Question on NEC 210.20

[Mila15]

My question is regarding a branch breaker in a large 480v Power Panel. The circuit feeds a 36A heating load. Can a 40A breaker be used, knowing the following:
- This is at a small generating Station
- Space Heater is supposed to turn on only when ambient temperature is below 50F (10C)
- #8 AWG cable is rated for 90C, but rated for 50A (about 140% of load) at 75C insulation

Thanks!

 

[DRWeig]

424(B) makes your heater a continuous load no matter the how it operates.

So conductors must be 125% of 36A or 45A. Yours are 50A, assuming you don't have to do any ambient temperature derating. No problem.

A 40A breaker protects a 50A conductor just fine. No problem.

What part of your design did you think was questionable?



Best to you,

Goober Dave

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[Mila15]

Thanks DRWeig. I'm actually concerned about the breaker size. NEC 210.20 indicates OCP shall be rated not less than 125% of continuous load + 100% of non-continuous load. This means breaker shall be at least 45A (unless assembly is rated for 100% operation).

 

[wroggent]

424(B) makes your heater a continuous load no matter the how it operates.

So conductors must be 125% of 36A or 45A. Yours are 50A, assuming you don't have to do any ambient temperature derating. No problem.

A 40A breaker protects a 50A conductor just fine. No problem.

What part of your design did you think was questionable?

Shouldn't the breaker also be sized to 125%, i.e. 45A(see 424.19)? Keeping 424.3(B) in mind, what does 220.51 apply to?

 

[DRWeig]

Let's see...

424.19 is for the disconnecting means, not the overload protection. It would apply if this breaker is also serving as the disconnect.

220.51 applies to feeders, not branch circuits. However, it does not conflict with 424.3(B). They both call for 100 percent load for fixed electric heating in calculations. That is, fixed electric heaters cannot be considered non-continuous loads.

And on MICHAELUSA's original question about 210.20, he is right and I am wrong . It does need a 45A breaker.

Best to you,

Goober Dave

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[Mila15]

220.51 refers to load calculation, when considering calculating the demand load for fixed electric heaters. 210.20 addresses sizing OCPDs. In this case, 45A is a minimum size (unless the OCPD and assembly are listed for 100% operation). I believe if I want to stay with the 40A breakers, I need to make sure they are listed for 100% operation (plus the whole assembly of course). An other consideration is, since Power Plants DO NOT have to comply with NEC, would standard 40A breakers cause a safety and/or operational issues, especially that heaters will only come on during cold ambient conditions, which is way below the 40C ambient condition most breakers are rated at?

 

[wroggent]

90.2(4) may apply to you - where are you installing this?

 

[Mila15]

This is in an Electric Utility Generation Plant, where 92.B.5.b applies.