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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

EMT conduit fill quick reference (40% — three or more conductors)

Maximum count of same-size THHN copper conductors per trade size at the 40% fill limit in NEC Chapter 9, Table 1. One- and two-conductor pulls allow 53% and 31% — use the calculator above for mixed sizes.

Maximum THHN conductors in EMT at 40 percent fill
Trade size40% allowable area14 AWG max12 AWG max10 AWG max8 AWG max
1/2"0.122 in²12953
3/4"0.213 in²2116105
1"0.346 in²3525169
1-1/4"0.598 in²61442816
1-1/2"0.814 in²83613822
2"1.342 in²1381006336

Source: NEC Chapter 9, Table 4 (EMT internal area) and Table 5 (THHN conductor area). Verify against locally adopted code and manufacturer listings.

Quick answer

NEC Chapter 9 limits how much conductor area can occupy a raceway: 53% fill for one conductor, 31% for two, and 40% for three or more. Exceeding fill makes pulling difficult and can violate code. The smallest trade size that keeps fill at or below the limit is what you need for your conductor mix. Enter conduit type, size, and conductors below; fill percentage and compliance are calculated from NEC Chapter 9 Tables 4 and 5.

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.

Equipment grounding conductors count

Every conductor that occupies space inside the raceway counts toward fill, including equipment grounding conductors. A common mistake is sizing for the ungrounded and neutral conductors only and then discovering the ground pushes the fill over the limit. If the run is tight, upsize the raceway before you pull.

When to upsize before you pull

If fill is above 35% on a long run with multiple bends, plan the next trade size up. Pulling at the legal maximum on a 200-foot run with three 90° bends is a recipe for damaged insulation. The cost of one larger stick of conduit is trivial compared with fishing damaged conductors back out.

NM cable versus individual conductors

Nonmetallic cable is measured by its overall cross-sectional area, not by the conductors inside. A 12/2 NM-B cable is roughly 0.20 square inches — larger than two individual 12 AWG THHN conductors at 0.0133 square inches each. Cable fill calculations routinely require a larger raceway than the same conductor count in THHN.

Common questions

How many 12 AWG wires fit in 1/2 inch EMT?
Nine 12 AWG THHN conductors fit in 1/2 inch EMT at the 40% fill limit. That is 0.120 square inches of conductor area against 0.304 square inches of usable raceway area. Ten conductors would exceed the limit and require 3/4 inch EMT, which accepts up to 16.
How many 10 AWG wires fit in 3/4 inch EMT?
Sixteen 10 AWG THHN conductors fit in 3/4 inch EMT. Each 10 AWG THHN is 0.0211 square inches, so 16 conductors total 0.338 square inches against the 0.533 square inch fill allowance. Seventeen conductors would require 1 inch EMT.
What is the maximum conduit fill percentage?
NEC Chapter 9 Table 1 sets three limits: 53% for one conductor, 31% for two conductors, and 40% for three or more. Most residential and commercial runs have three or more conductors, so 40% is the figure you use most often.
Does the ground wire count in conduit fill?
Yes. Every conductor inside the raceway counts, including equipment grounding conductors. A 3/4 inch EMT with three 12 AWG hots, three 12 AWG neutrals, and one 12 AWG ground is seven conductors total — not six.
How many 6 AWG wires fit in 1 inch EMT?
Five 6 AWG THHN conductors fit in 1 inch EMT. Each 6 AWG THHN is 0.0507 square inches, so five conductors total 0.254 square inches against the 0.864 square inch fill allowance at 40%. Six conductors would require 1-1/4 inch EMT.
What size conduit do I need for 4 circuits of 12 AWG?
Four 120V circuits with a shared neutral typically need eight current-carrying conductors plus a ground — nine conductors total. That requires 3/4 inch EMT, which accepts up to 16 twelve-AWG THHN conductors. Half-inch EMT tops out at nine.
How many 14 AWG wires fit in 1/2 inch PVC?
Eleven 14 AWG THHN conductors fit in 1/2 inch Schedule 40 PVC at 40% fill. PVC has the same internal area as EMT for the same trade size. Twelve conductors would require 3/4 inch PVC, which accepts up to 21.
Can you put Romex in conduit?
Yes, when protection is required — but you size the conduit from the cable's overall diameter, not the wire gauge inside. One 12/2 NM-B cable is about 0.20 square inches, so only two cables fit in 1/2 inch conduit. Three cables need 3/4 inch.
How many 8 AWG wires fit in 3/4 inch EMT?
Three 8 AWG THHN conductors fit in 3/4 inch EMT. Each 8 AWG THHN is 0.0366 square inches, so three total 0.110 square inches against the 0.533 square inch allowance. Four 8 AWG conductors require 1 inch EMT, which accepts six.
What happens if conduit is overfilled?
Overfilled conduit fails inspection, conductors are difficult or impossible to pull without damaging insulation, and ampacity derating may apply on top of the fill violation. The fix is pulling everything out and installing a larger raceway — far more expensive than sizing correctly upfront.

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