MCB Size

[jim dungar]

Worrying about convenience /inconvenience of something like a 200A breaker tripping is not something I even think about whatsoever.

Really.

If the fault is big enough to trip a panel "main", quickly resetting it is usually not my first concern. OSHA and most safe work practices require troubleshooting the reason for the trip rather than immediate reclosing.

 

[kwired]

I once had a situation where my helper really tightened down the screws on a romex clamp entering the service entrance panel for a 10-3 cable on a 30 amp breaker. He learned not to super tighten those clamps again I guess.

First time turning the circuit on it not only tripped the 30 amp breaker but also the 200 amp main. Seems unlikely but this was underground service with the source being maybe about 100 feet away and IIRC was a 75 kVA transformer. Could been a 50 but I think was a 75. If the fault were near the load end of the 30 amp circuit instead of at the supply end I would guess circuit impedance would have limited fault current enough that only the branch breaker blows.