Mike Henderson
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- 1,405
- Location
- Villa Park, CA
Well, you're correct in that you only need to break one leg. But people, including us, make mistakes and forget to turn that other switch off before we muck around with the motor. Let me put it this way - with my memory and attention span, I want a double pole switch.Mike,
It's not necessary to switch both legs of a 240 volt motor circuit provided there is a separate disconnect. Normally, one would use the circuit breaker as the disconnect in which case it is OK to use a single pole switch to control the motor.
My other comment on the 120 vs 240 volt supply for motors is that there is a fair amount of latitude given as to what constitutes acceptable wiring. According to code as long as the voltage drop under full load is less than 3% you are OK. Unfortunately, AC motors can draw up to 6x their full load rating while starting which yields a voltage drop of 18% during startup. That is why there are many anecdotal stories of people having much faster motor startup when they switch the motor and circuit wiring to 240V.
Greg
I also want GFCI on all my shop circuits - but that's a different discussion.
The example you gave about the 3% drop works the same on both voltages. If you have a 3% drop on your 220 volt line and get 6x inrush current, the percentage voltage drop will be the same.
I think the reason you hear those stories is that people had marginal 110 volt circuits before they upgraded to a proper 220 volt circuit. When they do, the motor works better.
I hate to keep repeating myself, but if you feed the motor with a proper circuit, it will perform the same on either voltage.
Mike
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