calculating box fill

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Old 02-08-14, 11:16 PM
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calculating box fill

I am installing hardwired smoke detectors in my house and the first ceiling box in the run has a lot of conductors, so I wanted to make sure I'm not overfilling the box. I can stuff everything in, but it seems tight.

I am using a plastic 4 1/4" round Carlon old work box that is listed at 18 cubic inches.

I have one 14-2 cable entering the box from the panel, and three 14-3 exiting the box to the other detectors. There is a harness for the smoke detector that has 4 wires (hot, neutral, ground, plus the interconnect wire) - I'm not sure if you need to take this into account.

There are no cable clamps in the box.
 
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Old 02-08-14, 11:40 PM
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#14=2.00 Cu. In. You have a total of 12 each #14 @ 2.00 Cu. In. = 24 Cubic Inches. Can you run your smoke detector circuit differently to reduce the count? Install a larger box?

Mod Note: the box is overfilled.
 

Last edited by pcboss; 02-09-14 at 06:54 AM.
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Old 02-09-14, 06:28 AM
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1.
... it seems tight ...
This is enough to tell the whole story.

2. I'm pretty sure that the wires from the harness count also, although there may be a de-rating such as 1-1/2 ci instead of 2 ci for each of these conductors seeing that those are smaller than 14 gauge. For chandeliers you can get away with not counting the wires from the fixture because there is a dome or cone shaped canopy below the ceiling surface that gives more space.

Can you get an "extra deep" such as 3-1/2 inch deep box? Also they make extenders or "rings" that fit over the mouth of the box although you will now need longer screws to hold the smoke detector cover plate and all of the rings in place.

Mod Note: the wiring harness does not count towards box fill. Also the dome or canopy can only be used for space if the capacity of the dome is marked on the canopy.
 

Last edited by pcboss; 02-09-14 at 06:53 AM.
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Old 02-09-14, 06:36 AM
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Box Fill

Device counts 2, so total would be 28 cu. in.

Mod Note: neither the wiring harness nor the smoke alarm count towards box fill. The total capacity needed is 24 cubic inches.
 
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Old 02-09-14, 06:28 PM
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It sounds like you already have your cable ran, but could you not jump from box to box instead of having them all meet at one junction box? Then you would only have 5 wires in your first box, 6 in the next two, and 3 in the last. This also leaves you room in all boxes should you ever need to add more.
 
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Old 02-19-14, 03:34 PM
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Thanks for the help! I was able to move one of the junctions, so I don't have this overloaded box acting as a hub anymore.

To make sure I understand -- each 14 gauge conductor counts as 2 cubic inches, plus all the grounds together count as another 2 cubic inches?

And to check my work -- having removed one 14-3 cable my new total is:
one 14-2 = 2 conductors
two 14-3 = 6 conductors
all grounds = 1 conductor
TOTAL = 9 conductors = 18 cubic inches
 
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Old 02-19-14, 04:13 PM
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Nicely done.

And yay to me for not getting a mod note.
 
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