Electrician · Box Fill
Box Fill Calculator Practice Problems
Free, unlimited NEC 314.16 box fill practice for the Journeyman & Master Electrician exam. Every click builds a new box — conductors, device yokes, clamps, and grounding conductors — find the minimum required volume.
How to calculate it
Box fill, worked by hand
Add up one volume allowance per item — conductors, then yokes, clamps, and grounds — and compare the total to a standard box's listed volume.
Problem: A device box contains 4 current-carrying 12 AWG conductors, 1 device yoke (a receptacle), 1 internal cable clamp, and the circuit's grounding conductors. What is the minimum required box volume?
1. Volume allowance for 12 AWG = (NEC Table 314.16(B))
2. Conductors:
3. Device yoke (counts double):
4. Internal clamp (one allowance total):
5. Grounding conductors (one allowance total):
6. Total:
Minimum box volume = — a 4" × 1-1/4" square box (listed at exactly 18.0 in³, NEC Table 314.16(A)) is the smallest standard box that satisfies this.
FAQ
Box fill — frequently asked questions
What counts as a "device yoke," and why does it count double?
A device yoke is the metal strap on a switch or receptacle. Per NEC 314.16(B)(4), each strap counts as two volume allowances (not one), based on the size of the largest conductor connected to that device — devices take up more physical space in the box than a single conductor, which is why the code double-counts them.
Do all clamps inside the box count toward fill?
Internal clamps count as one volume allowance total for the whole box (not per clamp), based on the largest conductor present, per 314.16(B)(2). Clamps that are external to the box — like a connector on the outside — don't count toward box fill at all.
How are grounding conductors counted if there are several of them?
All equipment grounding conductors together count as one volume allowance total, based on the largest grounding conductor in the box, per 314.16(B)(5). If there's also a separate isolated (insulated) grounding conductor, that adds one more allowance on top.
Does a jumper or pigtail that never leaves the box count toward fill?
No. Per 314.16(B)(1), a conductor with no part leaving the box — like a short jumper spliced entirely inside — is not counted at all. Only conductors that originate outside the box or pass through it count.
If a box has mixed wire gauges, which allowance size do I use?
Each conductor is counted at its own gauge's allowance, but the box-wide allowances — clamps, device yokes, and grounding conductors — are all based on the largest conductor gauge present anywhere in the box, even if most of the conductors are smaller.