Conduit Fill Calculator — NEC Chapter 9

How conduit fill works

When to use this: deciding what size conduit to buy before a run, checking whether you can add another circuit to an existing raceway, or verifying an inspector's objection.

Three NEC Chapter 9 tables do all the work:

• Table 1 — what percentage of the conduit's cross-section the conductors may occupy. Depends only on how many there are.
• Table 4 — the internal area of each conduit type and trade size.
• Table 5 — the area of each insulated conductor.

Add up the conductor areas, divide by the conduit area, and compare against the Table 1 percentage. That is the entire calculation.

The fill limits — and the one that surprises people

Two conductors are limited to 31% — less than three or more. That looks like a misprint and it is not. Two round conductors side by side in a round conduit leave the most awkward unusable space, and they are the hardest case to pull because neither can shuffle past the other. Three or more pack better and move more freely.

Most people assume 40% applies universally. On a two-wire run — a switch leg, a 240 V circuit with no neutral — that assumption puts you 29% over the real limit.

Why 40% and not 100%?

The conductors physically fit at 70% or 80%. The limit is not about space, it is about two practical problems:

• Pulling. Above roughly 40%, conductors jam against each other and against the conduit wall. The pulling force needed rises sharply, and the insulation gets scraped off against the raceway — creating a fault that will not show up until the circuit is energised, or years later.
• Heat. Conductors in the middle of a tight bundle cannot get rid of their heat. That is a separate concern from ampacity derating, and both apply.

How many conductors fit? — EMT with THHN

At the 40% limit. This is the table most people actually want.

Note how fast large conductors eat a conduit. One 4/0 is 0.3237 in² — more than the entire internal area of 1/2 inch EMT.

Insulation type changes the answer

The conductor area in Table 5 is the insulated area, so insulation thickness matters as much as the copper:

THW 12 AWG is 0.0260 in² against THHN's 0.0133 — almost double. Nine THHN 12 AWG conductors fit in 1/2 inch EMT; only four THW do. Specifying the wrong insulation on a materials list can cost you a conduit size across an entire job.

Conduit type matters too

Internal area at the same trade size, square inches:

IMC is roughly 12% roomier than EMT at the same trade size; PVC Schedule 80 is about 28% tighter. "1 inch conduit" is not one thing.

Fill and derating are different limits

This is the most common misunderstanding, and both must be satisfied independently.

Derating factors once you pass three:

So a 3/4 inch EMT holding nine 12 AWG conductors is comfortably within fill — but those conductors are derated to 70%, and 12 AWG THHN at 30 A becomes 21 A, which no longer supports a 20 A breaker with the required margin. Legal on fill, non-compliant on ampacity.

Worked example

Two 20 A circuits plus a shared ground, in EMT:

4 × 12 AWG THHN (hots and neutrals) = 4 × 0.0133 = 0.0532 in²
1 × 14 AWG THHN (ground) = 0.0097 in²
Total = 0.0629 in², 5 conductors → 40% limit

1/2 inch EMT is 0.304 in², so 40% is 0.1216 in². The bundle at 0.0629 in² is 20.7% fill — comfortably compliant.

But four of those five are current-carrying, so derating applies at 80%. The 12 AWG THHN drops from 30 A to 24 A, which still supports a 20 A breaker. Both limits satisfied.

What this does not cover

• Bends. No more than 360° of bends between pull points, regardless of fill. Four 90° elbows and you must install a pull box.
• Different-diameter mixtures beyond three groups. For complex pulls, sum the Table 5 areas by hand.
• Cables rather than conductors. A cable assembly such as MC or NM uses its overall diameter, not the sum of its conductors.
• Jurisdiction. Local amendments exist, and some authorities are stricter than the NEC baseline.