Help adding Line Level T.Stat to Electric Heater

[wmshay6]

Hello everyone, long time lurker, infrequent poster. Finally bought a heater for my garage. Have been reading lots of threads, but still looking for a little guidance. I purchased a TPI Model HF686TC. 208/240V 4200/5600W.

I thought about adding a simple DP line level stat to bypass internal stat, but can't locate one locally with high enough amperage rating for 5600W. Then I thought about using a SP Stat but not sure how to calculate the amperage draw. Is one leg simply half of the overall amperage rating from both legs? (ie- max rating of 5600 divided by 2 is 2800W, divided by 120 is 23.3A for the switched leg- still a pretty high Amperage rating for a thermostat)

When looking at the heater, it appears that there is a simple rheostat setup on one leg of the 240V circuit and the other leg is directly connected.

My thought was I could bypass the internal Rheostat (using the simple lug connections shown) and hardwire this thing through a wall mounted SP thermostat. I'd rather a DP stat so I can definitvely cut power, but if push comes to shove can always throw the breaker on rare occasion if I need to.

What am I missing here? There is existing #10 NM-B cable in place that I would be working with (already installed from something else and since abandoned, so this would be only item hardwired on the 240V 30A circuit)

 

[rlitman]

Are you connecting this to a 240V circuit? If so, why are you using 120V in your calculations? Current flows through both legs. So it is 5600W/240V. Yeah, that still equals 23.3A.

I'm not sure what you're describing as a rheostat. I don't see a rheostat in your pictures. Is it a relay?

 

[wmshay6]

That's kind of my question. A single pole stat only connects to one of the legs of the 240V circuit. Hence, it only ever receives 120V, or is that incorrect?

It's kind of a moot question anyway- the Amperage rating is the same either way. I need a 23.3A rating and most of the local stuff maxes at 22A. So I guess, I can order online or keep looking locally.

That said, does my logic of bypassing the internal thermostat seems sound when looking at the wiring picture? Or, alternatively, just leave the internal stat on MAX and use the wall mount to control the unit.

 

[rlitman]

The single pole stat breaks the connection on one leg, but it sees the whole 240V. Yes, that part is moot.

I wouldn't bypass the internal thermostat. Just turn that up (all the way up if you want), and it will act as a safety cutoff.

Yeah, you'll need a 30A or larger thermostat. Or, use a relay.

 

[wmshay6]

There's no "cushion factor" in thermostats is there? (like you encounter when wiring a compressor motor) I ask because 5600W/240V is 23.3A.

I've found a double pole rated for 25A. So, I'm thinking that should work- I don't think I need to oversize the thermostat. Is this correct?

Shouldn't ever hit a 30A draw as it would be on a hardwired, dedicated circuit with 10Ga NM-B.

 

[rlitman]

That should work. While a contactor that breaks both legs simultaneously (your double pole) sees the same current, they last longer than ones that break just one leg.

But 25A line voltage themostats are stupid expensive. If you can find space for one of these:
https://www.amazon.com/dp/B004YW5CWQ/?tag=atomicindus08-20
you can use any 240V rated thermostat to trigger the relay, as the current through the thermostat will be insignificant.