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I'm considering a piece of shop equipment with a motor that can be wired as 110-volt (20-amp) or 220-volt (10-amp). I'm leaning towards 220v despite the additional cost of installing the electrical outlet.

The equipment vendor says the minimum circuit should be 15-amp. Initially, I was planning to request each leg be 20-amp (40-amp total) but after digging into this, I'm wondering if 10-amp on each leg would be sufficient considering the circuit minimum is 15-amp.

If I have the new outlet to power this machine wired as 220-volt, will the circuit breakers in the box need to be 15-amp total or 15-amp on EACH 110-volt leg?

  • Wait, where is that "40 amps" coming from? Those wires aren't wired in parallel. The current in a closed loop is the same at every point in the loop. – Eric Lippert Feb 9 '17 at 5:44
  • The equipment vendor says "the minimum circuit should be 15-A". Is this at 110 V or at 220 V? If all it requires is a 20-A, 110-V circuit (and you have that already), then it is a needless expense to install a 220 V circuit. Where are you located? What is the make and model of the equipment? Do you have copper wires in the branch circuits or aluminium? – Jim Stewart Feb 9 '17 at 10:06
  • If you decide to install a 2-pole 20-A breaker and you have a GE panel that takes the 1/2" wide breakers, be advised that a 2-pole breaker will not fit in every pair of adjacent side-by-side slots. This is because the two halves of the single unit 2-pole breaker must be on different legs. In my GE panel the first two 1/2" slots (going down) are on the SAME leg. the next two are on the other leg, and so on. The fitting on the panel and that on the GE 2-pole breakers only allows the 2-pole breaker to go across different legs. – Jim Stewart Feb 9 '17 at 10:53
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They are the same thing and you don't have a choice. Your breaker is going to be a 2-pole breaker rated 15A. When you look closely, it will very much resemble two 15A breakers bolted together. That's not far from the truth.

The breaker will mount in the service panel in two spaces, which could otherwise accommodate two 120V breakers. What is happening is there are two 120V buses in the panel, and the 2-pole breaker is designed to tap both of them. They are phased opposite (when one is -120V the other is +120V), so if you tap both, you get 240V.

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You already got all the "magic" you can get in going from 20A draw to 10A draw - same power, twice the voltage, half the current.

A 15-amp 240 volt breaker IS 15 amps on either leg.

"each leg 20 amps" is... 20 amps. Period. Not 40. Just 20 amps at twice the voltage.

240V circuit - 2 spaces, dual breaker. No magical mystery of adding the breakers because there are two of them - there are two of them because there are two hot legs, that's all. Do yourself a favor and use a proper dual breaker, not two handle-tied singles- that will reliably trip both sides at the same time for a fault in either.

For most shop equipment startup surge makes under-rating the breaker a bad idea.

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