What size wire for 70 amp sub-panel to garage?

Question

What gauge wire is need to make a 150 to 200ft run for a 70amp subpanel? I’m putting a 70 amp sub panel in the gargare which has no service at all. Going to be running lights, garage opener and a few outlets.

Answer 1

Minimum mandatory wire size is based on the breaker rating

There is a mininum wire size which is required based on the circuit breaker rating. That size is #4 copper or #2 aluminum.

You get this number by using a quality voltage drop calculator by specifying the breaker amperage and a wire length of 1 foot. e.g.

• 240V
• 3%
• 70A (the number on the breaker
• 1 foot (to keep it from upsizing for distance)

By the way, if the circuit was 55A (breaker 60A) that would be #6Cu/#4Al.

Be smart when you optionally upsize to reduce voltage drop

Over 100 feet of run, it's time to think about upsizing the wire to reduce voltage drop. This is a matter of using a larger-than-required wire to minimize voltage sag at the far end. It's not mandatory. **

However, the classic blunder is doing your voltage drop calculation based on the breaker number. Actually, you should be doing the voltage drop calc based on your computed loads.

You specified lights (2A if that), garage door opener (10A very intermittently) and vaguely "some outlets", so inconsequential loads, maybe a 1500W heater or something.

I don't know about you, but I don't use a garage door opener while using a saw. The lights are inconsequential. So let's oversize this thing and pretend you're using two common 120/15A at max load of 12A, and you have been halfway intelligent about balancing - putting each load on opposite poles. so you dual 12@120 = 12@240. Let's bump it to 20A for some headroom. Heck, let's bump it to 30A just for chuckles. Now we hit the voltage drop calc again. This time, we say

• 240V
• 5% voltage drop
• 30A (the typical load we expect to use ordinarily, even this is exaggerated)
• 200' (our actual distance).

OK. The result is 6cu/4al with 2.29% drop, which is smaller than our "minimum mandatory" size above -- so we must use the minimum mandatory.

Final answer: #4Cu or #2Al.

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** unless your application is expected to cause rather extreme voltage sag, definitely not the case here. Some people think there's a "3%" requirement in the NEC. [Citation Needed]. Actually, NEC has an informational note that suggests upsizing to limit voltage drop to 3-5% based on actual loads will provide reasonable efficiency of operation. That's all it says, it's not a mandate. NEC 215.2(A)(4).

Answer 2

#3 copper will provide 70 amps at 240v with a 2.74% voltage drop or #1 aluminum at 2.83% drop at 200 feet.

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