What is the fill limit for 3 or more conductors?

For 3 or more conductors, the NEC limits fill to 40% of the conduit's internal cross-sectional area.

How do device yokes count for box fill?

Each device yoke (switch, receptacle) counts as two conductor volumes of the largest wire connected to it.

Conduit Fill & Box Fill Calculator

Reference helper for common field checks. Results are guidance only; verify all work against the applicable code and manufacturer data.

Conduit Fill

Mode

Conduit internal diameter (in)

You can enter ID from a chart. Area is computed automatically.

Allowable fill by conductor count

Conductors

Add each conductor type by outside diameter (in). We sum areas.

Total conductors: 0

Results

Conduit area: 0.0000 in²

Allowable fill area: 0.0000 in² (at 0%)

Sum of conductor areas: 0.0000 in²

Status: Waiting for inputs…

Auto rule picks 53% for one conductor, 31% for two, and 40% for three or more.

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Print creates a job ticket with your inputs and results. This tool runs 100% in your browser.

Box Fill

Box internal volume (in³)

Use manufacturer data or measured internal volume.

Largest conductor gauge in box (AWG)

Used for device yokes and equipment grounds equivalences.

Conductor Groups

Each group: gauge and quantity of insulated conductors that originate/terminate or pass through.

Device yokes

Each yoke counts as two conductors of the largest gauge present on that device.

Equipment grounds (total)

All grounds together count as one conductor of the largest gauge present.

Pigtails (do not leave the box)

Pigtails typically do not add to count when spliced only within the box.

Results

Per-conductor volume (by gauge): 2.25 in³

Total conductor equivalents: 0

Required volume: 0.00 in³

Box volume: 0.00 in³

Status: Waiting for inputs…

Per-conductor volumes follow a common reference mapping for typical copper conductors. Verify against current requirements.

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About this free conduit & box fill calculator

This tool helps electricians and planners verify conduit fill and box fill compliance. It runs entirely in your browser, allowing you to save your state via URL or print a job ticket.

NEC Conduit Fill Rules (Chapter 9, Table 1)

The National Electrical Code (NEC) limits how much of a conduit's cross-sectional area can be occupied by conductors to prevent damage during pulling and ensure heat dissipation.

1 Conductor: Max 53% fill.
2 Conductors: Max 31% fill (due to jamming probability).
3 or more Conductors: Max 40% fill.

Note: Nipples 24 inches or shorter can be filled to 60%.

Box Fill Volume Calculations (NEC 314.16)

Box fill ensures there is enough free space for conductors, devices, and fittings without overcrowding.

Conductors: Each counts as 1 volume unit based on its size (e.g., 2.25 in³ for #12).
Clamps: All internal cable clamps together count as 1 volume unit (based on largest wire).
Support Fittings: Luminaire studs or hickeys count as 1 unit.
Device Yokes: Each strap/yoke (switch, outlet) counts as 2 volume units.
Equipment Grounds: All grounds together count as 1 volume unit (up to 4, then 1/4 volume for each additional).