Conduit Fill Calculator

Select conduit type and trade size, then enter up to three THHN/THWN conductor groups. The calculator checks the fill percentage against the NEC Chapter 9 limits: 53% for one conductor, 31% for two, and 40% for three or more.

Conductor groups

Select AWG and enter a count for each group you need.

Conductor areas are reference values taken from NEC Chapter 9 Tables 4 and 5. The applicable edition of the NEC governs for code compliance.

Select a conduit size and at least one conductor group to see the fill result.

NEC Fill Percentage Limits

NEC Chapter 9, Table 1 specifies the maximum fill as a percentage of the conduit's internal cross-sectional area:

Number of conductors Maximum fill
1 conductor 53%
2 conductors 31%
3 or more conductors 40%

Source: NEC Chapter 9, Table 1. Always verify against the edition having jurisdiction.

Conduit Fill Chart (THHN/THWN)

The maximum number of same-size THHN/THWN conductors permitted in each trade size of conduit. Counts are computed from NEC Chapter 9 Table 4 (conduit area) and Table 5 (conductor area) against the Table 1 fill limits, so a cell showing 1 or 2 conductors uses the 53% and 31% limits rather than 40%.

EMT conduit fill chart

Maximum number of THHN/THWN conductors in EMT conduit by trade size and wire gauge
Wire size 1/2" 3/4" 1" 1-1/4" 1-1/2" 2" 2-1/2" 3"
14 AWG 12 21 35 61 83 138 241 364
12 AWG 9 16 25 44 61 100 176 266
10 AWG 5 10 16 28 38 63 111 167
8 AWG 3 5 9 16 22 36 64 96
6 AWG 1 4 6 11 16 26 46 69
4 AWG 1 2 4 7 9 16 28 42
3 AWG 1 1 3 6 8 13 24 36
2 AWG 1 1 2 5 7 11 20 30
1 AWG 1 1 1 3 5 8 15 22
1/0 AWG — 1 1 3 4 7 12 19
2/0 AWG — 1 1 2 3 6 10 15
3/0 AWG — 1 1 1 3 5 8 13
4/0 AWG — — 1 1 1 4 7 10

PVC conduit fill chart

Maximum number of THHN/THWN conductors in PVC conduit by trade size and wire gauge
Wire size 1/2" 3/4" 1" 1-1/4" 1-1/2" 2" 2-1/2" 3"
14 AWG 11 20 34 59 81 135 193 299
12 AWG 8 15 25 43 59 98 141 218
10 AWG 5 9 15 27 37 62 89 137
8 AWG 3 5 9 15 21 35 51 79
6 AWG 1 4 6 11 15 25 37 57
4 AWG 1 1 4 7 9 15 22 35
3 AWG 1 1 3 5 8 13 19 29
2 AWG 1 1 2 5 6 11 16 25
1 AWG — 1 1 3 5 8 12 18
1/0 AWG — 1 1 3 4 7 10 15
2/0 AWG — 1 1 2 3 5 8 13
3/0 AWG — 1 1 1 2 4 7 10
4/0 AWG — — 1 1 1 4 5 8

These counts are computed from the Chapter 9 areas rather than transcribed from NEC Annex C, so a cell may occasionally land one conductor either side of the Annex C figure where rounding differs. Annex C governs where a code official is counting. Counts assume all conductors are the same size and insulation type, with no nipple allowance (a raceway 24 inches or shorter may fill to 60% under NEC Chapter 9 Note 4). Mixed conductor sizes must be worked out by total area, which is what the calculator above does. Ampacity derating under NEC 310.15(C)(1) applies once more than three current-carrying conductors share a raceway, so the count that physically fits is often more than the count you should actually pull.

How It Works

The fill percentage formula

Conduit fill is the ratio of the total cross-sectional area of all conductors inside the conduit to the conduit's total internal area:

Fill (%) = (Sum of conductor areas ÷ Conduit internal area) × 100

The conductor areas are taken from NEC Chapter 9, Table 5 (total area including insulation). The conduit areas come from Table 4 (100% fill column). The applicable fill limit from Table 1 is then compared to the calculated fill.

Worked example

Three 12 AWG THHN conductors in 1/2" EMT:

  • Conduit internal area (1/2" EMT, Table 4): 196.1 mm²
  • Conductor area (12 AWG THHN, Table 5): 11.68 mm² each
  • Total wire area: 3 × 11.68 = 35.04 mm²
  • Fill: 35.04 ÷ 196.1 × 100 = 17.9%
  • Limit (3 conductors): 40% — PASS

If the same conduit held 10 conductors at 12 AWG (total area 116.8 mm²), the fill would be 59.6%, exceeding the 40% limit — a FAIL that requires a larger conduit.

Frequently Asked Questions

What are the NEC conduit fill percentage limits?

The NEC (Chapter 9, Table 1) sets three limits: 53% of the conduit internal area for a single conductor, 31% for two conductors, and 40% for three or more conductors. The lower two-conductor limit exists because two round conductors wedge against each other less efficiently than three or more, which can rotate to pack more freely.

How do I count conductors for conduit fill?

Count every current-carrying conductor plus the equipment grounding conductor (EGC) that runs inside the conduit. Neutral conductors are counted. Equipment bonding jumpers (EBJs) outside the conduit and conductors in separate raceways do not count. When conductors of different sizes are mixed, add each individual conductor's cross-sectional area (from NEC Table 5) and compare the total to the conduit's internal area.

What is THHN insulation and why is it common?

THHN stands for Thermoplastic High Heat-resistant Nylon-coated. It is the most widely used building wire in the US for conductors pulled through conduit, rated 90 degC dry and 75 degC wet (as THWN-2). Its thin, slippery nylon jacket makes it easy to pull and gives it a small cross-sectional area relative to its current rating, which helps conduit fill.

Why does conduit fill matter?

Exceeding the fill limit makes conductors difficult or impossible to pull without damaging the insulation, and it traps heat generated by current flow. Heat build-up reduces conductor ampacity, which is why the NEC ampacity tables already derate conductors in conduit. Staying within fill limits keeps pulling practical and prevents additional derating beyond what the tables already account for.

When should I choose EMT versus PVC conduit?

EMT (Electrical Metallic Tubing) is lightweight, easily bent, and is the standard choice for most interior exposed or concealed work in commercial and residential buildings. PVC Schedule 40 is used where corrosion or moisture is a concern, in underground runs, and where metal is prohibited. PVC has slightly different internal areas to EMT at some trade sizes, so always use the correct column in NEC Table 4 for your conduit type.

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