Wiring 10,000 watt heater

[BowtieNut]

Trying to heat the garage in my new house. I'll only heat it very occasionally when I need to change oil or some other small project so I decided to go with electric. Garage is 872 sq ft and I bought the Thermosphere 10,000 watt heater from Northern Tool. It will be fully insulated, 2x4 walls, blown in ceiling, insulated doors, & shares one wall with house.

Heater says it's 42 amps. On the box it says use 8ga wire. I bought a 50 amp breaker. Reading the manual it says I have to use a 55 amp breaker (apparently breakers for heaters need to be sized 125% of the load from what I read elsewhere?). I can't find a 55 amp breaker for my Eaton circuit panel so I guess I'd have to step up to 60 amp breaker? Except I'm not allowed to put a 60 amp breaker on 8 ga wire am I?

I'm not sure what options I have here. I already ran the wire and can't return it. I haven't opened the heater yet so I could return that and get a 7500 watt instead. The walls are not sheetrocked yet so I'm considering running another circuit/wire to the other side of the garage for another 7500 watt heater later if needed (the 7500 is almost half the price of a 10k anyways). Besides it would be more heat, more spread out, and more versatile. Anyone have any advice or other thoughts? Thanks!

 

[pattenp]

You need #6Cu with a 60A breaker.

 

[wyliesdiesels]

Trying to heat the garage in my new house. I'll only heat it very occasionally when I need to change oil or some other small project so I decided to go with electric. Garage is 872 sq ft and I bought the Thermosphere 10,000 watt heater from Northern Tool. It will be fully insulated, 2x4 walls, blown in ceiling, insulated doors, & shares one wall with house.

Heater says it's 42 amps. On the box it says use 8ga wire. I bought a 50 amp breaker. Reading the manual it says I have to use a 55 amp breaker (apparently breakers for heaters need to be sized 125% of the load from what I read elsewhere?). I can't find a 55 amp breaker for my Eaton circuit panel so I guess I'd have to step up to 60 amp breaker? Except I'm not allowed to put a 60 amp breaker on 8 ga wire am I?

I'm not sure what options I have here. I already ran the wire and can't return it. I haven't opened the heater yet so I could return that and get a 7500 watt instead. The walls are not sheetrocked yet so I'm considering running another circuit/wire to the other side of the garage for another 7500 watt heater later if needed (the 7500 is almost half the price of a 10k anyways). Besides it would be more heat, more spread out, and more versatile. Anyone have any advice or other thoughts? Thanks!

Since the heater is considered a continuous load the wire needs to be size @ 125%. Therefore you need 52a rated wire. This means you need #6 wire.

Since you already bought the wire, just return the heater and go with the 7,500w model.

 

[MattT]

What kind of wire did you run? Romex is only good for 40A so if that's what you've used you should definitely step down to a 7.5kW unit and breaker it at 40A. Even THHN isn't right for 10kW but would probably work breakered at 50A.

 

[BowtieNut]

I used NM-B. I thought I saw somewhere that it was good to 50 amps. That's so weird that the heater would say right on the box to use 8ga wire. Oh well, I'll try the smaller 7500 watt heater instead. Who knows, that one might be enough on its own for all I know. I see many reviews that say it should be fine for a well insulated garage of this size. Thanks guys!

 

[wyliesdiesels]

I used NM-B. I thought I saw somewhere that it was good to 50 amps. That's so weird that the heater would say right on the box to use 8ga wire. Oh well, I'll try the smaller 7500 watt heater instead. Who knows, that one might be enough on its own for all I know. I see many reviews that say it should be fine for a well insulated garage of this size. Thanks guys!

The devil is in the details.

Check t310.15(B)(16)

Yes #8 THWN is good for 50a.

NM-b is limited to 60* c amapacity so this means 40a for #8 NM-b.

Best to get the correct info before buying the wire.

 

[MattT]

I used NM-B. I thought I saw somewhere that it was good to 50 amps.

You probably saw a 75*C ampacity table, which is what'll usually pop up in a web search. Like Wylie said romex is limited to 60*C so the ampacities are lower.

That's so weird that the heater would say right on the box to use 8ga wire.

They probably googled the same table you did

 

[BowtieNut]

Thanks guys, I think I'll go exchange for the 7500 watt heater instead. That one uses 31.3 amps, so 125% is 39 amps. Then I can use a 40 amp breaker on my 8ga wire. And just to safely future-proof it I think I'll spend the $50 to run another 8ga wire to the other side of the garage.

The 7500 watt instructions say to use #10 wire with 45 amp fuse? I hope the heater was designed by someone with more sense than whoever wrote the manual. It has pretty favorable reviews so I'll cross my fingers.

 

[MattT]

The 7500 watt instructions say to use #10 wire with 45 amp fuse?

At least they're consistent on the wire sizing

The idiots have also caused a code compliance problem by putting fuse in the instructions. Code requires that those instructions are followed and doesn't allow subsituting breakers for fuses. No way that thing needs to be fused

 

[u3b3rg33k]

Read instruction manuals, then defer to NEC/NFPA.

 

[mike93lx]

If you are sealing the walls up, I would run 6/2 to the other side and be ready for a larger unit, if ever needed. Or conduit and leave the wire out until you know what you need