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Branch Circuit Component Sizing
by Eduexpert Sat May 13, 2017 3:57 pm
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?
source
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?
source
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