Skip to content
ElectraKit
Open menu

How to Calculate Conduit Fill

One-size conduit-fill charts answer one insulation and one conductor size. Real pulls mix both. This workflow sums conductor area group by group, applies the right percent limit, and routes cable assemblies that the tables do not cover.
Code basis
Chapter 9 Tables 1, 4, 5
Method
Area sum, not count charts
Default limit
40% for 3+ conductors
Hard case
Mixed sizes and cables

The Procedure

5 steps, in the order the code applies them.

  1. Step 1

    List every conductor group

    Group by size and insulation type. A pull with 12 AWG THHN and 10 AWG XHHW is two groups, not one average size.

  2. Step 2

    Look up each group's area

    Use Chapter 9 Table 5 or 5A for the actual insulation and stranding. Compact conductors only get compact areas when they are actually compact.

  3. Step 3

    Multiply area by quantity and sum

    Total conductor area is the sum of every group. Include the equipment grounding conductor — fill counts it even though derating does not.

  4. Step 4

    Select the percent fill limit

    One conductor uses 53 percent, two use 31 percent, three or more use 40 percent. A nipple not over 24 inches may use 60 percent.

  5. Step 5

    Compare to raceway internal area

    Read Chapter 9 Table 4 for the raceway type and trade size. If total conductor area exceeds the permitted area, enlarge the raceway or reduce the pull.

Calculate area, not count

Each conductor group contributes its own tabulated area multiplied by quantity. Mixed pulls are summed group by group.

After total area is known, compare it with the permitted area for the raceway trade size and conductor count.

When tables are not enough

Annex-style count tables are useful only for one conductor size and one insulation type. They do not answer a mixed-size pull.

Cable assemblies such as UF need manufacturer dimensions. Borrowing a similar single-conductor value can understate fill.

Decision Guide

The choices that change the answer, and how to make them.

  • Skip annex count tables for mixed pulls

    Count charts assume one size and one insulation. They are planning shortcuts, not the answer for a mixed pull.

  • Cables need manufacturer dimensions

    UF, MC and similar assemblies are not interchangeable with single-conductor Table 5 rows. Use the published OD or area from the maker.

  • Fill pass is not pull ease

    A raceway can meet percent fill and still be a hard pull because of bends, length and jam ratio. Plan lubrication and pull points separately.

FAQs

How do I calculate conduit fill with different wire sizes?
Sum the tabulated area of each size and insulation group, then compare the total with the permitted area for that raceway trade size and conductor count under Chapter 9 Table 1.
Does the ground wire count in conduit fill?
Yes. Fill counts every conductor in the raceway, including the equipment grounding conductor. Derating uses a different count.
When can I use 60 percent fill?
For a nipple with a maximum length of 24 inches, Chapter 9 Note allows 60 percent. Longer raceways use the 53/31/40 percent limits.

Calculated against the NEC 2023 edition using Chapter 9 Table 1, Chapter 9 Table 4, Chapter 9 Table 5. Confirm the edition adopted by your AHJ, which may differ and may carry local amendments.

Not covered by this tool

  • Motor, air-conditioning and welder circuits, which are sized under their own articles.
  • Conductors in free air and photovoltaic circuits.
  • Parallel conductor sets, taps and busbars.
  • Local amendments, which can be stricter than the published code.

Data last verified