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Correct way to control Sidewalk Heating Mats
(lemmy.today)
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If they don't trust their specifications, do you? Pick up a Kill-a-Watt or some other way to measure them.
The only thing that comes to mind is that most resistive heaters eventually fail and might draw significant additional current when they do. But you should have plenty of extra margin with a 15A switch.
I will say, I've had an old mechanical window AC trip the ZEN15's overload detection when the thermostat short cycled and the compressor was stalled. So I would trust it for that in a pinch, and especially with a resistive load. But for an outdoor heating element application, I still would oversize the switching device (and get the 40A contactor), and put 15A circuit breakers before the contactor. Also, that 40A device can drive either a two pole 240V device (unison contacts) or two 120V devices, so you could just put each heater on its own pole and heat up your wiring a little less. Or probably just wire only one pole, I haven't read the manual.