Wall Mounted Fan Forced Electric Heater

[infinity]

Someone asked me today about the branch circuit for a 10kw, 240 volt, thermostatically controlled electric garage heater. I immediately said 60 amp circuit. Then when I looked in the codebook I thought well maybe not so fast. 424.4(B) says the branch circuit conductors shall be sized at 125% of the rating of the heater load. What about the OCPD? Can a 50 amp OCPD be used with #6 conductors? 424.4(A) seems to say that the OCPD can be 50 amps for this load. Obviously if you've already run 65 amp conductors it would be easy to simply use a 60 amp OCPD but is it required if the load is non-continuous? Also for this installation no NM cable will be used.

424.4(B) Branch-Circuit Sizing.
The branch-circuit conductors for fixed electric space-heating equipment and any associated motors shall be sized not smaller than 125 percent of the load.

424.4 Branch Circuits.
424.4(A) Branch-Circuit Requirements.
An individual branch circuit shall be permitted to supply any volt-ampere or wattage rating of fixed electric space-heating equipment for which the branch circuit is rated.

 

[Jaybone812]

Someone asked me today about the branch circuit for a 10kw, 240 volt, thermostatically controlled electric garage heater. I immediately said 60 amp circuit. Then when I looked in the codebook I thought well maybe not so fast. 424.4(B) says the branch circuit conductors shall be sized at 125% of the rating of the heater load. What about the OCPD? Can a 50 amp OCPD be used with #6 conductors? 424.4(A) seems to say that the OCPD can be 50 amps for this load. Obviously if you've already run 65 amp conductors it would be easy to simply use a 60 amp OCPD but is it required if the load is non-continuous? Also for this installation no NM cable will be used.

the 50 amp max only applies to infrared heaters in other than a dwelling if I’m not mistaken
Your heater is forced air correct?


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[infinity]

Your heater is forced air correct?

It's a Qmark MUH102.

 

[Jaybone812]

I just looked that model up and definitely a forced air ,not infrared . so the Branch circuit is for the heater is not limited to 50 amps or less.
So the Branch circuit conductors and ocpd should be factored at 125% of the kw rating on the nameplate

 

[kwired]

Someone asked me today about the branch circuit for a 10kw, 240 volt, thermostatically controlled electric garage heater. I immediately said 60 amp circuit. Then when I looked in the codebook I thought well maybe not so fast. 424.4(B) says the branch circuit conductors shall be sized at 125% of the rating of the heater load. What about the OCPD? Can a 50 amp OCPD be used with #6 conductors? 424.4(A) seems to say that the OCPD can be 50 amps for this load. Obviously if you've already run 65 amp conductors it would be easy to simply use a 60 amp OCPD but is it required if the load is non-continuous? Also for this installation no NM cable will be used.

You only referred to the first sentence of A. It goes on to state branch circuit limitations for when there is multiple units on one branch circuit.

An individual circuit can be any size depending on rating of unit, but all electric space heating is also considered to be a continuous load therefore the 125% requirement for conductor ampacity.

As long as the 125% combination of the heating load and the blower is not more than 55 amps NM cable is acceptable and can be protected with a 60 amp overcurrent device.

 

[infinity]

So the Branch circuit conductors and ocpd should be factored at 125% of the kw rating on the nameplate

My questions about the OCPD and is it required to be 125% of the nameplate?

 

[Eddie702]

I always sized the conductors and wire at 125% on all HVAC equipment. How can you know it will not be a continuous load?

 

[infinity]

How can you know it will not be a continuous load?

Good question. For one this is a 10 KW unit going in a small shop the size of a 2-car garage and it is thermostatically controlled. It might be possible that if you set it to 90° and it was -5° it might not shut off for 3 hours but with normal use it will not run at maximum output for 3 hours or more.

 

[wwhitney]

My questions about the OCPD and is it required to be 125% of the nameplate?

Article 424 is surprisingly coy about explicitly stating that the OCPD is required to be 125% of the load. But it is at least implied: for example 424.22(B) refers to resistance type heating elements being protected at not more than 60A, with any loads larger than 48A being subdivided into loads not more than 48A.

424.19 requires the disconnecting means to be rated at 125% of the "total load of the motors and the heaters." So if you are using the OCPD as your disconnecting means as per say 424.19(B)(1), the OCPD would have to be 125% of the nameplate [assuming the nameplate doesn't already include a 125% factor for either of those loads.]

Cheers, Wayne

 

[wwhitney]

P.S. The 2017 NEC is clear on this question, 424.3(B) said "Fixed electric space-heating equipment and motors shall be considered continuous load."

A public comment on the 2020 NEC second draft proposed changing this to "The branch circuit and overcurrent protection for fixed electric space-heating equipment and any associated motors shall be sized not smaller than 125% of the load."

This was partially accepted to give the language in 2020 NEC 424.3(B) which refers only to branch circuit conductors and not the OCPD. The panel statement was "Text revised for clarity related to conductor sizing. Text related to overcurrent protection not accepted as overcurrent protection requirements are covered in 424.22 (A)."

But I think the panel overlooked the fact that 424.22(A) does not explicitly state that the OCPD shall be sized at 125% of the load . . .

Cheers, Wayne

 

[infinity]

But I think the panel overlooked the fact that 424.22(A) does not explicitly state that the OCPD shall be sized at 125% of the load . . .

Yes that's what I was getting at. No reason for the OCPD to be smaller than the conductors but the words to require it simply aren't there.