Suppose one is doing junction box fill calculations for a typical NM-B installation such as the one shown below for a switch box with one normal and one three-way switch. Which of the wires count towards conductor fill?
Section 314.16(B) of the 2011 NEC states (in part):
314.16(B)(1) Conductor Fill
Each conductor that originates outside the box and terminates or is spliced within the box shall be counted once, and each conductor that passes through the box without splice or termination shall be counted once. Each loop or coil of unbroken conductor not less than twice the minimum length required for conductors in 300.14 shall be counted twice. The conductor fill shall be calculated using Table 314.16(B). A conductor, no part of which leaves the box, shall not be counted.
and
314.16(B)(5) Equipment Grounding Conductor Fill
Where one or more equipment grounding conductors or equipment bonding jumpers enters a box, a single volume allowance in accordance with Table 314.16(B) shall be made based on the largest equipment grounding conductor or equipment bonding jumper present in the box. Where an additional set of equipment grounding conductors, as permitted by 250.146(D), is present in the box, an additional volume allowance shall be made based on the largest equipment grounding conductor in the additional set.
So how many conductors, and how many equipment grounding conductors are present in the box shown here?
Best Answer
Please see this answer, for more detail. Basically if you have equipment grounding conductors enter the box, you add 1 fill unit.
Example Breakdown
In the example you've provided you have 12 if it's a box without internal clamps, and 13 if it has internal clamps.
Current Carrying Conductors Ungrounded (hot) 4 Grounded (neutral) 3 ---------------------------------- 7 Equipment Grounding Conductor 1 Clamps (maybe) 1 Devices 2 x2 ---------------------------------- 4 ---------------------------------- Total 13