A device box has internal cable clamps and is fed by three 14-2 cables with ground, so six circuit conductors and three 14 AWG equipment grounding conductors enter it. It holds one single-pole switch on a strap, and two 14 AWG pigtails that begin and end inside the box. The volume allowance for 14 AWG is 2.00 cubic inches, one or more clamps count as a single allowance, and a device strap counts as double the largest connected conductor. What minimum box volume is required?
- A24.00 cubic inches
This counts the two pigtails as conductors. A conductor that neither enters nor leaves the box has no volume counted against it, because it displaces space the entering conductors were already credited for.
- 20.00 cubic inches
- C18.00 cubic inches
This leaves out the allowance for the internal cable clamps. Clamps inside the box take one allowance based on the largest conductor present, however many clamps there are.
- D22.50 cubic inches
This uses the 2.25 cubic inch allowance that belongs to 12 AWG throughout. Every conductor here is 14 AWG, which carries the 2.00 cubic inch allowance.
Why B is correct
Six conductors entering the box take 6 x 2.00 = 12.00 cubic inches. The three grounding conductors count once, adding 2.00. The clamps add one allowance of 2.00. The switch strap adds 2 x 2.00 = 4.00. Conductors that begin and end inside the box are not counted at all, so the pigtails add nothing. The total is 20.00 cubic inches.
What this question is testing
Tests whether you can assemble a box fill total from several different kinds of allowance at once and whether you correctly exclude conductors that both originate and terminate inside the enclosure while still counting clamps, the device strap and the grounding conductors.
On the job
Pigtails are everywhere in modern work, since a device is not permitted to be the splice point for a circuit that continues on, and grounding pigtails to metal boxes are routine. Crews who count them condemn perfectly good boxes and cut in larger ones for no reason. The rule is easy to remember once you see the logic: volume allowances follow conductors that come from somewhere else, not ones born and buried in the same box.
Memory technique
Born in the box, buried in the box, counted nowhere.
Exam tip
If a conductor never leaves the box, it takes no volume allowance. Everything else does.
Where to look it up
The box fill provisions of Article 314 list the allowances one at a time: conductors, clamps, support fittings, devices and grounding conductors.