Conduit Fill Chart — NEC Chapter 9

How many wires fit in a conduit?

The answer is never a single number per conduit size, because it depends on the conductor size, the insulation type and how many conductors you are installing. What the code does fix is the percentage of the conduit's internal area the conductors may occupy, and then Chapter 9 gives you the two pieces of arithmetic needed to turn that into a count: the fill area of the conduit (Table 4) and the area of one conductor (Table 5).

For the most common case — three or more THHN conductors in EMT — the numbers worth memorising are small. Half-inch EMT holds nine #12s. Three-quarter-inch EMT holds sixteen. One-inch EMT holds twenty-six. Those three figures answer the great majority of "how many wires in 3/4 conduit" searches, and every one of them comes straight out of the tables on this page rather than from a rule of thumb.

The conduit fill formula, shown explicitly

The calculation in Chapter 9 is short enough to hold in your head once you have seen it written out:

The conduit's total area is the last column of Table 4 — the 100% figure. The 40%, 60%, 53% and 31% columns in the same table are already worked out for you, so you can skip the first step and divide straight out of the 40% column whenever you are installing more than two conductors. Everything on this page is computed the same way: from the total area and the percentage, never transcribed from a secondary chart.

The fill percentages — Chapter 9 Table 1

The three percentages are not arbitrary. A single conductor sits near the centre of the raceway and pulls easily, so it may use more of the area; two conductors naturally align and jam more readily, so they are held to a tighter 31%; and above two, the 40% figure is the working limit. A nipple of 24 in. or less between enclosures is the one exception that may be filled to 60% (Note 4).

Table 4 — conduit and tubing fill areas

Table 4 lists, for every raceway type and trade size, the nominal internal diameter and the area at each percentage. The tables below give the 40%, 60%, 53% and 31% areas plus the total (100%) area, in square inches, for the raceways that carry most residential and light commercial work. Where a trade size is not manufactured in a given type the row is simply absent, exactly as in the code.

Table 5 — conductor areas for the wire you actually pull

The conductor side of the calculation depends on the insulation. THHN, THWN and THWN-2 are the types installed in raceway almost all of the time, and Table 5 gives their approximate area by size. Other insulations are larger — RHH, RHW and RHW-2, for instance, have areas roughly twice those of THHN at the same gauge, and XHHW has its own column in Table 5A. If you are pulling one of those, use its actual dimensions rather than the THHN figures below.

Quick counts — THHN in EMT, ready to read

The single table below answers the questions people actually type, without any division: the maximum number of THHN conductors in each EMT trade size, for the six conductor sizes used in branch circuits. The numbers are computed from the two tables above using Note 7, not copied from a manufacturer flyer.

Worked example — checking three-quarters EMT by hand

You are pulling #12 THHN for a lighting circuit and want to know the largest conduit a single run will take at the 40% limit.

• Conduit total area. Table 4 gives 3/4 EMT a 40% area of 0.213 in² and a total (100%) area of 0.533 in². Check: 0.533 × 0.40 = 0.213. ✓
• One conductor. Table 5 gives a #12 THHN an approximate area of 0.0133 in² (8.581 mm²).
• Divide. 0.213 ÷ 0.0133 = 16.0. The quotient is just over 16, so sixteen #12 conductors land inside 40% of the raceway, and a seventeenth would not. Answer: 16.
• Check the same in millimetres. 137 mm² (the Table 4 40% figure) ÷ 8.581 mm² = 15.97 — a decimal of 0.97, which Note 7 rounds up to 16. The rounding rule is exactly what rescues this case: 15.97 is not 16 by truncation, it is 16 by the code's own note.
• Swap to #10. 0.213 ÷ 0.0211 = 10.1, so ten #10s. To the next size up: 0.213 ÷ 0.0366 (a #8) = 5.8, which the 0.8 rule rounds to 6.

Common mistakes with conduit fill

• Leaving out the equipment grounding conductor. Note 3 counts it, at its actual dimensions. A run of nine #12s plus a green ground is ten conductors, and at 3/4 EMT that is still fine — but nine hot conductors plus the ground is ten, and it is the ground that people forget to count.
• Using the wrong fill column. The 40% column applies to more than two conductors. If you are installing exactly two, the limit is 31%; a single conductor gets 53%. Reading the 40% column for a two-wire circuit overstates what fits.
• Counting each conductor in a cable separately. Note 9 makes a multiconductor cable, optical fiber cable or flexible cord of two or more conductors count as a single conductor for fill. An assembly of individual insulated conductors with no overall jacket is not a cable and is counted conductor by conductor.
• Mixing insulation types in your head. A #12 is not a #12 is not a #12 — RHH and RHW-2 take far more area than THHN at the same gauge. Use the row for the insulation you are actually installing.
• Applying the 60% nipple allowance to a real run. The 60% figure is for a nipple of 24 in. or less between boxes and enclosures. It is not a general allowance for a crowded pull.
• Stopping at the physical count. Passing the fill check does not settle ampacity. More than three current-carrying conductors in a raceway triggers the 310.15(C)(1) adjustment — see the ampacity chart for the factors and the subpanel wire size guide for what that does to a feeder.

Conduit fill FAQ

Where these numbers came from

Chapter 9 Tables 1, 4 and 5 were read on 2026-10-02 from the NFPA-licensed single-user copy of the NEC Chapter 9 Tables PDF and cross-checked two ways: against the Western Tube conduit-wire fill chart (itself referenced from Annex C of an earlier edition) and against the NECA/IBEW Annex C Table C.1 reproduction, which agree on every small-conductor cell on this page. The Tables 4 areas and the Table 5 THHN areas have been stable for decades, but the code is adopted and reissued, so the page states the method rather than a single printed count. Confirm the figures against the edition your authority having jurisdiction enforces.

This is a working reference, not engineering. Fill is a starting check: it tells you how many conductors physically fit, not whether the circuit is correctly sized. Conductor ampacity, the 310.15 adjustment factors, box fill and the termination temperature rules all apply on top of it.

Related pages

• Wire ampacity chart — NEC Table 310.16 — the current a conductor may carry, and the ambient and bundle derating factors
• Breaker size chart — NEC 240.6(A) & 240.4(D) — the device at the head of the circuit and the wire it protects
• Voltage drop calculator — sizing a run for distance once the conduit is chosen
• Subpanel wire size guide — a full 100 A feeder worked through with the code sections
• Reference tables — every code table, with its source and verification date