Conduit Fill Calculator
Check conduit fill against NEC Chapter 9 (53%, 31%, 40%) for EMT, PVC, RMC, IMC and flex with THHN, XHHW and bare grounds, and find the smallest size that passes.
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About this toolWhat is the Conduit Fill Calculator?
Every raceway has a legal limit on how much of its inside area the wires may take up. The National Electrical Code sets it in Chapter 9, Table 1: 53% for a single conductor, 31% for two, and 40% for three or more. This conduit fill calculator adds up the cross-sectional area of every conductor you list, compares it with the internal area of the raceway you pick, and tells you whether the fill passes, how much room is left, and the smallest trade size of that raceway that would work.
You can mix sizes and insulation types in one run (for example six 12 AWG THHN, three 10 AWG THHN and a bare 10 AWG ground), add a multiconductor cable by its outside diameter, and switch between nine raceway types: EMT, ENT, FMC, IMC, LFNC-B, LFMC, RMC, PVC Schedule 40 and PVC Schedule 80. A second panel gives an Annex C style count: the most conductors of one size that fit in every trade size.
- Nine raceway types from NEC Chapter 9 Table 4, 1/2 in to 6 in
- THHN/THWN-2, XHHW-2, TW/THW and bare copper from Tables 5 and 8
- Mixed sizes, cables by diameter in inches or mm, and the 60% nipple rule
- Fill bar, spare area and the smallest passing trade size
- Annex C style table with the Note 7 rounding
- Results in in² or mm², shareable link, nothing sent to a server
How to use the Conduit Fill Calculator
- Pick the raceway type and trade size. Tick the nipple box only for a length of 24 in or less.
- List each conductor group: insulation type, size and count. Include neutrals and every equipment grounding conductor.
- Add a cable by diameter for NM, MC or other multiconductor cable; it counts as one conductor.
- Read the fill percentage, the limit that applies to your conductor count and the spare area. The table shows every trade size and marks the smallest that passes.
- Use the Annex C panel to see how many conductors of one size fit in each trade size.
Worked examples
Branch circuits in 3/4 EMT
Input
6 × 12 AWG THHN, 3 × 10 AWG THHN, 1 × 10 AWG bare stranded, 3/4 EMT
Output
0.1541 in² of 0.533 in² = 28.9% fill, limit 40%, passes with 0.0591 in² spare
Ten conductors means the 40% column applies, giving 0.2132 in² allowed. The bare ground counts toward fill at its Table 8 area of 0.011 in².
Most 12 AWG THHN in EMT
Input
12 AWG THHN, EMT, Annex C panel
Output
9 in 1/2, 16 in 3/4, 26 in 1, 45 in 1-1/4, 61 in 1-1/2, 101 in 2
For 1-1/4 EMT, 0.5984 ÷ 0.0133 = 44.99, which Note 7 lets you round up to 45. These match Informative Annex C Table C.1.
Feeder in PVC Schedule 40
Input
4 × 4/0 AWG THHN + 1 × 4 AWG THHN ground
Output
1.3772 in²: 2 in is 41.8% (fails), 2-1/2 in is 29.3% (passes)
The smallest Schedule 40 PVC that holds this set is 2-1/2. Switching to Schedule 80 for a run exposed to damage would need a larger trade size again.
Two conductors and the 31% rule
Input
2 × 6 AWG THHN in 1/2 EMT
Output
0.1014 in² against 0.0942 in² allowed (31%): over the limit
Two round conductors are the tightest case. One 6 AWG fits at 53%, three do not fit at 40%, and two fail at 31%.
One cable in a sleeve
Input
One 0.5 in OD cable, 3/4 EMT
Output
0.1963 in², 36.8% fill, 53% limit, passes
A cable counts as a single conductor, so the 53% column applies.
Background and insight
The calculation uses three tables from NEC Chapter 9. Table 4 lists the internal diameter and total area of each raceway type by trade size; the areas differ a lot between types, so a 1/2 in PVC Schedule 80 has only 0.217 in² against 0.304 in² for 1/2 in EMT. Table 5 gives the approximate area of insulated building wire, which depends on the insulation as much as the copper: 12 AWG THHN is 0.0133 in², while 12 AWG XHHW and THW are 0.0181 in². Table 8 gives bare conductor areas for equipment grounding conductors. The calculator holds the values from the 2020 and 2023 editions, which did not change for these rows.
The percentages are not arbitrary. One conductor can be pulled through 53% because it lies along the bottom of the pipe. Two conductors are limited to 31% because two round wires side by side are as wide as the pipe allows before they jam. Three or more settle into a bundle and can occupy 40%. Note 4 allows 60% in a nipple of 24 in (600 mm) or less, because there is no long pull. Note 7 lets same-size conductors round up when the count comes out at .8 or more, which is how Informative Annex C, the familiar "how many wires fit" tables, was built. Note 9 treats a multiconductor cable as one conductor, using its outside diameter.
Fill is only one of several checks. More than three current-carrying conductors in a raceway reduce each conductor's ampacity under 310.15(C)(1), so a pipe that passes fill can still need bigger wire. Three conductors with a diameter ratio near 2.8 to 3.2 can jam on a pull, and long runs with many bends need pulling-tension calculations. Equipment grounding conductors always count toward fill, even though they do not count as current-carrying.
Common use cases
Electrical estimating and design
Pick raceway sizes for branch circuits and feeders before ordering material or drawing a one-line.
Field checks
Confirm that one more circuit will fit in an existing run before pulling it.
Exam preparation
Practise Chapter 9 fill questions for journeyman and master electrician exams with the working shown.
Plan review and inspection
Check a submitted conduit schedule quickly, including mixed sizes and insulation types.
Tips and limitations
- XHHW and THW insulation is thicker than THHN at small sizes, so the same circuits need more room. Use the actual insulation on the reel.
- Leave spare capacity for future circuits; many engineers design to 30 to 35% rather than the 40% limit.
- Ampacity adjustment for more than three current-carrying conductors is separate from fill. A full 3/4 EMT with nine circuits can force an upsize in wire, not pipe.
- 3/8 in FMC and LFMC are filled from NEC Table 348.22, not Chapter 9, so they are not listed here.
- Annex C Table C.1 as commonly reprinted shows 2 conductors in a few cells where two wires exceed 31% (for example 6 AWG THHN in 1/2 EMT). This calculator applies Table 1 strictly and reports 1 there.
- Local amendments and the code edition your jurisdiction adopted govern. Treat the results as a design aid and check with the authority having jurisdiction.
Frequently asked questions
What is the 40% conduit fill rule?
NEC Chapter 9 Table 1 limits three or more conductors to 40% of the raceway's internal area. One conductor may use 53% and two conductors 31%. Nipples of 24 in or less may be filled to 60%.
How many 12 AWG THHN wires fit in 3/4 inch EMT?
Sixteen. 3/4 EMT has 0.533 in², 40% of that is 0.2132 in², and 12 AWG THHN is 0.0133 in², so 0.2132 ÷ 0.0133 = 16.03.
Does the ground wire count toward conduit fill?
Yes. Equipment grounding and bonding conductors count toward fill, using their Table 5 area if insulated or Table 8 area if bare. They do not count as current-carrying conductors for ampacity adjustment.
How do I calculate conduit fill with different wire sizes?
Multiply each conductor area from Table 5 by its count, add the results, and divide by the raceway area from Table 4. Compare the percentage with 53%, 31% or 40% depending on the total number of conductors.
Why is two-wire fill only 31%?
Two round conductors side by side are the most likely to jam in a pipe because their combined width approaches the inside diameter. The lower limit keeps pulls practical.
How is a cable counted in conduit fill?
Chapter 9 Note 9 treats a multiconductor cable as one conductor with an area based on its outside diameter. For an elliptical cable such as flat NM, use the larger dimension as the diameter.
Is PVC Schedule 80 smaller inside than Schedule 40?
Yes. Schedule 80 has thicker walls with the same outside diameter, so 1 in Schedule 80 has 0.688 in² against 0.832 in² for Schedule 40. Always use the area for the exact raceway type.
What is the Note 7 rounding rule?
When all conductors are the same size and the calculated count ends in .8 or more, the next whole number is permitted. 12 AWG THHN in 1-1/4 EMT works out at 44.99, so 45 are allowed.
Is there an in-depth guide to the Conduit Fill Calculator?
Yes. “NEC Conduit Fill Explained: The 53, 31 and 40 Percent Rules” covers the background, how it works, common pitfalls and worked examples. Read the full guide.
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