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I purchased a residential property that has a garage/workspace constructed in 2000 with a building permit and it has a 100amp sub panel. The space has three 72" electric baseboard heaters (1500 watts each) wired to one thermostat. One heater is disconnected. I hooked up the third heater and smelled wires overheating in the thermostat and quickly disconnected the third heater.

I see the baseboard heater wiring is hooked up to a gang of two 20 amp breakers, everything is wired with 12/2 cable, and my Honeywell thermostat (RLV4305) is rated up to 3500 watts @ 240 VAC/60Hz.

Question: Can I buy a thermostat that can handle three 72" electric baseboard heaters at 4500 watts total, or will this overload the existing 12/2 wiring?

I found something online that showed I might need 10/2 wiring for three 72" heaters - see attached. Thank you in advance for your comments and I appreciate your help very much.

enter image description here

I like the smaller heater idea.

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Yes, 3 1500W heaters will overload both your 3500W thermostat and your 20A 240V circuit.

You either need an additional double pole circuit and a second thermostat, or a 30A breaker and 10 gauge (copper) wiring and a different thermostat (or relay(s) /contactor(s) to switch the load(s) on command of a thermostat.)

(4500W/240V)/0.8 = 23.4375A, which is more than 20A. Divide by 0.8 (or multiply by 1.25) is required for coninuous loads, which electric heaters are defined as being.

Should have been obvious before you chose to add the disconnected one that 4500W is more than 3500W for the thermostat. As the person choosing to reconnect it, that was your responsibility to check. If you melted insulation there, you probably need to replace it, anyway.

If you'll use the heat more than a few hours at a time, rarely, the operating cost should drive you in the direction of using a cold-climate heat pump, where your 20A circuit can support about 9000W-equivalent of heating by moving heat, rather than making it directly. Or you can pay the price of one of those and more to the power company for operating 3000-4500W of resistance heat on a regular basis - your call.

You could also use a smaller (500W) third heater. With a properly rated (3750W or more) new thermostat, 750W. 1000W would be too big for the 12AWG wiring.

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  • Re: Double pole circuit. I mentioned the heaters are hooked up to a gang of two 20 amp breakers -as in (2) 20 amp breakers.
    – Victor
    Commented Dec 6, 2023 at 23:22
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    What about it? Utterly normal for any 240V circuit in the USA/Canada and a 1500W heater is going to be a 240V heater. You don't get a X2 multiplier on the amps becasue it's a double-pole circuit breaker - It's still 20A, but at 240V instead of 120V. You might do well to visit the Library and check out some basic starter books on home wiring before you tackle more, if you're messing with 240V heaters and don't know that in the first place.
    – Ecnerwal
    Commented Dec 6, 2023 at 23:26
  • Thank you for the clarification on all issues. Thinking it is best to just go with a smaller 500W or 750W setup and a new thermostat. I appreciate the help.
    – Victor
    Commented Dec 6, 2023 at 23:32

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