Conduit Fill Calculator — NEC Chapter 9 Table 1, Table 4 and Table 5

Background.

Conduit fill is the rule that stops a raceway from being packed so tightly that the conductors cannot be pulled without stripping their insulation, and cannot shed the heat they make once they are in. It is governed by NEC Chapter 9, and the whole of Table 1 is three lines long: one conductor may occupy 53 percent of the raceway's cross section, two conductors 31 percent, and more than two 40 percent.

That sequence surprises people, because it is not monotonic. Two conductors get the smallest share of the three. The reason is mechanical rather than thermal: a pair of conductors twists around itself as it is pulled and jams, wedging against the conduit wall in a way that three or more do not, so the code gives a pair less room. A single conductor can lie centrally and move freely, so it gets the most.

The arithmetic is then a matter of two lookup tables. Chapter 9 Table 4 gives the internal diameter and cross-sectional area of every raceway type and trade size, along with pre-computed 31, 40, 53 and 60 percent columns. Chapter 9 Table 5 gives the approximate area of each insulated conductor. Add up the conductor areas, look up the allowance for your raceway, and compare. Trade size is a name rather than a measurement throughout — 3/4 inch EMT has an internal diameter of 0.824 inches and a total area of 0.533 square inches, of which 40 percent is 0.213.

Two notes to those tables change answers often enough to be worth knowing by number. Note (4) permits a conduit or tubing nipple not exceeding 600 mm — 24 inches — installed between boxes, cabinets and similar enclosures to be filled to 60 percent. That is a short-run allowance, not a general one, and it is the reason a gutter-to-panel nipple can carry conductors that would never be legal in the run feeding it. Note (7) says that when you calculate the maximum number of conductors all of the same size and the result has a decimal fraction, you take the next higher whole number if that fraction is 0.8 or larger. Note the threshold: 0.8, not 0.5. It is the only place in the code that rounds this way, and it is why 0.610 divided by 0.0211 — twenty-eight point nine conductors — becomes 29 rather than 28.

This calculator implements all four rules and reports both sides of the comparison, because the two sides do not carry the same rounding. The conductor area is exact. The allowance is the printed Table 4 column, which the code rounds to three decimals, and that is deliberately the number used: it is what an inspector reads. Where a mixed-size pull lands within 0.0005 square inches of the limit — inside half of that printed rounding — the result says the pull is marginal and names the next trade size, because Note (7) cannot rescue a pull whose conductors are not all the same size.

What passing this check does not mean is worth stating before the answer rather than after it. Conduit fill is a mechanical and pulling rule; it says nothing about ampacity. NEC 310.15(C)(1) derates every current-carrying conductor once there are more than three of them in a raceway, so a pull that satisfies Table 1 comfortably can still be illegal on ampacity, and that calculation is a separate one. Fill also says nothing about whether a pull is physically achievable: bends, run length and pulling tension are their own problem, and a 40-percent-full four-inch run with four ninety-degree bends is a bad idea that the code does not forbid. This page covers EMT, rigid metal conduit and rigid PVC Schedule 40 in trade sizes 1/2 inch through 4 inch, with THHN/THWN-2 or XHHW/XHHW-2 conductors in up to two sizes. The dimensions come from NFPA 70, National Electrical Code — read from a reproduction of the 2014 edition and cross-checked against the 2008 edition, which agree. Local amendments govern, the edition adopted in your jurisdiction may differ from the ones these tables were read from, and a licensed electrician or electrical engineer must sign off before any work proceeds.

What is conduit fill calculator?

Conduit fill is the fraction of a raceway's internal cross-sectional area that its conductors are permitted to occupy. It is a limit on crowding, imposed for two reasons at once. Mechanically, conductors have to be pulled in without their insulation being scraped off against each other or against the conduit wall, and a crowded raceway multiplies the tension needed to do it. Thermally, conductors bunched together cannot shed heat to the surrounding air, which is a separate rule but the same underlying physics.

The vocabulary is table-driven. Trade size is the nominal name of a conduit — 3/4 inch EMT is not 0.75 inches inside. Total area is the 100 percent column of Chapter 9 Table 4. The allowable area is the 53, 31, 40 or 60 percent column of that same table. Conductor area is the approximate area including insulation, from Chapter 9 Table 5, and it depends on the insulation family as much as on the copper: THHN and XHHW of the same AWG are different sizes. A nipple is a raceway of 24 inches or less between enclosures. Note (7) is the 0.8 round-up for same-size pulls.

The calculator is valid for EMT, rigid metal conduit and rigid PVC Schedule 40, trade sizes 1/2 inch to 4 inch, with THHN/THWN-2 or XHHW/XHHW-2 conductors in up to two sizes. It is not valid for flexible or liquidtight conduit, ENT, PVC Schedule 80, multiconductor cables, bare conductors, or a raceway mixing insulation families, each of which has its own row or its own note. It does not perform the ampacity adjustment of 310.15(C)(1) and it is not a pulling-tension calculation.

How to use this calculator.

1. 01Pick the raceway type first. EMT, RMC and PVC Schedule 40 of the same trade size have genuinely different internal areas — 1 in. gives 0.864, 0.887 and 0.832 in² respectively — and PVC is the tightest of the three.
2. 02Pick the trade size you intend to use. The result names the smallest trade size that would take the pull, so you can check whether you are oversizing.
3. 03Choose the insulation family. THHN/THWN-2 is ordinary building wire; XHHW/XHHW-2 is cross-linked polyethylene, and the two are not simply related — XHHW is fatter at small sizes and thinner in the middle of the range.
4. 04Enter the main conductor size and how many. Count every conductor in the raceway, neutrals and equipment grounding conductors included — Table 1 counts conductors, not circuits.
5. 05Add the second size if the pull is mixed, which it usually is because the ground is smaller. Set its count to 0 if everything is the same size, which is also what enables the Note (7) round-up.
6. 06Say whether the run is a nipple of 24 in. or less between enclosures. If it is, Note (4) raises the limit to 60 percent.
7. 07Read the verdict before the percentage. It states which Table 1 row applied, both areas, and whether the pull is marginal or is being carried by the Note (7) round-up.
8. 08Then do the ampacity adjustment separately. More than three current-carrying conductors in the raceway derates all of them under NEC 310.15(C)(1), and a pull that passes here can still fail that.

The formula.

Step one is the applicable percentage, and it comes from the conductor COUNT, not from their size. NEC Chapter 9 Table 1 gives 53 percent for one conductor, 31 percent for exactly two, and 40 percent for more than two. A nipple of 24 inches or less between enclosures takes 60 percent instead, under Note (4), whatever the count. Because the sequence dips at two, adding a third conductor to a pair raises the allowance from 31 to 40 percent of the same raceway, and a pull that fails as a pair can pass as a trio.

Step two is the used area. Every conductor's approximate area comes from Chapter 9 Table 5 and includes its insulation, so the insulation family matters as much as the AWG. The worked example is 3 × 6 AWG THHN at 0.0507 in² each and 1 × 10 AWG THHN at 0.0211 in², which is 0.1521 + 0.0211 = 0.1732 in². Nothing is rounded here.

Step three is the comparison. Four conductors means the 40 percent row, and the 40 percent column for 3/4 inch EMT in Chapter 9 Table 4 reads 0.213 in². Since 0.1732 is less than 0.213, the pull passes with 0.0398 in² to spare. As a percentage of the raceway's 0.533 in² total area, that is 32.4953095685 percent fill.

The comparison uses the printed Table 4 column rather than a recomputed percentage of the total area, and the difference is real if small. Those columns are π·d²/4 times the percentage, rounded to three decimals — a fact this page verifies rather than assumes: of the 150 numbers in the three raceway tables it carries, 149 reproduce exactly from their own printed internal diameter, and every one of the 48 conductor areas reproduces exactly from its printed diameter. The single exception is rigid metal conduit at trade size 3 and 53 percent, where the code prints 3.974 in² and the exact geometry gives 3.9745, which would round to 3.975. The printed 3.974 is what this page uses, because it is what the code says.

Note (7) is the last piece and it applies only to conductors all of the same size. Divide the allowance by one conductor's area; if the fractional part of the result is 0.8 or larger, take the next whole number up. In 1-1/4 inch rigid metal conduit the 40 percent column is 0.610 in², and 0.610 ÷ 0.0211 = 28.909, whose fractional part is 0.909 — so 29 conductors of 10 AWG THHN are permitted, even though 29 × 0.0211 = 0.6119 in² is fractionally above the column. That overrun is the code's own allowance, and this page reports it as such rather than hiding it or refusing it. Mix two sizes in the same raceway and the note no longer applies, so a mixed pull is judged strictly on area.

A worked example.

Frequently asked questions.

References& sources.

1. [1]NFPA 70, National Electrical Code, Chapter 9, Table 1 'Percent of Cross Section of Conduit and Tubing for Conductors' — the source of the three fill limits: 1 conductor 53 percent, 2 conductors 31 percent, over 2 conductors 40 percent. Authority: NFPA. Document revision: 2014 edition, as reproduced. Locator: Chapter 9 Table 1. Access: gated at NFPA (free-access registration); open at the reproduction. Independence: primary text, secondary host. Retrieved 2026-07-29.
2. [2]NFPA 70, Chapter 9, Table 4 'Dimensions and Percent Area of Conduit and Tubing' — the numeric source for every internal diameter, total area and 31/40/53/60 percent column used on this page, for Article 358 EMT, Article 344 rigid metal conduit and Article 352 rigid PVC Schedule 40, trade sizes 1/2 in. through 4 in. Authority: NFPA. Document revision: 2014 edition, as reproduced. Locator: Chapter 9 Table 4. Access: gated at NFPA; open at the reproduction. Independence: primary text, secondary host. Retrieved 2026-07-29.
3. [3]NFPA 70, Chapter 9, Table 4 — the same table as reproduced for the 2008 edition, used as a cross-edition check on the trade size 2-1/2 in. and 3-1/2 in. rows of rigid metal conduit and rigid PVC Schedule 40. Every value agrees with the 2014 reproduction, confirming that these dimensions did not move between the two editions. Authority: NFPA. Document revision: 2008 edition, as reproduced. Locator: Chapter 9 Table 4. Access: open. Independence: secondary-check. Result: agree. Retrieved 2026-07-29.
4. [4]NFPA 70, Chapter 9, Table 5 'Dimensions of Insulated Conductors and Fixture Wires' — the numeric source for the approximate area and approximate diameter of every THHN/THWN/THWN-2 and XHH/XHHW/XHHW-2 conductor from 14 AWG to 1000 kcmil used on this page. Both the areas and the diameters were taken, so that each area could be independently recomputed as π·d²/4; all 48 reproduce exactly. Authority: NFPA. Document revision: 2014 edition, as reproduced. Locator: Chapter 9 Table 5. Access: gated at NFPA; open at the reproduction. Independence: primary text, secondary host. Retrieved 2026-07-29.
5. [5]Wiring Done Right, 'Conduit Fill Calculation' — the source consulted for the wording and numbering of NEC Chapter 9 Note (7): 'When your calculations results land in a decimal of point .8 or larger you must adjust to the next higher number of conductors. NEC Chapter 9 Notes to tables Note 7', together with the nipple rule that 'a nipple less than 24 inches long shall be calculated at 60% fill allowed instead of the usual 40% fill'. Authority: Wiring Done Right (electrical training). Locator: Notes to Tables, Notes 4 and 7. Access: open. Independence: secondary-check. Retrieved 2026-07-29.
6. [6]ExpertCE, 'How to Use NEC Chapter 9, Table 1 for Percent Conduit Fill' — independent confirmation of the 53/31/40 percent limits, of the mechanical reasoning behind the dip at two conductors (jamming during a pull), and that the nipple allowance is Note 4 to Chapter 9. Cites the 2023 NEC for its worked example, which is how this page checks that Table 1 is unchanged from the edition its dimensions were read from. Authority: ExpertCE (electrical continuing education). Document revision: 2023 NEC. Access: open. Independence: secondary-check. Result: agree. Retrieved 2026-07-29.
7. [7]WireRef, 'EMT Conduit Fill & Wire Fill Chart' — a reproduction of NEC 2023 Annex C, Table C.1, used as the independent check on this page's maximum-conductor arithmetic (BUILD-BRIEF §10.5). Fifty-four cells were compared for EMT with THHN/THWN-2 conductors from 14 AWG to 1 AWG in trade sizes 1/2 in. to 2 in.: 52 agree exactly, and the two that do not are both in the 6 AWG row, where this page returns the lower, more conservative count. Both disagreements are asserted in conduit-fill.test.ts and analysed in the dossier. Authority: WireRef. Document revision: NEC 2023 Annex C. Access: open. Independence: primary check. Result: agree on 52 of 54, conflict on 2. Retrieved 2026-07-29.