What is the NEC conduit fill limit for 3 or more conductors?

NEC Chapter 9, Table 1 limits conductor fill to 40% of the conduit's cross-sectional area when 3 or more conductors are present. For 2 conductors the limit is 31%, and for 1 conductor it is 53%.

How do I calculate conduit fill for mixed wire sizes?

Look up each conductor's cross-sectional area in NEC Chapter 9, Table 5 by AWG and insulation type (THHN areas used here). Sum all conductor areas. Compare to the 40% fill area for your conduit type and trade size from Table 4. If total area is less than or equal to the Table 4 value, the conduit size works.

Does the conduit fill rule apply to EMT, RMC, and PVC?

Yes. The 40% rule for 3+ conductors applies to all conduit types: EMT, IMC, RMC, PVC Schedule 40, PVC Schedule 80, LFMC, and LFNC. Interior areas differ by type and trade size (Table 4) but the 40% percentage is the same.

Do equipment grounding conductors count in conduit fill?

Yes. All conductors in the conduit count for fill — including equipment grounding conductors. There is no NEC exception that allows grounds to be excluded from conduit fill calculations.

Free Calculator

Conduit Fill Calculator — NEC Chapter 9

Select conduit type, enter conductor sizes and counts — get an instant fill percentage, pass/fail result, and minimum conduit size. Works for EMT, IMC, RMC, and PVC. Mixed conductor sizes supported. Free, no account needed.

✓ EMT, IMC, RMC, PVC Sch 40/80 ✓ 40%/31%/53% rules ✓ NEC Tables 4 + 5 ✓ Mixed conductor sizes

The Fill Rules — NEC Chapter 9, Table 1

NEC Chapter 9, Table 1 sets maximum fill percentages based on the number of conductors in the conduit:

53%

1 conductor only

31%

2 conductors exactly

40%

3+ conductors — most common

These percentages apply to the conduit's total internal cross-sectional area from NEC Table 4. For mixed wire sizes, sum conductor cross-sectional areas from Table 5 (THHN areas used here) and compare to the 40% fill area for your conduit type and trade size.

Equipment grounding conductors count

Grounds occupy physical space in the conduit and must be included in the fill. Add them as a separate wire group in the calculator using the appropriate AWG.

EMT vs IMC vs RMC vs PVC

All trade sizes ("½ inch", "¾ inch") have different actual interior areas depending on wall thickness. EMT has the largest interior area for a given trade size — meaning you can fit more conductors. RMC has the thickest wall and smallest interior. You may need a larger trade size in RMC compared to EMT for the same conductor package.

EMT — thin wall, most common for interior commercial. Largest interior area per trade size.
IMC — intermediate wall. ~90% of EMT's interior area at most sizes.
RMC — thick wall, required for service entrance and high-abuse locations. Smallest interior area.
PVC Schedule 40 — non-metallic, direct burial and wet locations. Area similar to EMT.
PVC Schedule 80 — thicker wall than Sch 40, smaller interior. Common where exposed to physical damage.

Worked Examples

Example 1 — 3 × 12 AWG THHN in ½" EMT

3 conductors × 12 AWG THHN — ½" EMT

3 × 12 AWG THHN @ 0.0133 in²0.0399 in²

Total conductor area0.0399 in²

½" EMT — 40% fill area: 0.122 in²PASS — 32.7% fill

Example 2 — Mixed sizes, 1" EMT

4 × 10 AWG + 2 × 12 AWG + 1 × 12 AWG ground = 7 conductors total, 40% rule applies.

Mixed conductors — 1" EMT

4 × 10 AWG THHN @ 0.02110.0844 in²

3 × 12 AWG THHN @ 0.01330.0399 in²

Total conductor area0.1243 in²

1" EMT — 40% fill area: 0.346 in²PASS — 35.9% fill

¾" EMT — 40% fill area: 0.213 in²ALSO FITS — 58.3% of allowable

Example 3 — 200A service, 2" RMC

2 × 2/0 THHN ungrounded + 1 × 2/0 neutral + 1 × 4 AWG ground = 4 conductors.

200A service — 2" RMC, THHN

3 × 2/0 AWG THHN @ 0.22230.6669 in²

1 × 4 AWG THHN @ 0.08240.0824 in²

Total conductor area0.7493 in²

2" RMC — 40% fill area: 1.316 in²PASS — 56.9% of allowable

1½" RMC — 40% fill area: 0.792 in²FAIL — exceeds 0.792 in²

FAQ

Why is the 2-conductor limit (31%) more restrictive than 3+ (40%)?
Two large conductors side by side in a round conduit are harder to pull than those same conductors plus a smaller third. The geometry of how round conductors pack inside a round conduit changes with count — 31% reflects the pulling force required for two conductors through a run with standard bends.
Can I use the NEC Annex C tables instead of this calculator?
NEC Annex C provides pre-calculated maximum counts for same-size same-insulation conductors in each conduit type and trade size. For mixed sizes, you must use the Table 4 + Table 5 manual method — which is what this calculator does.
Does conduit fill and ampacity derating interact?
Yes — they're separate calculations. A conduit that passes the 40% fill rule with many current-carrying conductors may still require ampacity derating per NEC 310.15(C)(1). Always do the derating calculation separately when more than 3 current-carrying conductors are present.
Why does DesignedWire use THHN areas?
THHN/THWN is the most common insulation type for conductors pulled in conduit. If you're using XHHW or another insulation type, the conductor areas are slightly different — check NEC Table 5 for exact values. XHHW typically has a larger OD than THHN, so THHN areas give a slightly optimistic result for XHHW conductors.

Studying for the journeyman or master electrician exam?

NEC Chapter 9 conduit fill is one of the most frequently tested calculation topics on state licensing exams — most states require you to know the 40%/31%/53% rules and how to use Tables 4 and 5 for mixed conductor sizes. Use this calculator to check your hand-worked answers. The conduit fill guide has three full worked examples including a 200A service run.