Box fill.
Count the conductors. Each yoke is two of the largest in the box. All grounds together count as one. Internal clamps add one of the largest. Then compare against Table 314.16(A) volume.
Uses common box volumes from NEC 314.16(A) and conductor volume allowances from Table 314.16(B). The calculator covers AWG 14 through AWG 6.
Box fill is a counting problem before it is a math problem. Count every conductor that uses space, add yokes, grounds, and internal clamps the right way, then compare the required cubic inches to the box. If you searched for NEC box fill calculator, NEC box fill chart, or NEC box fill table, this is the 314.16 count that those searches are pointing at.
Four 12 AWG conductors, one yoke, grounds, and one internal clamp
Inputs
- 4 in. square x 1-1/2 in. box
- Four 12 AWG conductors
- One device yoke
- Equipment grounds in the box
- One internal clamp
Result
Four 12 AWG conductors use 9.00 cu in. The yoke adds 4.50 cu in. Grounds add 2.25 cu in. The clamp adds 2.25 cu in. Required volume is 18.00 cu in. A 21.00 cu in box has room.
Here is the trap: the grounds are not four more counts. All the equipment grounds together count once. On the other side, the yoke is not one count. It is two. Miss either rule and the answer moves.
The box-fill mistakes that show up on exam questions
- Counting every ground. Grounds count once as a group. Five grounds do not mean five separate volume allowances.
- Forgetting the device yoke. A switch or receptacle yoke counts as two allowances. That is enough to make a small device box fail.
- Missing the internal clamp. If the clamp is inside the box, it takes one allowance. If the question says external clamp, read carefully before adding it.
- Using one size for every wire. Mixed-size boxes need mixed-size allowances. Count each conductor by its own AWG, then size the special allowances correctly.
After you run the calculator, drill the count
Box fill calculator questions
How do grounds count for box fill?
All equipment grounding conductors in the box count as one allowance, sized by the largest equipment grounding conductor present. Do not count every ground wire separately or the required volume will come out too high.
How does a device yoke count in box fill?
Each device yoke counts as two conductor allowances, sized from the largest conductor connected to that yoke. On exam questions, that yoke count is one of the easiest details to skip.
Do internal clamps count for box fill?
Yes, internal cable clamps take one conductor allowance based on the largest conductor in the box. External clamps do not use box volume the same way, so read the question before adding the clamp allowance.
Which NEC section covers box fill?
Box fill is handled in NEC 314.16. The calculator uses common box volumes from 314.16(A) and conductor volume allowances from Table 314.16(B).
Is this based on the NEC box fill table or chart?
Yes. Electricians often search for NEC box fill chart, NEC box fill table, and NEC box fill calculator. The table work is the same: find the conductor volume allowance, total the counts, and compare required cubic inches to the box volume.
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