Posted By: Nebraska
Subpanel Calculation - 01/04/04 09:56 PM
I recently added a sub panel to my house It contains 3 15 amp breakers, 2 15 amp tandem breakers, 1 20 amp breaker and 1 2 pole 30 amp breaker for a water heater, all the 15's and the 20 are for general purpose receps. How do you calculate the size of breaker to put this sub panel on. Taking 80% of all the breakers added together just doesn't seem right, maybe it is though. Please help me. Thank you.
Posted By: earlydean
Re: Subpanel Calculation - 01/04/04 10:16 PM
You got the cart before the horse. First, you compute the combined loads, using diversity factors allowed by the NEC. Second, you choose a panel of a size adequate for the loads just computed. Third, you choose a feeder of ample size. Fourth, you choose a circuit breaker sized to protect the feeder and panel.
Or, take the shortcut: size the breaker for the size of the new panel (usually 100 or 60 Amps)and feed with 100 or 60 Amp wires.