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Conduit Fill Calculator

Check conductor fill against NEC Chapter 9 limits and find the trade size you need.

Raceway

Areas come from NEC Chapter 9, Table 4.

Conductors

Add a row for each distinct conductor size and insulation type in the raceway. Count grounds and neutrals here — every conductor occupies area, even if it does not count for ampacity derating.

Conductor group 1

Conductor group 2

Fill of 3/4" EMT

22.9%of 40% allowed

8 conductors fit this raceway.
Total conductor area8 conductors counted
0.1220 in²
Allowable fill area40% of 0.533 in² internal area
0.2132 in²
Area remainingSpace left before the limit
0.0912 in²
Smallest compliant sizeEMT
3/4"

Compliant fill

8 conductors at 0.1220 in² occupies 22.9% of the 3/4" EMT, inside the 40% limit from Chapter 9, Table 1. Fill is one constraint among several — bends between pull points are capped at 360° and pulling tension can still govern a long run.
Fill across every EMT trade size
Trade sizeInternal areaAllowableFillStatus
1/2"0.304 in²0.1216 in²40.1%Over limit
3/4" — selected0.533 in²0.2132 in²22.9%Compliant
1"0.864 in²0.3456 in²14.1%Compliant
1-1/4"1.496 in²0.5984 in²8.2%Compliant
1-1/2"2.036 in²0.8144 in²6.0%Compliant
2"3.356 in²1.3424 in²3.6%Compliant
2-1/2"5.858 in²2.3432 in²2.1%Compliant
3"8.846 in²3.5384 in²1.4%Compliant
3-1/2"11.545 in²4.6180 in²1.1%Compliant
4"14.753 in²5.9012 in²0.8%Compliant
Conductor area accounted for
ConductorArea eachQuantityTotal area
THHN 12 AWG0.0133 in²60.0798 in²
THHN 10 AWG0.0211 in²20.0422 in²
Total80.1220 in²

What this assumes

  • Conduit areas come from NEC Chapter 9, Table 4; conductor areas from Table 5.
  • Fill limits follow Chapter 9, Table 1: 53% for one conductor, 31% for two, 40% for three or more.
  • Does not account for conductor bundling ampacity adjustment, which is a separate calculation.

Important

Fill is only one constraint. Total bend degrees between pull points are limited to 360° by NEC 358.26 and equivalent articles, and pulling tension can govern long runs.

Understanding the result

The 40% rule

For three or more conductors, NEC Chapter 9 Table 1 limits fill to 40% of the raceway's internal cross-sectional area. The remaining space is not wasted — it is what allows conductors to be pulled without damaging insulation and lets heat dissipate.

Why one and two conductors are different

A single conductor may occupy 53% and two conductors only 31%. The two-conductor limit is stricter because two round conductors jam against each other in a round raceway, creating far more pulling friction than one or three.

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