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In my house, can I upgrade a 30 amp breaker to a 40 amp breaker in my main breaker panel, if I ran the right gauge wire (8-gauge I think )? I have a 100 amp panel...

Don't have any any slots available in the panel but I have a 240v 30amp not doing anything. That I can use in the basement if were able to upgrade to a 240v 40amp breaker and just run new wire in the basement

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    This sounds ok, but with an old panel, you need to make sure you can even get a 40A breaker for it. – JPhi1618 Feb 5 at 18:01
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    What will the new circuit be used for? Are you sure you need a 40 AMP breaker? – JACK Feb 5 at 18:51
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    Are you planning on using copper or aluminum wire? Is the 100A your main panel or the size of a subpanel? What is the 40A feeding? – NoSparksPlease Feb 5 at 19:24
  • What make and model is your panel? Can you post photos of it for that matter? – ThreePhaseEel Feb 6 at 0:15
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Yes, if you are running new wire, and ensure it is the proper gauge for the material (copper or aluminum), there is no reason you can't swap out to a higher amp breaker if you have an unused one.

I would suggest that you go to 50 amp if you're going to this much trouble. 40 amps is a bit of an odd duck, so you might as well have 50 amps available because you could end up with an appliance or tool that needs it. The most common residential outlets will be either 30 amp, or 50 amp for 240. If you're running this to another subpanel I guess that doesn't matter, but I'd still suggest scaling the circuit for 50 amps.

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Yes you can do what you want to do. You would need to feed a device 40 amp or a sub panel, a 4 wire feeder is needed in both cases. A sub panel can be main lug or main breaker. The neutral buss must be isolated from the grounding buss in a sub panel make sure to get a large enough panel you can put in larger on that 40 amp feeder if you wanted you could install a 100 amp panel, but your actual load will be limited by the feeder size. But yes you can do it.

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