Conduit fill is one of the most frequently referenced calculations in electrical work. NEC Chapter 9 limits how many conductors you can pull through a raceway to prevent damage during installation and to allow adequate heat dissipation during operation. Overfilling conduit makes pulling difficult, risks insulation damage, and violates code.
The rules seem simple — one conductor gets 53%, two conductors get 31%, and three or more get 40% fill. But the real-world calculations involve mixed conductor sizes, equipment grounding conductors, cable assemblies versus individual conductors, and the difference between conduit types that share the same trade size but have different internal areas.
This guide walks through NEC Chapter 9 Tables 1, 4, and 5, explains how to calculate fill for mixed conductor sizes, and covers the mistakes that trip up even experienced electricians on inspections.
NEC Table 1: Fill Percentage Rules
NEC Chapter 9 Table 1 establishes the maximum percentage of a conduit's internal cross-sectional area that conductors may occupy. For one conductor, the limit is 53% fill. For two conductors, 31% fill. For three or more conductors, the limit is 40% fill. These percentages apply to all raceway types including EMT, IMC, RMC, PVC, and flexible metallic conduit.
The reason two conductors have a lower percentage than three is geometry. Two round conductors sitting side by side in a round conduit create a worst-case packing arrangement with maximum contact against the conduit wall. Three or more conductors can nest more efficiently, so the fill allowance increases.
These percentages are based on the total cross-sectional area of the conductors including insulation, not just the copper or aluminum. A 12 AWG THHN conductor has a cross-sectional area of 0.0133 square inches. That includes the 0.019-inch-thick insulation and 0.004-inch nylon jacket around the copper.
Equipment grounding conductors count toward fill. Spare conductors pulled for future use count toward fill. Cable assemblies like MC or AC cable use their overall diameter, not individual conductor areas, and follow different fill rules under NEC 392 and the cable article.
1 conductor: 53% of conduit internal area
2 conductors: 31% of conduit internal area
3 or more conductors: 40% of conduit internal area
These apply to the total area of all conductors (including insulation) versus the internal area of the raceway.
NEC Conduit Fill Calculator
Calculate NEC Chapter 9 conduit fill for mixed conductor types. Check percent fill against code limits, get violation flags, and find the next size up recommendation.
EMT, IMC, and RMC: Same Trade Size, Different Internal Area
A 3/4-inch trade size conduit is not the same inside diameter across conduit types. EMT (Electrical Metallic Tubing) has thinner walls than IMC (Intermediate Metal Conduit), which has thinner walls than RMC (Rigid Metal Conduit). The internal cross-sectional area determines how many conductors fit.
For 3/4-inch trade size: EMT has an internal area of 0.213 square inches, IMC has 0.235 square inches, and RMC has 0.213 square inches. At 1-inch trade size: EMT is 0.346 in², IMC is 0.384 in², and RMC is 0.355 in². The differences matter when you are right at the fill limit.
NEC Chapter 9 Table 4 provides the internal area and allowable fill area for each conduit type and trade size. Always use the correct table for your conduit type. Using EMT values when installing RMC, or vice versa, can result in a code violation even though the trade size is the same.
PVC conduit comes in Schedule 40 and Schedule 80. Schedule 80 has thicker walls and less internal area. A 1-inch Schedule 40 PVC has 0.327 in² inside, while Schedule 80 has 0.275 in². That difference can change your conduit size selection.
Calculating Fill with Mixed Conductor Sizes
When all conductors are the same size, NEC Chapter 9 Table C (Appendix C) gives you a direct answer: the maximum number of conductors of that size in each conduit type and trade size. But real jobs often mix sizes — three 10 AWG THHN circuits with a 12 AWG ground, or a mix of 8 AWG and 6 AWG feeders.
For mixed sizes, look up each conductor's cross-sectional area in NEC Chapter 9 Table 5, add up all the areas, then compare against the 40% fill area for your conduit from Table 4. If the total conductor area is less than or equal to the 40% fill area, the conduit size works.
Example: You need to pull four 10 AWG THHN (0.0211 in² each) and two 12 AWG THHN (0.0133 in² each) with one 12 AWG bare ground (0.0060 in²). Total area: (4 × 0.0211) + (2 × 0.0133) + (1 × 0.0060) = 0.0844 + 0.0266 + 0.0060 = 0.1170 in². A 3/4-inch EMT has a 40% fill area of 0.085 in² — too small. A 1-inch EMT has 0.138 in² at 40% fill — that works.
1. Look up each conductor area in Table 5 (includes insulation)
2. Multiply each area by the number of that size
3. Sum all conductor areas
4. Compare total to 40% fill area from Table 4 for your conduit type
Total conductor area ≤ 40% conduit area = code compliant
Common Conduit Fill Mistakes That Fail Inspection
The most common mistake is forgetting to count equipment grounding conductors. Every EGC takes up space and counts toward fill. On a conduit with six 12 AWG THHN circuits plus three 12 AWG grounds, the grounds add 27% more conductor area. That can push you from a compliant 3/4-inch to requiring a 1-inch conduit.
Another frequent error is using the wrong conductor area. THHN and THWN-2 have different insulation thicknesses and different cross-sectional areas even for the same wire gauge. A 6 AWG THHN is 0.0507 in², while a 6 AWG XHHW is 0.0590 in². Using the wrong value can result in an overfilled conduit.
Nipple fill is a separate rule that catches people. NEC 344.22 and similar sections allow 60% fill for conduit nipples 24 inches or less in length. This exception exists because short runs do not have the heat dissipation concerns of long runs, and pulling difficulty is minimal. But the nipple must connect two enclosures or boxes — a conduit body does not count.
Finally, over-40% fill is not always a violation if it is a single run with no bends. The fill limits assume standard pulling conditions. If the inspector sees a heavily loaded conduit with multiple 90-degree bends and a total pull length over 100 feet, expect a conversation even if you are technically at 39%.
1. Forgetting EGC in fill calculation
2. Using THHN areas for XHHW or other insulation types
3. Exceeding 360° of bends between pull points (NEC 344.26)
4. Mixing Table C (same-size) with mixed-size methods
5. Claiming nipple fill (60%) on runs longer than 24 inches
How Conduit Fill Interacts with Ampacity Derating
Conduit fill and ampacity derating are related but separate requirements. NEC 310.15(C)(1) requires ampacity adjustment when more than three current-carrying conductors are in the same raceway. The adjustment factors reduce allowable ampacity: 4–6 conductors to 80%, 7–9 conductors to 70%, 10–20 conductors to 50%.
Neutral conductors carrying only unbalanced current from line-to-neutral loads on 3-phase, 4-wire systems do not count as current-carrying for derating purposes. Equipment grounding conductors never count as current-carrying. But both still count for physical fill.
This creates a dual constraint. Your conduit must be large enough for physical fill compliance under Chapter 9, and your conductors must be large enough for ampacity after derating under 310.15. Sometimes derating forces you to upsize conductors, which increases their cross-sectional area, which can force you to upsize the conduit.