Heater OCPD Sizing

[fifty60]

I have equipment that can either be ran at 230V single phase or 208VAC single phase. I have a 3KW heating branch within this equipment.

3000W/230V = 13.04A 13.04 * 1.25 = 16A. So for 230V I can use a 20A fuse.

For 208V, the heater Watts derate to around 2.45KW

2.45KW/208 = 11.8A 11.8A * 1.25 = 14.74. So for 208 I can use a 15A fuse.

I would really like to use the 20A fuse for 208 because I am so close to 15A at 1.25 * de-rated current. NFPA 79 is only concerened with heaters above 16.7A.

NEC 424 references me back to 210. I get a little lost here. Is 210 saying that I can use a 15, 20, 30, 40, or 50A OCPD as long as the conductors are rated for this OCPD? Or am I stuck with derating the heaters * 1.25 and then rounding up.

 

[GoldDigger]

Clearly the heaters can safely handle the higher current they will carry at 230 (240). So as long as the circuit wires are protected by the 20A breaker, the heaters also be fine.
Unless the equipment manufacturer specifies a maximum breaker size, the NEC is not directly involved in the maximum, just the minimum.

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[Jraef]

Size the conductors for the load, larger conductors is perfectly fine, then size the OCPD for the conductors, and OCPD that is smaller than the conductor rating is perfectly fine too. You just cannot size the conductors for less than 125% of the maximum load current, and you cannot size the OCPD for larger than the conductor rating.

Don't worry about protecting the load itself on something simple like a heater, they covered that when it was UL listed and if they needed a specific OCPD ahead of it to get that listing, they would have a nameplate that told you that on the heater, i.e. "MOCP = 15A", something like that. MOCP = Maximum Over Current Protection. If you don't see that, you don't worry about it as the installer. (It's slightly different for motors and transformers by the way, that's why there are specific code sections dedicated to motors and transformers.)