Branch Circuit Component Sizing

[rin9137]

Hopefully you guys can assist me in settling a debate at my office. We design control centers that run pumps and resistive electric heaters. The question at hand is where in NFPA70 / 79 addresses how to size the 480VAC 3P components?

There is no question that the overcurrent protection is to be sized no less that 125% of the FLA of the circuit with the wire sized such that it has an ampacity (after elevated ambient air derating) equal or greater to this protection rating. The question is what about the 3P contactor or SCR power controller? My understanding is if for a heater branch circuit sized at 48FLA / 60A Fusing, then the contactor should also be sized for an AC1 rating of 60A or greater. My coworker believes this to be wrong and that the contactor needs to have an AC1 rating of 48A or greater. Same for the SCR power controller. What about downstream terminal blocks? 48A or 60A?

Which sizing method is correct?

 

[buzzp]

I didn't look up in the NEC but the 60A seems more logical. If the heater was pulling above FLA, say 55 amps, then it can sit there all day long and never trip (motor might trip its overload but were talking heater) with a 48A contactor. The contactor will probably fail.

 

[jraef]

There is no question that the overcurrent protection is to be sized no less that 125% of the FLA of the circuit with the wire sized such that it has an ampacity (after elevated ambient air derating) equal or greater to this protection rating.

That right there is the fly in your ointment, which is why you have an issue. There is NO requirement for the PROTECTIVE DEVICES to be sized at 125% of the FLA, only the CONDUCTORS. While it's then true that you CAN size the circuit breaker for the conductor size, you don't HAVE TO. Smaller is just fine. What you CAN'T do is have components in the power circuit that are not being protected by the protection devices. So if you have a heater that draws 48A, then your CONDUCTORS must be sized for 60A, but a contactor rated for 50A resistive is perfectly fine for handling 48A heaters, so if you have used that contactor, your device protecting that contactor must be sized to protect THAT device, so a 50A fuse or breaker. There is absolutely nothing wrong with having conductors rated for 60A and an OCPD rated for 50A.

Now, if the heaters are going to run for 3 hours or more CONTINUOUSLY, then another factor looms with regard to using a circuit breaker, that being that the breaker will only be capable of 80% of it's rating continuously, so you are actually back to a 60A breaker. In that case you would need to use a contactor rated 60A minimum.


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