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The subpanel is fed from a 50 ampere 240 volt two pole breaker.

Currently there are 3 20 ampere 120 volt breakers, running on it (two freezers and an air compressor). Would like to add a 30 ampere 240 volt two pole breaker for a plasma cutter, but I don't want to overload the 50 amp breaker.

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I don't see any way to answer this without knowing peak (not just sustained) load from the freezers and compressor. If you do have those numbers, then if their sum exceeds 20A you're potentially in trouble. If it doesn't, you should be OK. – keshlam Jun 4 '14 at 16:03
how will you be using the plasma cutter? Will you be running it while the any of the other things are running? You could if nothing else was running. If the compressor or freezers kick on while using your plasma cutter there will be a spike in draw and probably trip the breaker on your main that feeds the sub panel – squinny Jun 4 '14 at 19:23

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