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Subpanel Calculation

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.
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