I originally planned to install a subpanel and feed it 90 amps. I thought I could do this by putting a 90A double-pole breaker in my main panel and feeding off that. What I was told was "This will supply 90A on each leg, so your shop will really have 180A", but I don't think that is right. Does that truly supply 90 amps on each leg or does this get reduced to 45A on each leg (i.e. if I'm running equipment at 240 V then I have 90A available, but all the 120V stuff is limited to 45A)?
I think where this could bite me is if I'm running a welder at 240V and 50A (25A per leg) and somebody else is running the table saw and the compressor kicks on (assuming saw and compressor are on the same leg). This scenario will trip the breaker, correct?
Best Answer
Common misunderstanding. If you supply your sub-panel with 90 amps at 240 volts, that's what it will have. You'll still have 90 amps available on 2 legs at 120 volts. So yes, you'll have 180 amps available at 120 volts (90 amps on each hot (leg)).
If possible, can you run your table saw and compressor at 240? Many motors can easily be converted from 120 to 240 with a simple wiring change. See the face plate on the motor for instructions. If not, be sure they are on separate 120 v legs. And just because the instructions for a table saw says it needs a certain amperage, the reality is tools like a table saw rarely draw their full load. I know others here tend to over-kill the supply, but I'm a bit more realistic to real-world considerations.