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I know that normally a receptacle or device in the gang box counts as 2 conductor volumes. Does a line voltage thermostat just count the same as any other device? I'm replacing an old mechanical 240V thermostat with a programmable digit one (Honeywell TL8230A1003) and I've found that this new thermostat takes up significantly more space in the gang box than the old one and it is a very tight fit. Does the box fill calculation account for a device like this differently?

The thermostat is on a 20A breaker and 12/2 wire is coming into the box. There is also 12/2 wire going to heater and 12/2 wire going to another thermostat/heater for a total of 6 conductors. The 3 bare grounds count as another conductor and then the thermostat as 2 for a total of 9 conductors. This would indicate that I need a 9 * 2.25 = 20.25 cu. in. gang box unless the thermostat somehow counts as more.

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Does the box fill calculation account for a device like this differently?

No. The Code does not distinguish between particular devices.

If the device is yoke mounted it requires a double volume allowance of the largest conductor attached to it in the box.

Here is the pertinent 2014/2017 National Electrical Code Article:

314.16(B)(4) Device or Equipment Fill. For each yoke or strap containing one or more devices or equipment, a double volume allowance in accordance with Table 314.16(B) shall be made for each yoke or strap based on the largest conductor connected to a device(s) or equipment supported by that yoke or strap. A device or utilization equipment wider than a single 50 mm (2 in.) device box as described in Table 314.16(A) shall have double volume allowances provided for each gang required for mounting.

If the device was surface mounted (like the old bi-metallic style probably was) on the outside of the box, or is not technically "yoke mounted" then this requirement does not apply.

Your conductor count looks good. According to Table 314.16(A) the deepest 3" x 2" device box is only good for 18.0 cubic inches or 8-#12 wire count.

So, to be Code compliant, you would need to remove the pass-thru conductors and make those joints elsewhere, or change the box to a 4" x 1 1\2" square with a single gang plaster ring, or they make some single gang boxes with an expansion on the side that may have enough volume.

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  • Thanks. I can't move the wire to another location as it only goes to two places and that would just move the problem to the other gang box. The previous box was a super-packed 14 cu in and I replaced it with the biggest 1-gang "old work" box I could find at the local store, but it was only 20 cu in which is still just short of the requirement. Looks like I could use a MSB22+ (1 Gang Device Box Plus) Madison SmartBox, meproducts.net/smart.
    – Scott B
    Jan 2, 2018 at 17:10
  • @ScottB Try using a "surface conduit starter box" like Wiremold BW5. It's designed to mount over a flush-mount box, pushing it "out" an inch or two so you can attach surface conduit to its sides. However if you don't attach any surface conduit, it just adds cubic inches.
    – Harper - Reinstate Monica
    Jan 2, 2018 at 18:52
  • @Harper Thanks for the suggestion, I'll take a look at that option as well.
    – Scott B
    Jan 2, 2018 at 22:23
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Don't forget your cable clamp(s) -- NEC 314.16(b)(2). Similar to how all grounds count as one wire, all cable clamps count as one wire.

So I get 22.5 cubic inches, and that's a piece of cake with a common 4" square box that costs a dollar. It gives you a statutory 21 cubic inches. (314.16a). I mean the regular-depth 1-1/2" type, not the deep 2-1/8" type.

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You'll then need a cover plate anyway to hold the receptacle, so just get one that gives you a few more cubes and that's a buck or two at any sanely priced shop. And you're done.

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  • This is probably the best way to handle it, but I was hoping to avoid drywall work. I'd need to cut out additional dry-wall to install it and then patch other the mud/plaster plate. The current installation is PVC, not metal and it is the older style with the "punch out" tabs so I don't think it needs to include the cable clamp count (I assume then that it is stapled within 8 in of the box though I can't see that). For the newer PVC boxes with the cable tabs/tongues that act as a "clamp", does the stated cu in on the box already remove that volume or do I still have to account for it?
    – Scott B
    Jan 2, 2018 at 22:22

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