Raceway fill · NEC Chapter 9

Conduit Fill Calculator

Add one or more conductor groups by size and insulation type, pick a conduit type, and see the smallest trade size that stays within NEC Chapter 9, Table 1 fill limits. Areas come from NEC Chapter 9, Tables 4 and 5.

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NEC Ch. 9 Tables 1·4·5
Mixed conductor groups
EMT · RMC · PVC

Raceway fill

NEC Chapter 9 · Tables 1·4·5

Formula

Fill % = Σ conductor area ÷ conduit area

Field inputsRaceway fill

Conduit: EMT · RMC · PVC
Limits: 53 · 31 · 40%
Areas: Tables 4 · 5
Groups: multi-size

Returns the smallest trade size that stays under the fill limit.

Conductor groups

Build the pull

Conduit type

Group 1

Size

Insulation

Qty

Group 2

Size

Insulation

Qty

Assumptions: conductor areas from NEC Chapter 9, Table 5 by insulation type; conduit internal areas from NEC Chapter 9, Table 4; fill limits from NEC Chapter 9, Table 1 (53% for 1 conductor, 31% for 2, 40% for 3 or more). Nipples (≤24 in) and derating are not modeled.

Recommended conduit

Smallest fit

40% limit

Smallest trade size that fits

1/2"

Total conductors: 4
Total conductor area: 0.0610 sq in
Required area at 40%: 0.1525 sq in
Fill at 1/2": 20.07 %

All trade sizes (EMT)

1/2" (0.304 sq in)20.1% ✓

3/4" (0.533 sq in)11.4% ✓

1" (0.864 sq in)7.1% ✓

1-1/4" (1.496 sq in)4.1% ✓

1-1/2" (2.036 sq in)3.0% ✓

2" (3.356 sq in)1.8% ✓

2-1/2" (5.858 sq in)1.0% ✓

3" (8.846 sq in)0.7% ✓

3-1/2" (11.545 sq in)0.5% ✓

4" (14.753 sq in)0.4% ✓

Source: NEC Chapter 9, Tables 1, 4, and 5.

Safety & accuracy notice

This conduit fill calculator is a planning aid only. It is not engineering, a permit, or design approval, and it is not legal or code certification. It does not model short nipples, conductor derating, or every special condition.

Always verify against the current adopted NEC (National Electrical Code), the equipment listings and manufacturer instructions, and your local AHJ (Authority Having Jurisdiction — the inspector or department that enforces the code). The final raceway and fill should be reviewed and approved by a licensed electrician or professional engineer.

Conduit fill FAQ

What are the NEC conduit fill limits?

NEC Chapter 9, Table 1 limits raceway fill to 53% of internal area for one conductor, 31% for two conductors, and 40% for three or more conductors. This tool selects the limit automatically from the conductor count.

Where do the conductor and conduit areas come from?

Conductor cross-sectional areas come from NEC Chapter 9, Table 5 (by insulation type), and conduit internal areas come from NEC Chapter 9, Table 4 (by conduit type and trade size).