Conduit Fill Calculator

Check raceway fill, compare conduit sizes, and find the smallest conduit that fits your conductor combination.

Built for mixed-conductor sizing Add conductors by size, insulation and quantity, choose the raceway type, and get total conductor area, allowable fill, fill percentage and the smallest compliant conduit size.

Size Your Raceway

1. Raceway

Conduit type

Trade size

Nipple ≤ 24 in between enclosures (use 60% fill limit)

2. Conductors

3. Check

Planning aid only. Raceway fill is one code check; conductor ampacity, derating, bending space, pull conditions, grounding/bonding and local code requirements may also apply. Verify the adopted electrical code and the actual conductor/raceway manufacturer's data before installation.

How Fill Is Calculated

The calculator sums the cross-sectional area of every conductor, including the equipment grounding conductor, then compares that total with the permitted percentage of the raceway's internal area.

NEC Fill Limits

For the standard Chapter 9 fill calculation, the commonly used limits are 53% for one conductor, 31% for two conductors and 40% for more than two. A nipple 24 inches or less can use the 60% exception.

Formula

Fill % = total conductor area ÷ allowable raceway area × 100

The allowable area changes with conductor count and the selected raceway type and trade size.

Frequently Asked Questions

Yes. The equipment grounding conductor occupies physical space in the raceway and is included in the conductor-area total for the fill calculation.

The standard Chapter 9 Table 1 limits are 53% for one conductor, 31% for two conductors and 40% for more than two conductors. A qualifying nipple 24 inches or less between enclosures can use the 60% rule.

No. Fill controls physical space inside the raceway. Ampacity adjustment or derating is a separate check that can reduce allowable conductor current when multiple current-carrying conductors share a raceway.

Yes. This calculator is designed around conductor groups, so you can add different AWG sizes and insulation types and sum their individual areas before checking the raceway limit.

No. Raceway fill is only one part of an electrical installation. Check ampacity, derating, temperature, grounding and bonding, conductor protection, bending space, pull conditions and the code adopted by your jurisdiction.