Sizing MCB for Panelboard

[CopperYeti]

Hi All,

I have some questions about sizing a Panelboard MCB based upon the total connected kVA load. Panelboard is a 208/120v, 3Phase.

If I have a Panelboard with a total connected load of 78.3 kVA: 78300/208√3 = 217 Amps. How would you size this panelboard?

- Connected load amps (217 A) and add a 10% or 25% spare capacity margin?
- Size based on next C/B size up from 217 -> 225A MCB?
- Add a Spare Capacity margin and then go to the next C/B size up? 217 * 1.25 (25% Spare Capacity) = 272 Amps -> 300 Amp MCB?
- Should you ever size a Panelboard MCB based off of the 80% circuit breaker load rule?

If there are any more tips about sizing MCBs for Panelboards, it is greatly appreciated. I'm a younger design engineer trying to build good design practices.

Thanks in advance!

 

[electrofelon]

My philosophy is NEC calcs are so conservative that providing for future spare capacity is rarely needed. In your example, I would probably just go to 225, but I May modify it depending on panel board rating and breaker frame size cut offs. For example in your case, if I was using Siemens gear, their true panel boards will have a 250 amp bus, and the QR frame breakers go up to 250 amp so I might just go with everything 250 if the wire run isn't very long

I assume MCB is a main breaker in the panel? Unless specifically required because of being in a detached structure, an SDS or to achieve a series rating, I wouldn't be even providing a main circuit breaker. Waste of money IMO.