting.
Not likely you'll ever have a mill, lathe , compressor, welder, running at the same time.
I ran # 2 CU 60 feet to a 100 amp panel. Way more than I'll ever need, but I'll never have to dig it up to go to a larger wire size either

Also a waste of money to run copper. You won't see a power company doing that.

 

[aliva]

I don't trust aluminum and I got the copper for free

 

[mksj]

There are a number of considerations as to what wire you use and how you want to run it. If you are pulling through conduit, I prefer soft annealed copper wire, if you are doing direct burial or overhead, then use aluminum service entrance/direct burial type. A 100 amp breaker/sub panel, you would use 4AWG copper, 2AWG aluminum x 3 (neutral can be 1 size smaller), and a ground wire of at least 8AWG copper, or 6AWG aluminum. You want to make sure the wire is rated to 90C and also all the connections/breakers are also rated to this temperature. This may sound trivial, but often connection may be rated at a lower temp, and you need to derate the system. There also may be some derating for direct burial vs free air service entrance wire size. Code may vary a bit, conduit size for a 100A service is 1.25" Cu, 1.5" Alu. Voltage drop is about 0.8% for the size wire specified over 40 feet. Either wire would work, what you use depends on how you want to pull it, the cost difference at this level is minimal. Aluminum becomes a lot cheaper as you go to larger gauge wire. Terminals must be rated for aluminum, and I use a electrical gel on the wires which prevent corrosion. Also be sure to mark the wires at both ends before you pull it to denote red-hot (L2), white-neutral, green-ground (if wire is not already green).

I put in a 100amp sub panel service to my garage, it is about 38' from my main breaker box. I used a 1.5" plastic electrical conduit (a bit bigger than required, but easier to pull wire), and use three 4AWG copper THHN wires (1.19/foot at home depot), and one copper 6AWG THHN green ground. There is no reason to go bigger, if you have a 100 amp service. You can pull off both 240V and 120V on separate breakers in the sub-panel, so you should not need to pull any other wires in the conduit coming from the main breaker.

 

[hcdozier]

One thing I have NOT seen mentioned and that is making sure that the grounding is good for any subpanel.

I ran CU#2 (3 wires) to my 100A subpanel in the utility barn I have for a shop. Only about a total 25 foot run, but overkill is not only recommended, but a safety factor as well to assure myself and wife that nothing would overheat in the runs under any circumstances. I buried it 30" below ground level and also placed a series of salt treated timbers on top for safety for the future.

Then I also through the same 1 1/2" plastic conduit pulled a CU#4 ground wire.

The subpanel is UNBONDED, meaning that although the 3 CU# 2 wires are connected to the two bus sides and neutral, the grounding bolt is removed so that there is a separate ground for the barn to a pair of ground rods in the earth outside near the load center panel.
The CU#4 from the house ground connects to the subpanel ground, which then connects to the two local ground rods.

This places all grounds together at the same potential so no loops.

Then the subpanel provides both 220V 1-phase and 110V (using the common neutral back to the main panel). That is sourced from a 100A breaker in the main panel. The subpanel "main" is a local 100A breaker for the whole panel.

This is what the NEMA code specifies for subpanels.

Hank

 

[Eddyde]

how much is 40 feet of # 2? I'm trying to find the best place to buy cable. I too like overkill

I just ran a 100 Amp sub panel with 2,2,2,4 (2 hot legs, 1 neutral & 1 ground) aluminum cable from Home Depot, cost less than $2 a foot here in NY.
Be sure to use anti oxidant paste on the connections.