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Old 03-11-2019, 01:13 PM   #1
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Help Determining Proper Junction Box Size


I am doing some updating in our kitchen and wanted to make sure my box is large enough to contain all the splices, etc. and am having a hard time calculating it.

I plan to use a 3-gang old work box with these specs: 5.72-Inch Length by 2.79-Inch Width by 3.69-Inch Depth. I believe it's 55 cubic inches.

I plan to take the incoming 12/2 line and pigtail off of it to
1 GFCI outlet (end run)
2 separate switches
1 hardwired undercabinet light
(obviously the 12/2 running from the switches to the lights)

The lights were previously wired with a switch loop. I didn't want to go that route, primarily because I don't have any 12/3 and wanted to stay up to code having a neutral available in all switch locations.

Any help would be appreciated and sorry if this is confusing.
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Old 03-11-2019, 01:46 PM   #2
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Re: Help Determining Proper Junction Box Size


Google NEC 370-16 (a)
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Old 03-11-2019, 02:05 PM   #3
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Re: Help Determining Proper Junction Box Size


I would never mix power and lighting on the same circuit. You are only asking to be in the dark if you happen to kick a breaker. I would also use one of the device boxes as my junction box. I would never have a junction box any where in the circuit other than a device box. I would gang 2 or maybe three boxes together to ensure I have plenty of room.



But I like making thing easy for myself when it comes to electrical hardware and circuits.


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Old 03-11-2019, 03:30 PM   #4
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Re: Help Determining Proper Junction Box Size


I wouldn't waste time calculating. I'd use a plastic box and forget about it.
It'll easily be big enough for your needs.

Note: I'm assuming that plastic boxes are larger than metal where you are,
same as they are here where I am.

Edit: another note - plastic boxes often have "wings/tabs" on the end of the
box (away from the stud) that make their mounting simpler since you don't
have to make/add anything to the end of the box to prevent it from pulling
forward when the devices are screwed in.

Last edited by CodeMatters; 03-11-2019 at 03:39 PM.
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Old 03-11-2019, 03:41 PM   #5
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Re: Help Determining Proper Junction Box Size


Quote:
Originally Posted by HandyAndyInNC View Post
I would never mix power and lighting on the same circuit.

Mixed circuits in residential is something about 1 in a hundred people care about........I'm not one of 'em.
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Old 03-11-2019, 04:11 PM   #6
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Re: Help Determining Proper Junction Box Size


If the GFCI is part of a small appliance branch circuit, it cannot be part of a lighting circuit.
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Old 03-11-2019, 05:48 PM   #7
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The plastic.box will be stamped with the number of conductors and capacity.The

As above the small appliance circuits should not have lighting on them.
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Answers based on the National Electrical Code. Local amendments may apply. Check with your local building officials.
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