NEC box fill calculator (314.16(B))

Enter the conductors, device yokes, grounding conductor and fittings of the box and we show the volume required by NEC 314.16(B) in in³, with the working line by line. If you enter the box volume, we also show whether they fit. An informative calculation; NEC mode is in beta.

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What does the box fill calculator compute?

According to NEC 314.16(B) it gives the volume (in³) a junction box needs for the conductors and devices, and, if you enter the box volume, whether they fit in it:

• Conductors: each takes the volume for its size (18 AWG 1.50 in³, 16 AWG 1.75, 14 AWG 2.00, 12 AWG 2.25, 10 AWG 2.50, 8 AWG 3.00, 6 AWG 5.00 in³).
• Equipment grounding conductors: together they count as one conductor, by the largest size.
• Device yokes (switch or receptacle): each counts as two conductors.
• Internal cable clamps: together they count as one conductor.
• Fittings: together they count as one conductor, of the largest conductor size.

The result is shown line by line, so that it can be checked.

Worked example: switch box with two 14/2 cables and one switch

This fits in an 18 in³ box and does not fit in a 10 in³ box.

What it does not compute

The calculator gives the conductor volumes of 314.16(B); you enter the box volume yourself (from the 314.16(A) table or the manufacturer's marking). It does not check other items in the box or special cases. It is an informative calculation, not a permit document: compliance is decided by the authority having jurisdiction (AHJ) and the NEC edition in force, and the values must be checked against the official text. The program's NEC mode is in beta.

Related calculator

To size the conduit for the conductors use the conduit fill calculator; for a whole plan see the US electrical plan page.

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