Conduit Fill Calculator
A conduit fill calculator determines the minimum conduit size needed to contain a group of electrical conductors while complying with NEC fill percentage limits. The NEC limits conduit fill to prevent overheating, facilitate wire pulling, and allow future wire additions. Fill limits are 40% for 3 or more conductors, 31% for 2 conductors, and 53% for a single conductor in most conduit types. This calculator uses conductor cross-sectional area data from NEC Table 5 (THHN insulation) and conduit internal area data from NEC Table 4 to find the minimum conduit trade size.
Conduit fill formula
Total wire area = sum of (Count x Area per conductor) for each wire type
Required conduit area = Total wire area / 0.40 [for 3 or more conductors]
Select conduit trade size with internal area >= Required conduit area
Conductor areas from NEC Chapter 9 Table 5 (THHN 90C insulation). EMT internal areas from NEC Chapter 9 Table 4. For 1 conductor: use 0.53 fill limit. For 2 conductors: use 0.31. For 3 or more: use 0.40.
Frequently asked questions
What is conduit fill percentage?
Conduit fill percentage is the ratio of the total cross-sectional area of all conductors inside a conduit to the interior area of the conduit. The NEC limits fill to 53% for 1 conductor, 31% for 2 conductors, and 40% for 3 or more conductors, to allow for pulling and heat dissipation.
What conduit size do I need for four 12 AWG THHN wires?
Four 12 AWG THHN conductors each have a cross-sectional area of 0.0133 sq in (NEC Table 5). Total area = 4 x 0.0133 = 0.053 sq in. At 40% fill, the required conduit area = 0.053 / 0.40 = 0.133 sq in. A 1/2-inch EMT (area 0.304 sq in at 40% = 0.122 sq in available) is slightly under; use 3/4-inch EMT (0.213 sq in at 40%).
What types of conduit does the NEC cover?
Common NEC-covered conduit types include EMT (electrical metallic tubing), IMC (intermediate metallic conduit), RMC (rigid metal conduit), PVC schedule 40 and 80, and LFMC (liquidtight flexible metallic conduit). Each type has different internal area, cost, and application suitability. EMT is the most common for commercial buildings.
Can I mix different wire sizes in the same conduit?
Yes. You must sum the actual cross-sectional areas of all conductors (from NEC Chapter 9 Table 5) regardless of size or insulation type. Add the individual areas together and size the conduit to keep the total fill at or below the NEC maximum percentage for the number of conductors.
Does conduit fill affect ampacity derating?
Yes. When 4 to 6 current-carrying conductors are in a conduit, NEC Table 310.15(C)(1) requires derating ampacity to 80%. For 7 to 9 conductors, derate to 70%; for 10 to 20, derate to 50%. Neutral conductors in balanced 3-phase circuits do not count as current-carrying conductors for derating purposes.
Official sources
- NFPA: NFPA 70 NEC - Chapter 9 Tables 1, 4, and 5: Conduit Fill and Conductor Area.
- NFPA: NEC Annex C - Conduit and Tubing Fill Tables for Conductors.
Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. See our methodology.