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Conduit fill: how many wires fit in a conduit?

Calculate the percentage of a conduit's cross-section filled by your wires and compare it with the National Electrical Code limits.

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Results

Conduit fill

13.1%

Within the limit

Maximum number of type 1 wires alone

9

Maximum fill allowed (NEC Chapter 9, Table 1)
40.0%
Conduit inside area
0.304 sq in
Area taken by the wires
0.040 sq in

Why conduit fill is limited

Electrical conduit protects wires, but packing too many into a raceway causes two problems. Wires are harder to pull, so insulation can be scraped and damaged during installation. And bundled conductors shed heat poorly, so they run hotter. The National Electrical Code (NEC) therefore limits the share of a conduit’s cross-section that conductors can occupy.

The NEC limits

From NEC Chapter 9, Table 1:

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

The odd 31% for two conductors exists because two round wires side by side can jam in a bend.

The calculation

Fill = total wire area ÷ conduit inside area

with each area calculated from diameter as π × d² ÷ 4. Use the inside diameter of the conduit (NEC Chapter 9, Table 4) and the outside diameter of each wire including insulation (Table 5). The calculator handles two wire sizes, for example circuit conductors and a smaller ground.

Worked example

Three 12 AWG THHN conductors (0.130 in outside diameter) in 1/2-inch EMT (0.622 in inside diameter):

Result Value
Conduit fill 13.1%
Maximum allowed 40%
Maximum 12 AWG THHN wires alone 9
Conduit inside area 0.304 sq in
Area taken by the wires 0.040 sq in

That matches NEC Annex C, which lists 9 conductors of 12 AWG THHN for 1/2-inch EMT.

Typical values

Conductor (THHN/THWN-2) Outside diameter
14 AWG 0.111 in
12 AWG 0.130 in
10 AWG 0.164 in
8 AWG 0.216 in
6 AWG 0.254 in
Conduit (EMT) Inside diameter
1/2 in 0.622 in
3/4 in 0.824 in
1 in 1.049 in

Other conduit types (PVC Schedule 40 and 80, rigid metal, flexible) have different inside diameters for the same trade size, so look up the right table.

Beyond fill

  • Ampacity adjustment: with more than three current-carrying conductors in a raceway, NEC 310.15(C)(1) reduces their allowable current, to 80% for 4 to 6 conductors and 70% for 7 to 9.
  • Nipples: conduit runs of 24 inches or less may be filled to 60%.
  • Bends: no more than 360 degrees of bends between pull points.

Electrical work is regulated. Check local code amendments and have work inspected as required.

Frequently asked questions

What is the maximum conduit fill?

The National Electrical Code (Chapter 9, Table 1) allows 53% for one conductor, 31% for two and 40% for three or more.

How many 12 AWG wires fit in a 1/2-inch EMT conduit?

Nine THHN 12 AWG conductors, based on an inside area of 0.304 square inches and a wire area of 0.0133 square inches at 40% fill.

Where do I find the wire diameter?

Use the conductor's outside diameter including insulation, from NEC Chapter 9, Table 5 or the manufacturer's data. Insulation type matters: THHN is much thinner than THW.

Do I need to derate for many wires?

Yes. When more than three current-carrying conductors share a conduit, their ampacity must be reduced under NEC 310.15(C)(1), for example to 80% for 4 to 6 conductors.

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Results are estimates provided for information purposes. They do not replace professional advice.

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