Article 424 Supplementary OCPD / Subdivided Loads

[wwhitney]

2023 NEC 424.22(B) says "Resistance-type heating elements in electric space-heating equipment shall be protected at not more than 60 amperes. Equipment rated more than 48 amperes and employing such elements shall have the heating elements subdivided, and each subdivided load shall not exceed 48 amperes."

Now various sections refer to "the supplementary over-current protective devices for the subdivided loads," like 424.22(C). But I don't actually see a statement in Article 424 of the form "Subdivided loads shall be provided with supplementary over-current protection devices." Did I miss it? Or is it acceptable to install a piece of fixed electric space heating equipment that uses say 2x 32A resistance-type heating elements, and supply it with 2x 40A branch circuits, with no supplementary OCPD?

[I have not looked at the applicable UL standard; if it requires supplementary OCPD for subdivided loads, the question would be moot, as such equipment would not exist. That would also explain why Article 424 seems to assume such supplementary OCPD for subdivided loads; still seems odd that the NEC would omit the explicit requirement.]

Thanks, Wayne

 

[mtnelect]

I have never seen "Subdivided Loads" for "Emergency Heat" in any HVAC manufacture equipment, only specific size elements for each size.

 

[wwhitney]

I have never seen "Subdivided Loads" for "Emergency Heat" in any HVAC manufacture equipment, only specific size elements for each size.

The "optional electric heat" is field installed, so I would think there would be no relationship between that choice and the heat pump rating. The subdivision requirement only applies to "electric resistance" heating, so it would apply to the heat strip. The max rating is 48A, so at 240V that would be 11.52 kW. So the 20 kW option, for example, could not be provided by a single electric resistance unit, but would need to be 2 * 10kW.

Cheers, Wane

 

[mtnelect]

The "optional electric heat" is field installed, so I would think there would be no relationship between that choice and the heat pump rating. The subdivision requirement only applies to "electric resistance" heating, so it would apply to the heat strip. The max rating is 48A, so at 240V that would be 11.52 kW. So the 20 kW option, for example, could not be provided by a single electric resistance unit, but would need to be 2 * 10kW.

Cheers, Wane

If you were to do that, I would think you would violate the manufacturer rating of that particular HVAC equipment.

 

[wwhitney]

If you were to do that, I would think you would violate the manufacturer rating of that particular HVAC equipment.

Do what?

Also, I don't see the point of your second image, it doesn't shown anything about the optional electric heat, just about the heat pump characteristics. Article 424 doesn't apply to heat pumps.

Cheers, Wayne

 

[mtnelect]

Do what?

Also, I don't see the point of your second image, it doesn't shown anything about the optional electric heat, just about the heat pump characteristics. Article 424 doesn't apply to heat pumps.

Cheers, Wayne

You are correct, I left out the Electric Heat page. Is this what you were referring to "Subdivided" under "3 Circuits".

 

[Dennis Alwon]

2023 NEC 424.22(B) says "Resistance-type heating elements in electric space-heating equipment shall be protected at not more than 60 amperes. Equipment rated more than 48 amperes and employing such elements shall have the heating elements subdivided, and each subdivided load shall not exceed 48 amperes."

Now various sections refer to "the supplementary over-current protective devices for the subdivided loads," like 424.22(C). But I don't actually see a statement in Article 424 of the form "Subdivided loads shall be provided with supplementary over-current protection devices." Did I miss it? Or is it acceptable to install a piece of fixed electric space heating equipment that uses say 2x 32A resistance-type heating elements, and supply it with 2x 40A branch circuits, with no supplementary OCPD?

[I have not looked at the applicable UL standard; if it requires supplementary OCPD for subdivided loads, the question would be moot, as such equipment would not exist. That would also explain why Article 424 seems to assume such supplementary OCPD for subdivided loads; still seems odd that the NEC would omit the explicit requirement.]

Thanks, Wayne

I have wired a 20 kw furnace that required 2 x 60 amp circuits. There weren't any supplementary overcurrent protective devices installed.

 

[Dennis Alwon]

Some units take one circuit at say 100 amps. Those loads would have to have supplementary overcurrent protective devices.