Free electrical planning tool

Conduit fill calculator

Estimate how much of an EMT or PVC Schedule 40 raceway is occupied by THHN/THWN-2 or XHHW-2 conductors.

Raceway and conductors

Describe one conductor set

Use one size and insulation type per calculation. For mixed conductor sets, calculate each occupied area and add them.

Conductor assumptionsTHHN/THWN-2 · NEC Chapter 9 dimensions

The result applies the Chapter 9, Table 1 limit of 53% for one conductor, 31% for two, or 40% for more than two.

Preliminary result

15.0%

The modeled set is within the 40% fill limit.

Occupied area
0.0798 in²
Allowed area
0.2130 in²
Fill limit
40%
Smallest modeled raceway
1/2 in.

Preliminary design aid: verify every conductor in the raceway, actual cable dimensions, nipple rules, wiring method, adopted code, and field conditions.

How to read the result

The fill percentage compares total conductor area with the raceway's full internal area. The allowed area applies the Chapter 9 limit for the number of conductors entered. The smallest modeled raceway is the first supported trade size with enough allowed area for that conductor set.

Conduit fill is an area calculation, not a diameter comparison. Two conductors that look as though they fit side by side can still exceed the permitted occupied area, especially when insulation makes the outside diameter larger.

A practical guide

How conduit fill is calculated

Start with the actual outside area of each insulated conductor. Multiply that area by quantity, then compare the sum with the permitted area of the selected raceway.

Conduit fillTotal conductor area ÷ full raceway area × 100

The permitted percentage changes with conductor count.

Insulation changes occupied area

Wire gauge describes the metallic conductor, but conduit fill uses the outside dimensions of the insulated conductor. The same gauge can occupy a different area with THHN/THWN-2 than with XHHW-2.

Count every conductor

Include grounded, ungrounded, control, and equipment grounding conductors when the applicable rules require them to count. This calculator models one size and insulation per run so the input stays auditable.

The limit changes at one and two

Chapter 9, Table 1 allows 53% for one conductor, 31% for two, and 40% for more than two. These limits address pulling and installation realities, not only whether the conductors can physically enter the raceway.

Mixed conductor sizes

For a mixed set, find the Table 5 area for each insulation and size, multiply each by its quantity, and add the results. Compare that combined area with the applicable Table 4 raceway allowance. ZapSketch performs that aggregation when circuits share a routed raceway.

  1. List every conductor. Group identical size and insulation combinations.
  2. Calculate each group area. Multiply the individual conductor area by its quantity.
  3. Add the groups. Include the modeled equipment grounding conductor allowance.
  4. Check the selected raceway. Compare total occupied area with its permitted fill area.

Modeled scope and source

This calculator uses conductor dimensions from NEC 2023 Chapter 9, Table 5 and raceway dimensions from Table 4, with the percentage rules in Table 1. Review the official NFPA 70, 2023 edition (opens in a new tab) for the source material and notes.

  • Supported raceways are EMT and PVC Schedule 40 from 1/2 inch through 4 inches.
  • Supported insulation families are THHN/THWN-2 and XHHW-2 from 14 AWG through 600 kcmil.
  • Cables, compact conductors, nipples, jam ratio, pulling tension, conductor bending, and other wiring methods are outside this calculator's scope.

From a fill check to a routed plan

See what happens when circuits share the same raceway.

Route home runs on the drawing and keep conductor choices connected to preliminary fill, voltage-drop, and material feedback.

Open ZapSketch and plan a raceway