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Old 02-21-2010, 09:47 AM   #1
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Box Fill Calcs


I am in the process of adding a subpanel and completely rewiring my attached garage. I didn't expect there to be quite so much detail involved, but I seem to be getting a handle on all of it. What volume outlet boxes are needed for each of the following dedicated circuits:

1) 220V window A/C unit - 10/2 AWG
2) 220V air compressor - 10/2 AWG
3) 220V heater - 10/2 AWG
4) 220V welder - 6/3 AWG

All outlets will be old work boxes and all cable is Romex. I have done my own calcs, but would like confirmation for my results.

Also, any suggestions for which side of the window the A/C outlet be installed? The A/C unit has not yet been purchased and I have no idea what I'll end up buying. Are the cords generally located on the left- or right-side?
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Old 02-26-2010, 11:04 AM   #2
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Could anyone help with #4 above? What size box do I need for the welder outlet? 6/3 has been run to future-proof the circuit.
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Old 02-26-2010, 11:25 AM   #3
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here is the volume attached to a given wire size:
Box Fill Calcs-snapwirefill.jpg

each end of a wire is counted once. If a wire passes through without excessive loop, it is counted once. All grounds together are counted as 1 of the largest EGC conductor.


for each device in the box, each yoke is counted as a double of the largest conductor connected to that device.

so, what did you come up with for each calc?

Last edited by nap; 02-26-2010 at 11:28 AM.
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Old 02-26-2010, 11:50 AM   #4
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30 cu. in. for the welder outlet. Could anyone offer insight as to whether the 2-gang 34 cu. in. PVC old work box I intend to use will work from a practical standpoint? Will the wires physically fit in the box using a NEMA 6-50P outlet?
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Old 02-26-2010, 12:15 PM   #5
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They will fit, but if you unplug the welder many times, I would consider cutting out the drywall and mounting a metal box to the stud.
It takes a lot of pressure to unplug a 6-50, so the drywall may fail over time.
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Old 02-26-2010, 12:30 PM   #6
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Quote:
Originally Posted by jbfan View Post
They will fit, but if you unplug the welder many times, I would consider cutting out the drywall and mounting a metal box to the stud.
It takes a lot of pressure to unplug a 6-50, so the drywall may fail over time.

ditto.

or a surface mounted box with some decent screws holding it in place.
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Old 02-26-2010, 02:03 PM   #7
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Thanks for the replies. I placed it next to a stud, as I did with the other 29 boxes I've added. A couple of screws will be driven through the boxes into the studs to hold them in place better. From what I can tell, doing so is acceptable as long as there are no exposed screw threads inside the box. The box used for the welder is one of those extra-thick heavy-duty ones.
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