Box fill calculations
A box fill problem is a count. Under 314.16(B) each item in the box is worth a number of volume allowances, each allowance is the volume the Code assigns to a conductor size, and the box must have at least the total.
What counts, and for how much
- Each conductor that enters the box and is spliced or terminated there counts as one. A conductor that passes through without a splice also counts as one.
- A conductor that starts and ends inside the box, such as a pigtail, counts as zero.
- All internal cable clamps together count as one, based on the largest conductor in the box.
- Each device yoke, a switch or a receptacle, counts as two, based on the largest conductor connected to it.
- Up to four equipment grounding conductors together count as one, based on the largest of them.
Find the volume for each conductor size in the table in 314.16(B), multiply, and add. The classic mistake is counting every grounding conductor separately.
Worked problems
Problem 1. Per 314.16(B), what minimum box volume in cubic inches is required for a box containing 11 current-carrying 12 AWG conductors passing through, 3 equipment grounding conductors, internal cable clamps, 1 device mounted on a yoke (receptacle or switch)?
Answer: 33.75 cu in
Look up the volume 314.16(B) assigns to 12 AWG in your Code book, then add each item: conductors: 11 x 2.25 = 24.75; clamps, all counted as one: 1 x 2.25 = 2.25; devices: 1 x 2 x 2.25 = 4.50; grounding conductors, ALL as one: 1 x 2.25 = 2.25. Total = 33.75 cu in. Up to four equipment grounding conductors together count as ONE volume allowance, and each device yoke counts as TWO.
Reference: NEC 314.16(B)
Problem 2. Per 314.16(B), what minimum box volume in cubic inches is required for a box containing 10 current-carrying 12 AWG conductors passing through, 2 equipment grounding conductors, 1 device mounted on a yoke (receptacle or switch)?
Answer: 29.25 cu in
Look up the volume 314.16(B) assigns to 12 AWG in your Code book, then add each item: conductors: 10 x 2.25 = 22.50; devices: 1 x 2 x 2.25 = 4.50; grounding conductors, ALL as one: 1 x 2.25 = 2.25. Total = 29.25 cu in. Up to four equipment grounding conductors together count as ONE volume allowance, and each device yoke counts as TWO.
Reference: NEC 314.16(B)
Problem 3. Per 314.16(B), what minimum box volume in cubic inches is required for a box containing 12 current-carrying 12 AWG conductors passing through, 2 equipment grounding conductors?
Answer: 29.25 cu in
Look up the volume 314.16(B) assigns to 12 AWG in your Code book, then add each item: conductors: 12 x 2.25 = 27.00; grounding conductors, ALL as one: 1 x 2.25 = 2.25. Total = 29.25 cu in. Up to four equipment grounding conductors together count as ONE volume allowance, and each device yoke counts as TWO.
Reference: NEC 314.16(B)
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Common questions
How many volume allowances does a receptacle count for in box fill?
Two, based on the largest conductor connected to that device.
Do equipment grounding conductors count in box fill?
Yes, but together. Up to four equipment grounding conductors count as a single volume allowance based on the largest one.