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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?

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  • If you are running a 240 V welder at 50 A, there is 50 A in each leg. In a 240 V circuit the same current is flowing in each leg. You would still have 90 - 50 = 40 A available in each leg. If you would then turn on a table saw drawing 25 A at 120 V, then one leg would have only 15 A available. With the table saw on if you would turn on another appliance drawing 25 A at 120 V, on the same leg as the saw. then the 90 A breaker would trip (both legs would trip since it is a common trip br). If additional appliance would be on the other leg from the saw, then the breaker would not trip. Commented Oct 9, 2020 at 22:00

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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.

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  • And when your tablesaw trips in the middle of a heavy cut, you'll have some real-world considerations...
    – Ecnerwal
    Commented Oct 9, 2020 at 21:13
  • Similarly, few welders are drawing their full rated current most of the useful time. Most are only good for 10-20 minutes of continuous use at full power, and are rarely run that hard for normal size jobs (my 250A (output) welder rarely sees over 150A output.)
    – Ecnerwal
    Commented Oct 9, 2020 at 21:34
  • @Ecnerwal I've had my table saw trip during a heavy cut and all that happened was the blade stopped very quickly. I don't see a lot of danger unless the lights are on the the same circuit(bad idea). Commented Oct 9, 2020 at 22:01
  • @GeorgeAnderson Thanks for clarifying. I think I understand now. I now understand why I need wire rated for 90 A...because it just might get the full 90 A! Great tip on changing motors to 240 where possible.
    – tnknepp
    Commented Oct 12, 2020 at 10:47
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240V @ 90A = 21.6 kW

Or 120V at 90A (twice) = 180A = 21.6 kW

If your compressor and tablesaw are 120V, consider changing them to 240V, unless you also haul them off to jobsites that don't have 240V available.

If you are running a 240V welder at 50A, that's 50A per leg (& nothing on neutral.) 240V uses both legs.

You would then have 40A left on your 90A breaker, whether at 120V or 240V.

If the tablesaw and compressor are both 20A 120V and both on the same leg of the service, it might be a near thing with startup surges - but technically it is within 90A total: 50 on one leg, 90 on the other leg, 40 on neutral.

If they are 20A 120V and on opposite legs, no problem. 70A total, nothing on neutral.

If they are 10A 240V, also no problem. 70A total, nothing on neutral.

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  • This is helpful. Your post exposed my ignorance regarding the role of neutral. Can you explain why the current on neutral changes, or would that make a better "new" question?
    – tnknepp
    Commented Oct 12, 2020 at 10:42
  • Apparently I cannot edit my last comment. To put it another way, from what you wrote I might expect the neutral to have 180 A on it if I am drawing 90 A on each leg, which would be over the neutral's rating.
    – tnknepp
    Commented Oct 12, 2020 at 10:50
  • No. The neutral has maximum current when you are only using one leg, so 90A at most. When you are using both legs, the neutral only carries the difference between the two hot legs. If you are using 20A on each leg, the 20A goes in one leg and out the other and the neutral is idle. If you are using 50 A on one leg and 20 on the other, one leg has 50, one has 20, and the neutral has 30.
    – Ecnerwal
    Commented Oct 12, 2020 at 13:39
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No no, it doesn't work that way! Loads don't divide!

There is no such thing as a 30A-nameplate load drawing 15A on two legs. That kind of "division" does not happen anywhere.

240V loads put full current on both legs.

If you have a 50A EVSE, that is pulling 50A from leg L1 and 50A from leg L2.

120V loads put full on one leg.

If you have a 12A heater, that is pulling 12A from leg L1 and 0A from leg L2.

You might think, "Well, that's a lot like pulling 6A from both legs". If you think that, you're starting to get in the ballpark, but still... that balancing isn't going to happen without a transformer being involved.

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  • Thanks. I think I was thinking of how current gets divided when going from 120 to 240. While the current goes down per leg, the overall current draw remains the same. Maybe better to think about Watts instead of amps. Thanks for clarifying.
    – tnknepp
    Commented Oct 12, 2020 at 10:45
  • @tnknepp No -- the current remains the same in 120V - it just travels on different wires. 12A@120V = 12A on one of the hot wires + 12A on the neutral. It is 12A everywhere it flows. It is never less than 12A. The difference billing-wise is that since 120V flows only on one leg, it only "spins the meter" on one leg. Meters are built so current on each leg advances the meter (and current on neutral does not). Thus 12A@240V spins the meter twice as fast as 12A@120V. Commented Oct 12, 2020 at 19:09

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