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.
Best Answer
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:
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.