Conduit Fill Calculator

Check NEC conduit fill for EMT, RMC, IMC, PVC and flexible metal conduit using Chapter 9 conductor and raceway areas.

Mix wire sizes and insulation types, see pass or fail, and find the smallest trade size that works.

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How NEC conduit fill is calculated

Conduit fill compares the total cross-sectional area of the conductors with the internal area of the raceway. Look up each wire's area including insulation in NEC Chapter 9, Table 5, multiply by the quantity, and add the rows together. Then look up the raceway's total internal area in Chapter 9, Table 4. Each raceway type has its own row because wall thickness differs: 3/4" EMT has 0.533 in², while 3/4" PVC Schedule 80 has only 0.409 in².

Chapter 9, Table 1 sets the limit: 53% for one conductor, 31% for two, and 40% for three or more. Note 4 allows 60% in a nipple 24 in. or shorter between enclosures. Two conductors get the lowest limit because two round wires can wedge across the pipe diameter and jam during a pull.

Worked example: 9 x 12 AWG THHN in 3/4" EMT

A 12 AWG THHN conductor is 0.0133 in². Nine of them total 9 x 0.0133 = 0.1197 in². The 40% limit for 3/4" EMT is 0.533 x 0.40 = 0.2132 in², so the pull passes at 0.1197 / 0.533 = 22.5% fill. The same nine wires also fit 1/2" EMT, where the limit is 0.304 x 0.40 = 0.1216 in².

Informative Annex C does this math ahead of time for identical conductors: 1/2" EMT holds 9 x 12 THHN and 3/4" EMT holds 16. Where the division leaves a decimal of 0.8 or more, Chapter 9 Note 7 lets you round up. That is why 1" EMT is listed at 26 x 12 THHN even though 0.3456 / 0.0133 = 25.98.

Common mistakes

  • Forgetting the equipment grounding conductor. Insulated and bare EGCs count toward fill. Use Chapter 8 tables for bare conductor areas.
  • Using EMT areas for PVC. Schedule 80 PVC has a noticeably smaller bore, so a pull that fits 1" EMT can fail in 1" Schedule 80.
  • Treating fill as ampacity. A raceway that passes fill but carries more than three current-carrying conductors still needs the adjustment factors in 310.15(C)(1).
  • Mixing insulation types. XHHW-2 is larger than THHN in small sizes. For example, 12 AWG XHHW-2 is 0.0181 in² against 0.0133 in² for THHN.
  • Filling to exactly 40% on long runs. The code sets the maximum. Bends and long pulls usually need room to spare, so check jam ratio and pulling tension with the wire manufacturer's guide.

Code references

Fill percentages come from NFPA 70 Chapter 9, Table 1 and its notes.

Raceway areas come from Table 4 and conductor areas from Table 5 (Table 5A for compact aluminum).

Each raceway article repeats the rule: 358.22 for EMT, 344.22 for RMC, 342.22 for IMC, 352.22 for PVC and 348.22 for FMC.

The dimensions have not changed from the 2017 through the 2023 editions.

Your authority having jurisdiction and the adopted code edition make the final call.

Frequently Asked Questions

Common questions about the Conduit Fill Calculator

NEC Chapter 9 Table 1 allows 40% for three or more conductors, 31% for two and 53% for one. A nipple 24 inches or shorter may be filled to 60%.

Sources & References

NFPA 70, National Electrical Code (2023 edition)

The code document itself; free read-only access is available after registering with NFPA.
→Source: National Fire Protection Association

NFPA 70 free access edition

Read-only online view of the 2023 NEC, including Chapter 9 and Article 314.
→Source: National Fire Protection Association

Mike Holt Enterprises code training articles

Widely used NEC training source with worked raceway fill and box fill examples.
→Source: Mike Holt Enterprises

Southwire building wire specifications and field calculators

Manufacturer spec sheets for THHN/THWN-2 and XHHW-2 building wire, including outside diameters used in fill calculations.
→Source: Southwire Company

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