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I have a 220V circuit running from my panel in the basement to my garage. It was installed (not by me) with a 20A breaker. The circuit was installed to run only my table saw, which requires 220V, which is the only reason I had the line installed. The 220V line ends in a receptacle in the garage with a single plug obviously.

My question: Can I replace the 20A breaker with a 30A breaker? I would like to use a space heater in the garage and the space heaters I'm looking at all draw 30A. If only a single appliance is running on the circuit, how would I know if this breaker swap will work?

I would not use the heater and table saw simultaneously of course.

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The answer is NO 99% of the time.

To use a 30 amp breaker the wires and the receptacle must be rated for 30 amps. The wires need to be a minimum of 10 gauge copper/8 gauge Al.

The possibility of you having 10 gauge wires and a 30 amp receptacle is very low. Most likely you have 12 gauge wires and a 20 amp receptacle.

Changing to a 30 amp breaker will be a fire risk, reducing the need for a heater.

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  • Makes sense, and I certainly don't want to burn the house down. There's a chance the electrician used 8 gauge wire since the box is on the far opposite corner of the house. IF 8 gauge was used, would the increase the odds of it working?
    – ms960
    Commented Dec 9, 2023 at 11:54
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    What do you mean you coudn't see what the wire was? The wire is inside the box so (with the breaker off) free the receptacle by removing screws and pull the receptacle out. Don't disconnect the wires from the receptacle. You might be able to see the wire and determine the size of the wire. If you really want a 30 A heater, this would not be too much trouble. But if YOU hired an electrician to install this circuit why not ask the electrician if he remembers what he installed? Would you take a pic of your current receptacle and include it in your question using the edit function?
    – Jim Stewart
    Commented Dec 9, 2023 at 12:43
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    @JimStewart Unfortunately, the cable runs up from the basement and goes through the ductwork underneath the first floor, and comes out in the garage, so there isn't any attic or other space to see the wiring. There is not even enough sheathing visible as it comes out of the box and disappears into the wall to see what the gauge is. All I can tell is it's yellow Romex, but nothing more. Thanks for the suggestion however. I'll probably just leave it alone to be safe.
    – ms960
    Commented Dec 9, 2023 at 13:46
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    @ms960 Yellow romex is usually 12 gauge wire, so unless you replace with orange/10 gauge wire, the answer becomes 100% No. There do seem to be 20 amp heaters.
    – crip659
    Commented Dec 9, 2023 at 14:03
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    @crip659 LOL "reducing the need for a heater"; I would go further and say eliminating the need, because it would be inefficient to attempt to heat a house that has burned down.
    – Jimmy Fix-it
    Commented Dec 9, 2023 at 16:25
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You have 12 AWG wire on that circuit. That makes sense, #10 would be superfluous for the original application, which is the table saw.

Aside from not burning your house down by overloading #12 wire, let's try for not burning something else down: your electric bill.

Space heaters are monsters. They burn $1-2 an hour of electricity. Run one for 700 hours or so and you've paid for a modern Asian style DIY heat pump. This MOVES heat instead of making heat. As such, it does so at 200-600% efficiency depending on the weather, so it costs pennies per hour to run.

Why am I off on this crazy tangent? That 20A circuit. It would be plenty for a heat pump of same BTU capacity as the electric heater you've been eyeing. So it solves your problem.

Did I mention that the heat pump will give you air conditioning in the summer? So OK, so maybe it might make your summer electric bill go up. But not by a whole lot - some of them are laughably more efficient air conditioners than a traditional house A/C.

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