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Old 12-03-2009, 01:58 PM   #1
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??? about calculating loads for a sub panel


Hi guys (& gals), First post to the forum. I have what may be a really elementary question for you. I have run a couple of searches but have not found this answered yet.

Anyway, here goes. I am planning to install a sub panel to an outbuilding for a workshop. It will have both 220v & 110v circuits on it. When I calculate the max potential load I might draw on the (1) 220v circuit (say 24 amps) and the (4)110v circuits (say 54 amps total), do I halve the 110v total & add to the 220v total to get the max. ie 54/2 = 27 + 24 = 51 total amps @ 220v for the sub panel, so I could use a 60 amp breaker in the main box to supply the sub?

I've done basic wiring (lights, recepticles, switches), but only on 110v circuits and nothing that would have a heavy load. I've never had to account for 220 & 110 before when calculating loads.

Thanks for the help.

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Old 12-03-2009, 02:33 PM   #2
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??? about calculating loads for a sub panel


How did you arrive at the figures you have listed?

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Old 12-03-2009, 02:48 PM   #3
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??? about calculating loads for a sub panel


I imagined what could be a possible (if rare) situation like the following:

A cold night in the shop:
Lights, radio, heater, air compressor, shop vac, air cleaner, drawing a total of ~54 amps on the various 110v circuits along with a dust collector and table saw running on the 220v curcuit drawing a total of 24 amps. The numbers made up, I don't have the exact numbers in front of me. It was just to see if my method of calculation was correct for combining the total load for the combined 220 / 110 circuits.
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Old 12-03-2009, 06:23 PM   #4
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??? about calculating loads for a sub panel


Sub panel will be a 240v sub, so all loads are calculated at 240v
54a @120v = 27a @ 240v
Yes - sounds like a 60a sub would work if your loads are as described
For a detached structure a 60a sub is the min to run
There is a code section that requires a 60a disconnect as a min
You will also need 2 grounds rods

Some discussion going on another board as someone ran a 50a breaker in the Main to a 100a sub in the shed
Inspector wants him to feed it with a 60a or remove the 100a sub & reduce it to a 30a & 2 circuits
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Old 12-03-2009, 07:58 PM   #5
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??? about calculating loads for a sub panel


Thanks Scuba_Dave.
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