Conduit Fill Calculator

Calculate conduit fill for single or mixed conductor pulls and find the minimum trade size that stays within the NEC fill limits. Uses NEC 2020 Chapter 9, Table 1 fill percentages with Table 4 conduit areas and Table 5 conductor areas.

Conductors
Conduit

How to use this calculator

  1. Select conductor insulation, size, and quantity. Add a row for each distinct size or type in the raceway.
  2. Choose the conduit type. Leave conduit size on "Auto" for the minimum size, or pick a size to check a specific run.
  3. Flag the run as a nipple (24 in or shorter) if applicable — it raises the limit to 60% per Note 4.
  4. Read the total conductor area, allowable fill area, fill percentage, and pass/fail status.

NEC reference

This calculator uses NEC 2020 Chapter 9: Table 1 (53% for one conductor, 31% for two, 40% for three or more; 60% for nipples per Note 4), Table 4 (conduit internal areas), and Table 5 (approximate conductor areas including insulation). THHN, THWN, and THWN-2 share identical Table 5 dimensions; XHHW is treated separately.

Results are for reference only. Verify against the applicable adopted edition of the NEC and consult a licensed electrician for code compliance.

Conduit fill formula

Conduit fill is the total cross-sectional area of all conductors in the raceway divided by the raceway's internal area, expressed as a percent. NEC 2020 Chapter 9 Table 1 sets the ceiling by conductor count:

1 conductor   → 53% max fill

2 conductors → 31% max fill

3+ conductors → 40% max fill

Nipple (≤24 in) → 60% max fill (Table 1, Note 4)

Conductor areas come from NEC Chapter 9 Table 5 — values that include the insulation, not just the copper. THHN, THWN, and THWN-2 share one column; XHHW has different dimensions in a separate column. Conduit internal areas come from NEC Chapter 9 Table 4 by trade size and type — EMT, RMC, IMC, PVC Schedule 40, and PVC Schedule 80 each publish different internal diameters for the same nominal trade size, so the fill math is not interchangeable across raceway types.

Worked example

Five #6 THHN copper conductors in EMT:

Table 5: each #6 THHN = 0.0507 in²

Total conductor area = 5 × 0.0507 = 0.2535 in²

Count = 5 → Table 1 fill limit = 40%

1/2 in EMT (total 0.304): allowable = 0.122 in² → fails

3/4 in EMT (total 0.533): allowable = 0.213 in² → fails

1 in EMT (total 0.864): allowable = 0.346 in² → passes at 29.3% fill

Minimum trade size is 1 in EMT. The same pull as a 20-inch nipple changes the calculation: Note 4 raises the limit to 60% and 3/4 in EMT works (allowable 0.320 in², actual fill 47.6%). For a pull longer than 24 in, the nipple allowance does not apply and 1 in is required.

Common mistakes

  • Skipping the EGC. The equipment grounding conductor counts toward fill. NEC Chapter 9 Table 1 applies to every conductor in the raceway, not just current-carrying ones. Forgetting it is the most common source of undersized conduit on inspection.
  • Using the wrong fill percentage. The limit shifts with count: 1 conductor = 53%, 2 = 31%, 3 or more = 40%. The drop at two conductors is counter-intuitive but it is what Table 1 says. Nipples 24 in or shorter get 60% per Note 4.
  • Treating THHN area as XHHW area. Table 5 publishes separate values for these insulation types. The gap is biggest on small conductors — a #12 XHHW is 0.0181 in² versus #12 THHN at 0.0133 in², a 36% difference that will undersize a conduit fast. On larger conductors the values are closer. Always read the correct Table 5 column.
  • Mixing Schedule 40 and Schedule 80 PVC areas. Schedule 80 has a thicker wall, so its internal area is smaller for the same trade size. Table 4 lists them separately — pulling a Sch 40 area for a Sch 80 run consistently undersizes the conduit.

Frequently asked questions

What is the maximum conduit fill allowed by the NEC?

NEC 2020 Chapter 9 Table 1 sets the maximum: 53% for one conductor, 31% for two conductors, and 40% for three or more. Nipples 24 inches or shorter get 60% per Table 1 Note 4. The percentage applies to the total internal cross-section of the raceway (Table 4) measured against the sum of the approximate areas of all installed conductors (Table 5, or actual outside diameter for cable assemblies not tabulated). Cable trays and surface raceways have separate rules in Article 392 and 386.

Do I count the equipment grounding conductor toward conduit fill?

Yes. NEC Chapter 9 Table 1 applies to every conductor in the raceway, current-carrying or not. The EGC counts the same as any phase or neutral conductor for fill purposes. Use Table 5 for insulated EGCs and Table 8 area for bare grounding conductors. The EGC also counts toward the conductor-count threshold that selects the 53/31/40% bracket — three current-carrying conductors plus a bare EGC is four conductors for fill, not three.

What is the 60% fill rule for nipples?

NEC Chapter 9 Table 1 Note 4 allows raceways no longer than 24 inches between boxes, cabinets, or similar enclosures to be filled to 60% of their internal area. The ampacity adjustment factors in 310.15(C)(1) also do not apply on nipples. This commonly comes up on short conduit runs between adjacent panelboards, transformer secondaries to panels, and short nipples used as wireways between junction boxes. Length is measured between the enclosures, not including the fittings.

How do I calculate fill for mixed conductor sizes?

Look up each conductor's area in NEC Chapter 9 Table 5 by its insulation type and size. Multiply each by its quantity. Sum the totals — that is your total conductor area. Compare against the raceway's allowable area: the Table 4 total cross-section for the trade size and type, times the Table 1 fill percentage (53, 31, 40, or 60 for nipples). If the conductor sum is at or below the allowable, the conduit passes. This calculator handles mixed bundles directly — add one row per distinct size and insulation combination.

Does THHN have the same fill area as XHHW?

No. NEC Chapter 9 Table 5 publishes separate columns. The gap is largest on small conductors: #12 THHN is 0.0133 in² versus #12 XHHW at 0.0181 in², a 36% difference. On larger conductors the values converge and at some sizes XHHW is slightly smaller than THHN. THHN, THWN, and THWN-2 share one Table 5 column because their insulation dimensions are identical; XHHW and XHHW-2 share a separate column. Always pick the column that matches the installed conductor.

Can I use 3/4 EMT for three #6 THHN conductors?

Yes. Three #6 THHN total 3 × 0.0507 = 0.1521 in². The 40% fill limit on 3/4 in EMT (Table 4 total 0.533 in²) gives 0.213 in² allowable, and 0.1521 fits at 28.5% actual fill. Add a #10 THHN EGC (0.0211 in²) and the total is 0.1732 in² at 32.5%, still within limits. Four #6 THHN at 0.2028 in² also fits 3/4 EMT. Five #6 THHN at 0.2535 in² does not — step up to 1 in EMT.

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