Box Fill Calculator
Estimate electrical box fill from conductor sizes, equipment grounds, cable clamps, device yokes, support fittings, and marked box volume.
Conductors and fittings
Use the volume marked on the box or listed in the manufacturer's data.
If sizes differ, select the largest and enter the total quantity.
One allowance is based on the largest conductor entered.
Count each distinct type once, not each individual fitting.
Device yokes
Group yokes by the largest conductor connected to each yoke.
Entered volume allowances fit within the marked box volume.
About this calculator
This calculator estimates required box volume from the conductor sizes and counts you enter. It also accounts for equipment grounding conductors, internal cable clamps, device yokes, and luminaire support fittings.
Use the marked volume for the exact box. The result is a screening estimate, not a complete code approval.
Method and assumptions
Each insulated conductor receives an allowance based on its size. Equipment grounds are grouped under one allowance through four conductors, with a quarter allowance for each additional ground. Internal clamps receive one allowance based on the largest entered conductor. Each device yoke receives two allowances based on the largest conductor connected to it. Each distinct luminaire stud or hickey type receives one allowance based on the largest entered conductor.
Allowance volumes follow the NEC 314.16 box-fill table. This tool uses the 2026 NEC as its baseline; adoption and amendments vary by jurisdiction. Confirm the applicable provisions locally.
FAQs
Where do I find the box volume?
Use the volume marked on the box or the manufacturer’s product data. This calculator does not infer box volume from the box dimensions.
How are equipment grounds counted?
Enter the largest equipment grounding conductor and the total number of grounds. The estimate applies one allowance through four grounds, then one-quarter of that allowance for each additional ground. Verify the adopted-code rules for your installation.
How are internal clamps and support fittings counted?
Internal cable clamps add one allowance based on the largest conductor entered. Enter the number of distinct luminaire stud or hickey types; the calculator adds one allowance per type. It does not count each individual fitting.
How are device yokes counted?
Each device yoke contributes two volume allowances based on the largest conductor connected to that yoke. Group yokes separately when their conductor sizes differ.
Does this confirm that the box installation complies with code?
No. This is a box-fill volume estimate only. It does not evaluate every conductor, equipment, box, or installation exception, or determine whether the box is otherwise suitable.
Sources
- 2026 NFPA 70 (NEC), Section 314.16, box-fill conductor volume allowances and equipment counts.
- Confirm the edition adopted by the authority having jurisdiction and the marked volume of the listed box.