outlet box volume issues


Old 03-10-05, 08:06 AM
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outlet box volume issues

I'm installing 2 dimmer switches to control lights in a large room. These are also 3 way dimmers.

So, in one of the boxes I'm going to have the following:

2 dimmers
2 14-3 wires
2 14-2 wires

Now, I have calculated the necessary volume and realize that I can use one of the 25 or 32 cu. in 2 gang boxes and be okay "technically". However, the dimmers are quite large themselves so the box fill is going to be even greater than calculated.

What I'm wondering is whether I could use a 3 gang outlet box and then cover it with a 2 gang plaster trim piece. I've seen 2 gang boxes used with a single plaster cover, but not 3 gang box and 2 gang plaster cover. Does that make sense?

If it is possible, then I would have plenty of room for all the wiring, not to mention some more air space for keeping the dimmers cooler.


Jeff Nichols
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Old 03-10-05, 08:27 AM
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You can get almost any combination of gang box covers, so in theory you could use a 'three gang' box combined with a cover that has two holes, and then leave a blank space between the dimmers for better cooling.

Take a look at a catalog of boxes; you are sure to find what you need:

I am partial to large square boxes for having room to work. I'd recommend using a 4-11/16 square box, 2.125" deep with a cover made for two devices. Loads of room to work; the box plus cover would give you in excess of 50 cubic inches. However mounting becomes more difficult; you would probably need to open the wall, mount to a stud, and then patch the drywall.

Old 03-10-05, 08:45 AM
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The minimum box volume would be 30 cu. in. You have the equivalent of 15 wires at 2 cu. in each.

Yes dimmers are physically larger than other devices but still only count as 2 wires.

Be aware that when dimmers are next to each other in the same box that they will need to be derated. The instructions will tell you how much to derate if one fin is removed. You may want to install each dimmer in a deep single gang box.
Old 03-10-05, 09:09 AM
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I considered two single gang boxes, but I don't think I'd like the look of two separate switches and covers--i would prefer that they be covered by the same switch plate.

The idea of a metal box and cover is interesting. I'm already having to repair drywall in various places due to previously crappy wiring, so that's not an issue. The only drawback with metal is the need for the cable clamps, adding to the calculation for volume, although at 50 cu. in it probably wouldn't matter.

As I mentioned previously, I would like to have the dimmers next to each other, and not put a gap between them--besides, having a gap would necessitate a strange switch cover, true? And thanks for the info on the derating--the switches are 600W and they get derated to 500W (according to the info with the switches) when placed next to each other. Not a problem since I'm only running 120-150 watts max on each switch.

What would be ideal would be a 3 gang box, but that was "closed off" on one end, such that the closed off portion would be behind drywall, but would allow space for the wires.

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