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?

example installation

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.


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?

  • You've got eight conductors.
    – Edwin
    Dec 20, 2013 at 23:41
  • 1
    I get 7 + the single grounding allowance, or 8. Don't be foolish though. Don't stuff it so full of pigtails and long leads such that the wires are really packed in there with little free air and pushing everything in could displace or damage something.
    – bcworkz
    Dec 21, 2013 at 3:02
  • @Edwin oh man, you're giving me flashbacks to the Star Trek: The Next Generation episode when Picard had that Code compliance disagreement with the Cardassian... Oct 30, 2019 at 19:29

3 Answers 3


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

Equipment Grounding Conductor       1

Clamps  (maybe)                     1

Devices                         2
Total                               13  

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It requires yet another section of the code, but I think you are actually low.

The EGC is clear - 1, at the biggest size, and done.

Current-carrying wires (hots and neutrals and travelers) count once per wire.

But I think there is another paragraph (or sentence) which indicates that a "device" in the box ALSO counts (2 each.)

From Tester101's resonse to another question (but I think he's just quoting the code here)

Clamp Fill

  • If there are any internal cable clamps, add 1 (not 1 per, just 1).

Support Fittings Fill

  • If there are any luminaire studs or hickeys, add 1 (not 1 per, just 1).

Device or Equipment Fill

  • For each yoke or strap containing one or more devices, add 2.

So you have 8 count of wires, but 12 count (at least) of "fill units" - 13 if you have clamps.


Equipment Grounding Conductors aren't Conductors

In the maddening language of the NEC, this is true. "Conductors" (generally) refers to non-grounds: meaning

  • Ungrounded Conductors (hots which are not neutral but normally carry current)
  • Neutral, whose NEC lawyer-name is "Grounded Conductor" and carries current

These two are called "conductors" because they carry current in normal operation.

The lawyer-name for neutral has driven me absolutely bat-crazy, because everyone goes "Oh! You mean ground!" No no, the "Grounded Conductor" is a conductor. Ground isn't, that's why they call the Equipment Grounding, uhhhh, Conductor. You can't fight in here. This is the War Room!

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