NEC 314.16 is the National Electrical Code section that governs the maximum number of conductors and fittings allowed inside an outlet, device, or junction box. It uses a cubic-inch volume method to prevent dangerous crowding that can damage insulation.

How is the volume allowance determined?

Table 314.16(B) assigns a fixed cubic-inch volume per item based on the largest conductor size in the box. For example, a #12 AWG conductor requires 2.25 in³ each, while a #14 AWG conductor requires 2.00 in³ each.

What counts as a "device" for box fill?

Each yoke or strap that holds a device (switch, outlet, GFCI, dimmer) counts as two conductor volumes based on the largest conductor connected to it. A combination device on one yoke still counts as two volumes.

How do equipment grounding conductors count?

All equipment grounding conductors (bare or green) in a box together count as a single conductor volume based on the largest EGC size present, regardless of how many there are.

Do wire connectors (wire nuts) count toward box fill?

No. Wire connectors, cable ties, and similar fittings do not count toward box fill under NEC 314.16(B). Only the listed items — conductors, clamps, support fittings, and devices — count.

What is the volume allowance for a cable clamp?

A single internal cable clamp assembly (one or more clamps within the box) counts as one conductor volume based on the largest conductor in the box. External clamps on connectors entering the box do not count.

Electrical Box Fill Calculator

Name: Electrical Box Fill Calculator
Availability: InStock
Author: Nham Vu

Enter conductor sizes, clamps, devices, and fittings to calculate total box fill volume and check NEC 314.16 compliance.

Conductors

Count each current-carrying conductor and neutral separately. Do not count EGCs here.

Equipment Grounding Conductors (EGC)

All EGCs together count as one conductor volume based on the largest EGC.

Number of EGCs Largest EGC size

Clamps, Devices & Fittings

Internal cable clamp sets (counts as 1 volume)

Support fittings / fixture studs (each = 1 volume)

Device yokes / straps (each = 2 volumes)

Reference Conductor Size

Clamps, fittings, and devices are sized to the largest conductor in the box. The tool detects this automatically from the conductor table, but you can override it here.

Largest conductor

Results

Fill in the form and click Calculate Box Fill to see the required volume.

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Required volume (in³)

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Reference conductor AWG

Check against a box size (in³)

Volume Breakdown

Common Box Sizes (NEC Table 314.16(A))

Box	Volume (in³)	Status

NEC Table 314.16(B) Quick Reference

Conductor Size	Volume per item (in³)
14 AWG	2.00
12 AWG	2.25
10 AWG	2.50
8 AWG	3.00
6 AWG	5.00
4 AWG	7.00
3 AWG	8.00
2 AWG	9.00

Clamps and support fittings = 1 × volume of largest conductor. Each device yoke = 2 × volume of largest conductor. All EGCs together = 1 × volume of largest EGC.

Summary