Wire Gauge/Type for 100amp detached Shop?

Question

I have a new woodworking shop that I built behind my house. The shop has a 100 Amp panel and I have wire the shop for 110v/20a plugs and 220v/20a plugs. My house has a 225amp service and already has a 220v/100a breaker that was feeding a garage subpanel (no longer used).

Panel and breaker are Square D and I want t run a direct burial cable to the shop. Distance is 65 meters (about 200 feet). Total draw in the shop would be max 60 amps if I had everything running (lights, heater/AC, power tools, radio, etc) and although there would be voltage drop over the distance of the run, I think a 2/3 direct wire would still suffice.

I asked the inspector and he was only concerned about depth of trench etc.

I also wanted to use aluminum as it is cheaper than copper.

Trying to look at what cable to get and 2/3 aluminum Tech cable seems like it will do.

Answer 1

If it were me I would probably run 4/0 and mount a 100amp double pole breaker to feed it in your main panel. I'd come out of the breaker with #2 copper and then use a barrel connector to transition to the 4/0 aluminum on each hot line.. Then go the 200 ft in a conduit. In theory you don't need it that thick but I bet you would appreciate the less flickery lights when you turn on heavy loads out there.. You could also do direct burial, they make service cable that is approved for burial and you get a higher rating when buried in dirt because it is basically water cooled. 600v insulation, etc. If you make that trade off you may get annoyed every time you kick on a motor on. They make ul approved barrel connectors for the transition from #2 coper to 4/0 and aluminum and Home Depot carries them. The county inspector approved this method for me. I would call your local inspector and ask him. They are often helpful for the Diy folks when not swamped. Try again in early mornings before they are about, and try more than one..

Jeff

Answer 2

For a 100 ampere circuit, the conductors will likely be required to be 3 AWG copper or 1 AWG aluminum. However, if you care about power quality, you're going to want to increase the size of the conductors to reduce voltage drop.

It's recommended to reduce voltage drop to below 3% of the supply voltage. So for a 240 volts supply, you'll want the drop less than 7.2 volts. When I calculate voltage drop for panel feeders, I always use the maximum ampere rating of the circuit. This is a worst case calculation, if you're pulling the maximum capacity of the circuit. Calculating it this way, means that I don't have to do a load calculation (which is usually impossible over the internet). It also reduces the chances of having power quality issues, if you get up near the maximum current limit of the circuit.

3 AWG copper and 1 AWG aluminum conductors, have a resistance of about 0.254 ohms per thousand feet. If I plug that into a voltage drop formula, I can figure out what the voltage drop across the conductors will be.

Voltage drop (VD) = length (L) * 2 * ohms/ft. * current (A)
VD = 200 ft. * 2 * 0.000254 ohms/ft. * 100 amperes
VD = 400 * 0.000254 ohms/ft. * 100 amperes
VD = 40000 * 0.000254 ohms/ft.
VD = 10.16 Volts

Since this is greater than 7.2 volts, I'll try the next larger size conductors (#2 cu. 1/0 al.).

VD = 40000 * 0.000200 ohms/ft.
VD = 8 volts

Getting closer, but the voltage drop is still too high. Once again with the next larger size (#1 cu. 2/0 al).

VD = 40000 * 0.000157 ohms/ft.
VD = 6.28 volts

6.28 volts is less than 3% of 240 volts, so I found the size conductors to use.

tl;dr

To feed a 100 ampere secondary panel 200 ft. from the main panel, you'll want to use 1 AWG copper or 2/0 aluminum conductors. This is large enough to safely carry 100 amperes, and to maintain power quality over the distance.

NOTES:

• You'll want to feed the panel using 4 conductors (hot, hot, neutral, ground).
• You'll have to isolate the grounded (neutral) conductors and bus bar in the secondary panel.
• You'll have to install an adequate grounding electrode system at the outbuilding, and bond the grounding conductor to it.

^{Conductor resistance values are from Table 8 Conductor Properties, from chapter 9 of the 2014 National Electrical Code}