How to calculate conduit fill under the NEC

The method in one pass

Conduit fill compares two areas. One is the combined cross-section of every conductor in the raceway. The other is the part of the raceway's inside area that Chapter 9 lets you fill. If the wires take up no more than the allowed area, the conduit passes. The conduit fill calculator does exactly this; the steps below are what it does behind the button.

1. Find the internal area of the conduit in Chapter 9, Table 4 for its type and trade size (EMT, PVC Schedule 40 or 80, RMC, FMC).
2. Find the area of each conductor: Table 5 for insulated wire by type and size, Table 8 for bare conductors.
3. Multiply each conductor area by how many of that conductor you are pulling, then add them up.
4. Pick the Table 1 percentage from the total number of conductors, and multiply it by the Table 4 area to get the allowed area.
5. Compare. Used area at or under the allowed area passes. For same-size conductors, Note 7 can still let a borderline count through.

fill % = (sum of conductor areas) / (conduit internal area) x 100
passes when fill % <= the Table 1 percentage for that conductor count

Table 1: how full the conduit may be

Table 1 sets the limit by the number of conductors in the raceway, not by the wire size. Fewer wires get a higher percentage because a single conductor or a pair jams more easily than a bundle. The percentages are the same in NEC 2023 and NEC 2026.

Almost every real pull is three or more conductors, so 40% is the number you will use most. The count includes every conductor in the raceway. Chapter 9 Note 3 counts equipment grounding conductors, insulated or bare, so the green wire goes into the total. Fill percentages in Table 1 apply to the whole raceway, so a 3/4 in. EMT with 40% fill leaves most of its area empty on purpose: that room is what lets you pull the wire without damaging insulation.

Where the areas come from

Table 4 lists the total internal area for each conduit type and trade size. Each type has its own table because the wall thickness differs: 3/4 in. EMT has a bigger bore than 3/4 in. Schedule 80 PVC. Table 5 gives the overall area of an insulated conductor, including the insulation, by type and size. Table 8 gives the overall area of a bare conductor, which Chapter 9 Note 8 lets you use for bare conductors. The calculator uses the stranded values from Table 8.

Areas are in square inches and are used exactly as printed. Do not round them before you add. The table below shows a few values the calculator reads for EMT and THHN.

Example 1: same-size conductors and Note 7

Say you want to pull 6 #8 AWG THHN conductors through 3/4 in. EMT. That is more than two conductors, so the 40% limit applies.

EMT 3/4 in. internal area = 0.533 in.2
allowed area = 0.533 x 40% = 0.2132 in.2
#8 AWG THHN area = 0.0366 in.2 each
used area = 6 x 0.0366 = 0.2196 in.2
fill = 0.2196 / 0.533 = 41.20%

41.20% is over 40%, so by area alone this fails. But every conductor is the same size and type, and Note 7 applies. Divide the allowed area by one conductor's area and you get 5.83 conductors. The decimal is 0.8 or more, so Note 7 lets you round up to 6. Six conductors are allowed, and the calculator reports the pass with the Note 7 flag set (true). Five conductors would sit at 34.33% with no rounding needed.

Note 7 only works when every conductor has the same insulation and size. It does not apply to a mixed pull, and the calculator does not apply it to one or two conductors. The decimal has to reach 0.8; a smaller decimal rounds down.

Example 2: mixed sizes with a bare ground

Mixed pulls are the common case on a feeder or a multi-circuit home run. Take 4 x #10 THHN, 2 x #12 THHN, 1 x #12 bare. Add each group's area, then the groups.

4 x 0.0211 (THHN #10) = 0.0844 in.2
2 x 0.0133 (THHN #12) = 0.0266 in.2
1 x 0.006 (bare #12) = 0.0060 in.2
total = 0.1170 in.2 across 7 conductors (40% limit)

The smallest EMT that passes is 1/2 in. (internal area 0.304 in.2, allowed 0.1216 in.2). Fill there is 38.49%. Because the sizes are mixed, no Note 7 rounding is available, so the area comparison is the whole test. If the same wires only ran through a nipple of 24 in. or less, the 60% limit would still call for 1/2 in. EMT. You can check either answer on the 1/2 in. EMT page.

Nipples, charts and shortcuts

Note 4 allows 60% fill for a conduit or tubing nipple of 24 in. or less between boxes, cabinets and similar enclosures. It is not a licence to run a long sleeve at 60%: past 24 in., you are back to the Table 1 percentages.

For same-size wire you rarely need to add anything. The conduit fill chart lists the maximum number of conductors per trade size, already worked out with the 40% limit and Note 7. Here is EMT with THHN, computed from the same functions. See the full EMT conduit fill chart for every wire size, and the charts for PVC, RMC and FMC under the conduit fill chart.

Charts only cover one wire size at a time. As soon as you mix sizes or add a ground of a different size, go back to adding areas, or use the calculator. A 3/8 in. flexible metal conduit is a separate case: 348.20(A) restricts where it can be used, so the calculator does not size it with the Table 1 percentages. Start from the 3/4 in. EMT page if you want a single-size answer for the common trade size.

Conduit fill is only one limit. Check ampacity, voltage drop and any manufacturer or local requirement as well, and confirm with your AHJ which NEC edition is adopted.