Wire for garage heater

[Colsberg2013]

I have a 7500 watt garage heater. Can I use 8/2 NM-B wire? I have the wire on hand and trying to find solid is impossible?

I am assuming that's since it's already wrapped it does need to go in conduit.

Planned on running it to a junction box and then use my extra AC whip to from the junction box up to the heater.

Total run is 30ft. Thanks

 

[Terry D]

7500/240 = 31.25 amps x 125% = 39 amps. But look at the plate on the heater or the manual.

#8 NM-b is good for 40 amps. So you are good with that wire. Your A/C whip will have to be #8 THWN with a stranded #10 THWN ground

Use a 2-pole 40 amp breaker

NM-b does not have to be in conduit. There are some jurisdictions that will not allow it to be exposed though

 

[Colsberg2013]

So why would I need to have 10 gauge ground wire I'd everything else is 8ga. Just asking

Thank you

 

[wyliesdiesels]

EGCs are sized by the breaker rating. A #10 can be used on a circuit with a MAX 60a breaker

 

[pattenp]

The ground in that 8/2 nm-b should be a #10.

 

[dcg9381]

7500/240 = 31.25 amps x 125% = 39 amps. But look at the plate on the heater or the manual.

#8 NM-b is good for 40 amps. So you are good with that wire. Your A/C whip will have to be #8 THWN with a stranded #10 THWN ground

You *really* need to read the name plate on that heater before deciding. If it's true 7500 watt continuous (39 Amps), I would not use that wire or a 40A breaker:

"NEC sizing rules. Secs. 210-22(c), 220-3(a), 220-10(b), and 384-16(c) all relate to the sizing rules for overcurrent protective devices (OCPDs). The first three all specify the same requirement: OCPD size = 100% of noncontinuous load + 125% of continuous load."


You don't want that breaker to trip after it runs for 2 hours...

 

[sparky 1971]

You *really* need to read the name plate on that heater before deciding. If it's true 7500 watt continuous (39 Amps), I would not use that wire or a 40A breaker:

"NEC sizing rules. Secs. 210-22(c), 220-3(a), 220-10(b), and 384-16(c) all relate to the sizing rules for overcurrent protective devices (OCPDs). The first three all specify the same requirement: OCPD size = 100% of noncontinuous load + 125% of continuous load."


You don't want that breaker to trip after it runs for 2 hours...

Huh? Are you trying to say he needs a 70 amp circuit for a 7500 watt heater? (31+39). It will be just fine on a 40 amp circuit, which by the way is sized correctly. A heater is a continuous load only, not both.

 

[dcg9381]

<corrected>

 

[mike93lx]

As I read it, if the heater is a continuous 7500 watt load, he's under-sized at 40amps.


Sorry, I think that's for current protection on a feeder with multiple loads. In this case, I think the math is (39A continueous x 1.25%) = 48.75A. I'd say he needs a 50A "standard" breaker. The feeder wire needs to follow the breaker. Apparently there are breakers that are rated for 100% operation, so if he got one of those the 125% rule would not apply.

I don't want to dive too deep into the NEC on this. I'll just say this: In my experience many breakers cannot do more than 80% rated load for more than a few hours. They get hot and flip after that... This is a case that appears to be right on that line. Needs a "continuous load" beaker or needs to be sized 125%.

Check your math. 7500w / 240v = 31.25a, as already stated above. 39amps is with the 125% already applied

 

[dcg9381]

Check your math. 7500w / 240v = 31.25a, as already stated above. 39amps is with the 125% already applied

Yep, missed it.. 31.25 * 125% = 39amps, all good! My apologies. 40A "standard" breaker.

 

[sparky 1971]

As I read it, if the heater is a continuous 7500 watt load, he's under-sized at 40amps with a standard breaker.

You quoted the math that shows it needs a 40 amp circuit.

 

[dcg9381]

Mik93 corrected me.. Needs a 40 amp circuit. I can remove my post.. The 125% was baked in.

 

[Terry D]

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