How to use this calculator
- Enter marked box volume. Use the volume marked on the box, extension ring or box assembly.
- Count insulated conductors. Enter each conductor size group that enters the box and is spliced, terminated or passes through as a counted conductor.
- Add grounds and devices. Enter equipment grounding conductors, device yokes and the largest conductor connected to the devices.
- Check fittings and margin. Add internal clamps or support fittings, then compare required volume, fill percent and remaining volume.
How it works
Box fill is a volume allowance check: Vreq = Vconductors + Vgrounds + Vyokes + Vclamps + Vsupports The result is compared with the marked box volume: fill % = 100 x Vreq / Vbox
Insulated conductors use the allowance for their AWG size. Equipment grounding conductors are screened as one grounding-conductor allowance when at least one is present. Each device yoke is screened as two allowances of the selected device conductor size. Internal clamps and support fittings are screened using the largest conductor allowance present.
Use the conduit fill calculator for raceway area, the NEC electrical box fill calculator page for adopted-code search wording, the conduit bend offset calculator for route offsets, the conduit pull tension calculator for longer or bend-heavy pulls and the voltage drop calculator for long conductor runs.
Worked example
Verified against the live calculator
A box with four #12 insulated conductors, one equipment ground
and one device yoke needs 4 x 2.25 = 9.00 in^3 for insulated
conductors, 2.25 in^3 for grounds and
2 x 2.25 = 4.50 in^3 for the yoke. The total is
15.75 in^3. An 18.0 in^3 marked box has
2.25 in^3 remaining in this screen.
Reference data
Volume allowances used by this calculator for #18 through #6 AWG conductors.
| Conductor size | Volume allowance (in^3) |
|---|---|
| #18 AWG | 1.50 |
| #16 AWG | 1.75 |
| #14 AWG | 2.00 |
| #12 AWG | 2.25 |
| #10 AWG | 2.50 |
| #8 AWG | 3.00 |
| #6 AWG | 5.00 |
Source: NEC 314.16(B) style conductor volume allowances for #18 through #6 AWG. Verify against the adopted code edition and box/device listing.
Frequently asked questions
How do you calculate electrical box fill?
Add the volume allowances for counted insulated conductors, equipment grounding conductors, device yokes, internal clamps and support fittings, then compare the required volume with the marked box volume.
How are device yokes counted for box fill?
Each device yoke is screened as two conductor volume allowances of the largest conductor connected to that yoke. Verify multi-device and special-device cases against the adopted code and device listing.
Do grounds count for box fill?
Yes. In this screen, any positive number of equipment grounding conductors contributes one volume allowance using the largest grounding conductor present.
Do pigtails count in electrical box fill?
Pigtails that originate and terminate in the same box are usually not counted. Conductors entering the box and spliced, terminated or passing through generally are counted. Verify the adopted code text for the exact case.
Method & assumptions
- Supports one, two or three insulated conductor-size groups from #18 through #6 AWG.
- Any positive equipment grounding conductor count contributes one grounding-conductor allowance using the selected grounding conductor size.
- Each entered device yoke contributes two allowances of the selected device conductor size.
- Internal clamps and support fittings are screened using the largest conductor allowance present; external clamps are not counted.
- This is a box-fill arithmetic screen only. Pull boxes, conduit bodies, conductors larger than #6 AWG, ampacity adjustment, raceway fill, device listings, local amendments and the adopted code text need separate verification.