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

Background.

Sixteen 12 AWG THHN conductors fit in 3/4-inch EMT; the seventeenth does not. The 40 percent column for that raceway in NEC Chapter 9 Table 4 is 0.213 square inches, each conductor occupies 0.0133 square inches in Table 5, and 16 of them come to 0.2128. Nearly every conduit-fill question is that sum: the area of the insulated conductors you plan to pull, set against the share of the raceway's cross section the code lets them occupy.

The share comes from Chapter 9 Table 1, which is only three lines long. One conductor may fill 53 percent of the raceway, two conductors 31 percent, and more than two 40 percent. The dip at two is deliberate. The usual explanation is mechanical: a pair can twist and wedge against the conduit wall during a pull in a way that a single conductor or a bundle of three does not, so the code gives a pair the least room. The rule exists to protect insulation from being scraped off as conductors are dragged in, and to keep a raceway from being packed so tightly that the conductors cannot be pulled at all.

Two notes to the tables change answers often enough to know by number. Note (4) lets a nipple, a raceway no longer than 600 mm (24 inches) between boxes, cabinets and similar enclosures, be filled to 60 percent. Note (7) applies when every conductor is the same size: if dividing the allowance by one conductor's area leaves a fraction of 0.8 or more, the next whole number is permitted. The threshold is 0.8, not 0.5, and it lets a same-size pull sit fractionally above the printed column.

Passing the fill check settles one question out of several. Fill says nothing about ampacity: under NEC 310.15(C)(1), more than three current-carrying conductors in one raceway reduces the allowable ampacity of every one of them, so a pull with plenty of room can still be illegal. It says nothing about pulling tension, bends or run length either. The figures here come from a reproduction of NFPA 70's 2014 edition, cross-checked against the 2008 edition; jurisdictions adopt editions on their own schedules and amend them locally, so confirm against the code in force where you work, and have a licensed electrician or electrical engineer sign off before any work proceeds.

What is conduit fill calculator?

Conduit fill is the percentage of a raceway's internal cross-sectional area taken up by the conductors inside it, insulation included. NEC Chapter 9 caps it by conductor count in Table 1 and supplies the two dimensions needed to check it. Table 4 gives each raceway type and trade size an internal diameter, a total area and pre-computed 31, 40, 53 and 60 percent columns. Table 5 gives the approximate area of each insulated conductor, which depends on the insulation as much as on the copper. Trade size is a name rather than a measurement: 3/4-inch EMT has an internal diameter of 0.824 inches and a total area of 0.533 square inches. This calculator covers EMT, rigid metal conduit and rigid PVC Schedule 40 from 1/2 to 4 inches, with THHN/THWN/THWN-2 or XHH/XHHW/XHHW-2 conductors in up to two sizes. Flexible and liquidtight conduit, ENT and Schedule 80 PVC have their own Table 4 rows and are not included, and nor are multiconductor cables, bare conductors or a raceway that mixes insulation families, each of which has its own note.

How to use this calculator.

1. 01Pick the raceway type. EMT, rigid metal conduit and PVC Schedule 40 of the same trade size differ inside: at 1 inch their total areas are 0.864, 0.887 and 0.832 square inches, so PVC is the tightest of the three.
2. 02Pick the trade size you plan to install. The result also names the smallest trade size of that type that would take the pull, which shows whether you are oversizing.
3. 03Choose the insulation family you will actually pull, then enter the main conductor size and how many of it. Count every conductor in the raceway, neutrals and equipment grounding conductors included; Table 1 counts conductors, not circuits.
4. 04If the pull has a second size, usually a smaller ground, enter it and its count. Set that count to 0 when every conductor is the same size, which is also the only case in which the Note (7) round-up can apply.
5. 05Choose Nipple only for a raceway of 24 inches or less between enclosures; everything else is an ordinary run at the Table 1 limit.
6. 06Read the verdict before the percentage, then work out the ampacity adjustment separately: a pass here does not cover conductor derating.

The formula.

The check runs in three steps. First, the conductor count picks the limit: 53 percent for one conductor, 31 percent for two and 40 percent for three or more, or 60 percent for a nipple under Note (4) whatever the count. Second, the used area is the sum of n × a over the conductor groups, where n is the number of conductors of a size and a is its Table 5 area; nothing in that sum is rounded. Third, the used area is compared with the printed Table 4 column for the chosen limit, not with a percentage recomputed from the total area, because the printed column is what an inspector reads off the page. The fill percentage reported alongside, used area ÷ total area × 100, is information; the verdict rests on the areas.

A pull of a single size is judged by count instead. Dividing the column by one conductor's area gives the most that fit, rounded down unless the fraction is 0.8 or more, in which case Note (7) allows one more. In 1-1/4-inch rigid metal conduit the 40 percent column is 0.610 square inches and 10 AWG THHN is 0.0211, so 0.610 ÷ 0.0211 = 28.91 and 29 conductors are permitted, even though 29 of them occupy 0.6119 square inches. The calculator applies this round-up to the 40 percent and nipple columns; when it tests whether one conductor fits under 53 percent or two under 31 percent, it compares areas without a round-up. A pull mixing two sizes is always judged strictly on area, and when the margin is under 0.0005 square inches, half the rounding step of the three-decimal column, the verdict calls it marginal.

The Table 4 columns are π × d² ÷ 4 times the percentage, rounded to three decimals. Recomputing all 150 raceway values from their own printed diameters reproduces 149 of them exactly; the exception is 3-inch rigid metal conduit at 53 percent, printed as 3.974 where the geometry gives 3.9745, and the printed value is used. All 48 conductor areas likewise reproduce from their printed diameters.

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. 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. 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. 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. 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. 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. 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. 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. Authority: WireRef. Document revision: NEC 2023 Annex C. Result: agree on 52 of 54, conflict on 2. Retrieved 2026-07-29.