What fill percentage am I allowed?

Per NEC Chapter 9 Table 1: 53% for a single conductor, 31% for two conductors, and 40% for three or more conductors. Nipples 24 inches or shorter are allowed up to 60%.

Does the equipment grounding conductor count toward fill?

Yes. NEC 314.16(B) and the Chapter 9 fill rules count every current-carrying and equipment grounding conductor in the raceway, including bare grounds, against the cross-sectional area limit.

Can I use this for cable tray or wireway?

No. Cable tray fill is governed by NEC Article 392 and wireways by Article 376, both of which use different rules. This calculator covers raceways listed in NEC Chapter 9 Table 4 only.

Conduit Fill Calculator

NEC Chapter 9

Calculate conduit fill percentage per NEC Chapter 9 Tables 1, 4, and 5. Verify code compliance for any conduit and wire combination.

Conduit

Conduit Type

Conduit Size

Conductors

Insulation

Wire Size

Qty

Use Qty for same wire • Add Wire for different sizes

Results

Fill Percentage

2.5%

Code Compliant

Total Wire Area

0.0133in²

Available Area

0.2825in²

Conductor Count

Max Fill Allowed

53%

1 conductor - 53% fill allowed (NEC Chapter 9, Table 1)

2.5% fill

About this calculator

Conduit fill is the percentage of a raceway cross-section taken up by conductors. NEC Chapter 9, Table 1 caps fill at 53% for one wire, 31% for two wires, and 40% for three or more. Pull too many wires through too small a conduit and you risk insulation damage, heat buildup, and a failed inspection. This calculator handles EMT, IMC, RMC, PVC, and ENT in trade sizes 1/2" through 4", and it pulls cross-section data from NEC Chapter 9 Tables 4 and 5.

Formula

Fill percent equals the sum of conductor cross-sectional areas divided by the conduit interior area, expressed as a percent. NEC Chapter 9 Table 5 lists the area for each conductor type and size; Table 4 lists the interior area for each conduit type and trade size.

Fill % = (Σ conductor areas in in² ÷ conduit interior area in in²) × 100

Reference: NEC Chapter 9, Tables 1, 4, and 5

How to use

Pick the conduit type and trade size you plan to install.
Add each conductor: insulation type (THHN, THWN-2, XHHW, etc.) and AWG or kcmil size.
Add the same conductor multiple times if you have several of that size and type.
Read the resulting fill percent and the calculator will flag whether it exceeds the NEC Chapter 9 Table 1 limit for your conductor count.
Bump up to the next trade size if you fail and recheck before you order material.

Worked example

Setup

Three #12 THHN conductors in a 1/2" EMT run. Three or more conductors means the 40% fill rule applies.

Calculation

Each #12 THHN is 0.0133 in². Three of them sum to 0.0399 in². A 1/2" EMT has 0.304 in² interior area. Fill = (0.0399 ÷ 0.304) × 100 = 13.1%.

Answer

13.1% is well under the 40% limit, so 1/2" EMT is compliant for this pull.

Related study guides

Frequently asked questions

What fill percentage am I allowed?: Per NEC Chapter 9 Table 1: 53% for a single conductor, 31% for two conductors, and 40% for three or more conductors. Nipples 24 inches or shorter are allowed up to 60%.
Does the equipment grounding conductor count toward fill?: Yes. NEC 314.16(B) and the Chapter 9 fill rules count every current-carrying and equipment grounding conductor in the raceway, including bare grounds, against the cross-sectional area limit.
Can I use this for cable tray or wireway?: No. Cable tray fill is governed by NEC Article 392 and wireways by Article 376, both of which use different rules. This calculator covers raceways listed in NEC Chapter 9 Table 4 only.