calculating box fill

[bcboy]

In junction box fill calculations, what is the "clamps" requirement that is counted as one conductor? I've read that single gang plastic boxes are exempt from this. Is this always the case?

Also, what is the correct way to count the fill for a switch or receptacle? I've read "they count as two conductors", and "count each stud", though I suspect both are wrong. Would you count studs that aren't connected? For instance, I have a two switch unit that can be connected with three for four wires depending on whether you knock out the tab connecting the "common" studs. How should it be counted?

[DT]

Metal boxes often had separate clamps inside (that could be removed) that take up space. Plastic boxes don't have an internal clamp (the plastic push-through holders don't need to be counted since they are already allowed for in the manufacturer's volume measurement). Regular romex clamps in metal boxes don't count either, since the clamping mechanism is outside the box.

Each 'yoke' or strap counts as a double volume (same as two conductors), multiplied by the correct factor for the wire size connected to the device (switch or receptacle) on each particular yoke, i.e, #14 = 2, #12 = 2.25, etc.

Dennis

[zxcvbob]

For switches and recepts, you count the straps. If it will mount in a single-gang box, it's one device. I don't remember if it counts as one wire or 2 for the box fill. If you're putting in something big like a GFCI receptacle, leave a little extra room over the minimum or you'll be cussing if/when you ever have to replace it.

Bob

[gfretwell]

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.

... so you use two conductor deductions of the LARGEST conductor in the box for each device

[Wayne Whitney]

As I read it, you use the deduction of the largest conductor attached to the device(s) on the yoke, which might not be as large as the largest conductor in the box. No?

Cheers, Wayne

[gfretwell]

True, if the wire is "just passing through" with no terminations it counts one of the appropriate size.

[Thomas D. Horne, Electrician]

Yes Wayne you're correct.