Main Breaker Sizing

charlie b

Moderator
Staff member
I am unfamiliar with the design processes for solar installations. But I can say that we never size a breaker on the basis of the sum of downstream breakers. Rather, we size conductors to support their load, and then size breakers to protect those conductors. In your case, the breaker in question will be protecting the bus. So you need to know the total load on the bus.
 

topgone

Senior Member
Hey Guys,

I am sizing a panel board, where 2 inverters with 150A (120A x 1.25) breakers supply the bus, and the same panel board have the provision for monitoring as well with 2 15 A breakers. So I sized the bus as the sum of all the breakers (150+150+15+15 = Round off to 400A) and my question is - can the main breaker in this panel board be 300A or should it be 400A? because only the inverters supply the bus and the monitoring devices are loads. (Please check attached)

Assuming your feeder breakers were properly size (80% of respective load amps), your main breaker will be 294A ([150] + ([150 +15 + 15] X 0.8) = 294A) ~ 300A
 

electrofelon

Senior Member
You must qualify the panelboard bus by 705.12(D)(2). You are looking at using (C)(3) under that section so you would need 330 amps of busbar, or a 400 amp panelboard. This is what you also came up with, I concur (just thought I would include the code references). Logic would seem to point to the 300 amp breaker being adequate, and I cannot come up with a code section that says otherwise.
 

ggunn

PE (Electrical), NABCEP certified
Hey Guys,

I am sizing a panel board, where 2 inverters with 150A (120A x 1.25) breakers supply the bus, and the same panel board have the provision for monitoring as well with 2 15 A breakers. So I sized the bus as the sum of all the breakers (150+150+15+15 = Round off to 400A) and my question is - can the main breaker in this panel board be 300A or should it be 400A? because only the inverters supply the bus and the monitoring devices are loads. (Please check attached)

You don't need a main breaker at all if there is OCP between the panel and the point of interconnection. All you need to do is size the panel such that the ratings of all breakers on every bus sum to less than the rating of the bus. 705.12 (B) or (D) (2)(3)(c).
 
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