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Box Fill Question....OK Quiz, not question


Jeff Remas
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*This is a box fill calculation question only and nothing else*

While inspecting the unfinished basement for a client you come across a 4" square x 1-1/2" metal box that is fed with one 14/4 NM cable and one 12/2 NM cable. The box has one duplex receptacle and one single pole switch. As you are noting the missing cover for your report you look inside and easily see that the equipment grounds are attached to the box with a ground screw and properly attached to the devices. The NM cables are secured with a single cable clamp.

The client states "There sure are a lot of wires in there." and this gets you wondering if the box fill is compliant with the NEC.

Is it?

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Alright, 54+ views and only one taker. Probably because it is beyond the scope of a HI and no one feels the need to reply other than one person. Jim K I am sure is kicked back waiting to see what happens next since I am also sure he knows the answer.

The answer from Carson2006 is correct.

Anyone learn anything yet?

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My answer is different. Here's how I'd figure it:

4x4x1.5 boxes have 21 cubic inches of space inside.

The required space for each component is as follows:

Four #14 wires 8

Two #12 wires 4.5

Device #1 sized for #12 wires 4.5

Device #2 sized for #14 wires 4

Largest ground 2.25

Total 23.25

That doesn't include the clamp. If it's an interior clamp, I'd add another 2.25 cubic inches. With NM, they probably used a romex clamp on the outside of the box, so it wouldn't add anything to the calc.

Without a mud ring, the box is too small. With a two-gang mud ring (a little under 8 cubic inches) it's fine.

Say . . . how did they get the switch & the receptacle mounted on a 4x4x1.5 box anyway? I can't recall ever seeing a 4x4 box with taps for yokes. Wouldn't you have to have a mud ring in order to attach the devices?

- Jim Katen, Oregon

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Originally posted by Jeff Remas

Jim, 1st of all I screwed up and posted 14/4 by accident.

Don't you just hate it when that happens?

It should read 14/3. Sorry about that. 2nd item, let us just assume the metal mud ring is missing and that is one of the reasons there is no cover. The person could not figure out how to secure the devices to the box. Seen that too.

I think that it doesn't matter. Either way, with a two-gang mud ring, there's enough room in the box.

Maybe the installer welded the yokes to the box. We've seen stranger things.

- Jim Katen, Oregon

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