How to size commercial kitchen breaker?

Question

This is related to an earlier question that I posted. I'm just curious about this. Here's what I don't understand. I saw this the other day. This commercial kitchen (3-phase) sub panel has two 100 amp (2 large multiple cooking units with it's own sub panel also) 3 pole breakers and about 15 other 20 amp single pole breakers in it, but it's being protected by just a 150amp 3 pole breaker outside in the main panel. How's that.? If only half of the current of the sub panel capacity is used, wouldn't the main breaker will always trip? But they say it never does. Can someone explain this? Thank you

Answer 1

The load of each breaker can not be more than 80%, so each 20amp breaker cant have more than 16amps, while most are never loaded to that 80%. Same with the sub panels, they are rated at 100amps, and protected for 100amps, but there load would not reach 80% of 100amps. They only make certain sizes of panels, so you use the next size bigger. (100, 150, 200, 400)For instance, they dont make 70 amp panels.(to get that when needed, you take a 100 amp panel with 70 amp fuses)

So you have breakers feeding breakers, and everything is oversized to 120%, plus taking into account that each 20 amp breaker can have 16 amps, but the load may be 2 amps.

Each of the 3 phase wires, will carry the load its protected by. For instance, each wire( 3 phases) feeding the 150 amp breaker, can carry 150 amps. So you could almost say there is 450amps there( 3x150 ). Same with all breakers and phases.

Answer 2

The circuits in the subpanel are never maxed out all at the same time, so the total load on the whole system is never enough to trip the outside breaker.