2

My 3 x cadet (King Electric PAW) forced air wall heaters (total of 3500 W) run off 240V 30 Amp circuit in my condo. My old line voltage thermostat from King Electric ESP-230 just broke and I want to replace it with a Mysa V2 smart thermostat. It supports up to 3800 W, but the new thermostat says 16 Amp max. The double breaker in the electrical panel for the heaters says 30 amp.

Is it ok and safe to wire in the Mysa thermostat? Or is this an absolute no no?

Doing a bit more research it sounds like it’s ok to put a 16 amp device (any amperage really) on a 30 amp circuit. As long as the device amperage is lower than the circuit it should be fine. Can you confirm or am I misinterpreting this?

4
  • the circuit breaker function is to protect the wires and the device. In you case it should be a 20 amp breaker, the 30 amp will not protect the device until it is to late
    – Traveler
    Commented Nov 29, 2022 at 7:48
  • 1
    Does this thermostat have an internal fuse to protect it or would it be destroyed by overcurrent? If it were to experience damaging overcurrent, what would be the consequences--just a ruined thermostat or could it start a fire? When these line thermostats fail do they fail in the closed position or the open position? Commented Nov 29, 2022 at 12:03
  • Is there enough room to install a larger box at the location of the thermostat? Commented Nov 29, 2022 at 12:44
  • 2
    The breaker is oversized. 3800W @ 240 V is 15.8A After derating for continuous loads, (divide by 0.8 or multiply by 1.25) 19.79A Or 18.23 if your actual load is 3500W
    – Ecnerwal
    Commented Nov 29, 2022 at 12:55

1 Answer 1

7

Is it ok and safe to wire in the Mysa thermostat? Or is this an absolute no no?

It is an absolute no-no.

Doing a bit more research it sounds like it’s ok to put a 16 amp device (any amperage really) on a 30 amp circuit. As long as the device amperage is lower than the circuit it should be fine.

That's not how any of this works. Something like the converse is true: the supply must be capable of supplying at least as much power as the connected load(s) require, otherwise it won't work. If you have a AC heater that requires 20A, you can't put it on a 15A circuit - the breaker will trip of the fuse will blow immediately. If you try to start a car with 6 AAA batteries in series, it just won't work (because the batteries just cannot supply enough current).

So you must connect your loads to a circuit that is at least big enough, in order for them to work.

However, there are configurations that would work, but are not safe. If you connect a table lamp (designed to be on a 15A or 20A circuit) directly to your main circuit breaker (usually 100A or 200A) and a short occurs in it, the wires could easily overheat and catch fire without tripping anything.

Generally you don't have to worry about this too much because legitimate plug-in household devices are engineered and tested to cause a breaker trip (or fuse blow) on a 20A circuit (the largest overcurrent protection allowed for standard 15A receptacles) even if the device uses only a fraction of an amp normally.

With hardwired devices, it's more important to follow the manufacturer's instructions in order to ensure safety.

Going back to this:

it’s ok to put a 16 amp device (any amperage really) on a 30 amp circuit

It's (generally) OK to put smaller loads on a larger circuit. But a thermostat is not a load. It's a switch. It must be rated to safely handle the loads that it is controlling (the heaters), otherwise it may fail catastrophically. Using a too-small switch (thermostat) is like using too-small wire. Larger loads may cause it to overheat and catch fire before the breaker trips or the fuse blows.

Now, you may not need a 30A circuit if you only have 3500W of heaters on the circuit. 3500W at 240V is 14.6A. Continuous loads like heaters may use only up to 80% of a circuit's rating but 80% of 20A is 16A - that's more than 14.6A so you could switch the breaker to 20A and safely use this new thermostat.

You should also check the heater manufacturer's instructions to see if this heater setup should be on a 30A circuit at all. https://king-electric.com/wp-content/uploads/kel_product_files/PAW_INSTALL.pdf (may nor may not be for your exact models) says 12.1A - 16A of heaters should be on a 20A circuit, not 30A.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.