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Conduit fill calculator

Fill is the easy half. The half that costs money is what happens when more than three current-carrying conductors share the pipe, because that triggers an ampacity adjustment that can force you up a wire size, which changes the fill again. This runs both checks at once.

The raceway

Type

Trade size what you planned to run

Current-carrying conductors leave out the EGC, and the neutral on a balanced 3-wire

What is going in it THHN / THWN-2

Conductor 1 size

Conductor 1 quantity

Conductor 2 size

Conductor 2 quantity

Conductor 3 size

Conductor 3 quantity

Conduit fill

0%

Area used

0

Allowed in this size

0

Smallest size that fits

—

Ampacity adjustment

100%

The math

Built on Chapter 9 Tables 1, 4 and 5, with the adjustment factors from 310.15(C)(1). This is a working tool, not a substitute for the codebook or your inspector. Local amendments override the national text, and the licensed person signing the permit owns the result. Check which edition your AHJ has adopted.

The two limits people mix up

Chapter 9 Table 1 gives you the fill percentage, and it depends on how many conductors are in the pipe, not how big they are. One conductor gets 53 percent of the interior. Two conductors get 31 percent, which is the one that surprises people, because two is stricter than three. Three or more get 40 percent.

That is the only thing fill tells you. It does not tell you the pull will go, it does not tell you the conductors are big enough, and it does not tell you anything about heat.

The adjustment that actually costs you

310.15(C)(1) is the rule that turns a fill question into a money question. Run more than three current-carrying conductors in the same raceway and every one of them loses ampacity: four to six conductors get 80 percent, seven to nine get 70 percent, ten through twenty get 50 percent.

Put four 12 AWG circuits in one pipe and your 20 amp circuits are running on conductors adjusted down to 24 amps at the 90 degree column, which still works. Put ten in and you are at 15 amps of adjusted ampacity on a conductor you intended to protect at 20, and now you are pulling 10 AWG for a general purpose circuit. The conduit got bigger, the wire got bigger, and the labor went up, all because of a decision that looked like it was about pipe size.

Counting correctly matters here. The equipment grounding conductor is not a current-carrying conductor. The neutral of a balanced three-wire circuit generally is not counted either. The neutral of a three-phase four-wire circuit feeding nonlinear load is, and that one catches people on lighting jobs.

Four quarter bends, not four bends

358.26 and the parallel sections for the other raceway types cap you at the equivalent of four quarter bends, 360 degrees total, between pull points. Two offsets and a ninety is already 270 degrees before you have left the wall. Fill and bend count are separate limits and the one that stops a pull is usually the bends.

What this uses

Areas are the THHN and THWN-2 values from Chapter 9 Table 5, and raceway interiors are Table 4. If you are pulling XHHW-2, USE, or anything with a different jacket, the conductor areas change and you want the row for your actual insulation out of Table 5. The raceway numbers do not change.

Common questions

What is the maximum conduit fill percentage?

It depends on the number of conductors, not their size. Chapter 9 Table 1 allows 53 percent of the interior area for one conductor, 31 percent for two, and 40 percent for three or more. Two conductors being stricter than three surprises people, but that is what the table says.

Why is the fill limit for two conductors lower than for three?

The percentages come from the geometry of how round conductors pack inside a round raceway plus the room needed to pull them. Two conductors of the same size leave an awkward amount of unusable space between them, so the usable fraction is lower. It is a packing problem, not a heat problem.

Does the equipment grounding conductor count toward conduit fill?

Yes for fill, no for ampacity adjustment. Every conductor in the raceway takes up area, so the EGC counts in the fill calculation. It is not a current-carrying conductor, so it does not count toward the number that triggers the 310.15(C)(1) adjustment.

When do I have to derate for conductors in a raceway?

When more than three current-carrying conductors run in the same raceway or cable for more than 24 inches. 310.15(C)(1) gives 80 percent for four to six, 70 percent for seven to nine, 50 percent for ten to twenty, 45 percent for twenty-one to thirty, 40 percent for thirty-one to forty, and 35 percent above that.

Does the neutral count as a current-carrying conductor?

Usually not on a balanced circuit. The neutral of a two-wire circuit counts. The neutral of a balanced three-wire single-phase or four-wire three-phase circuit generally does not. It does count on a four-wire three-phase circuit where the load is nonlinear, which covers a lot of LED and electronic lighting work.

How many bends can I have between pull points?

The equivalent of four quarter bends, 360 degrees total. That is in 358.26 for EMT and in the equivalent section for each other raceway type. Offsets count toward it, so two offsets and a ninety puts you at 270 degrees already.

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Link to this calculator

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Instructors, JATC training coordinators, and apprenticeship programs: use this in class, put the link on a syllabus, or print the output for a handout. If you want an input added or a print-friendly version for your curriculum, tell me what you need and I will build it.

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Conduit Fill Calculator

Conduit fill calculator

The math

The two limits people mix up

The adjustment that actually costs you

Four quarter bends, not four bends

What this uses

Common questions

Link to this calculator

More free tools

Related reading

Conduit Fill Calculator

Conduit fill calculator

The math

The two limits people mix up

The adjustment that actually costs you

Four quarter bends, not four bends

What this uses

Common questions

Link to this calculator

More free tools

Related reading

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