Conduit Fill Calculator

Conduit Fill formula

Conductor area = Number of conductors x Area per conductor (Table 5)   (12 AWG THHN = 0.0133 sq in)
Fill % = Conductor area / Conduit inside area (Table 4) x 100
Limit = 53% for 1 conductor, 31% for 2, 40% for 3 or more (Table 1)
Maximum conductors = Allowed area / Area per conductor, rounded down unless the fraction is 0.8 or more (Note 7)

Worked example

Eight 8 AWG THHN conductors in 3/4 in EMT: 8 x 0.0366 = 0.2928 sq in. The 3/4 in EMT inside area is 0.533 sq in, so the fill is 0.2928 / 0.533 = 54.9%, well over the 40% limit for three or more conductors. Step up to 1 in EMT: the 40% area is 0.346 sq in, and 0.346 / 0.0366 = 9.45, so nine 8 AWG conductors fit and eight of them fill 0.2928 / 0.864 = 33.9%.

Two 4/0 THHN conductors in 1-1/2 in EMT: 2 x 0.3237 = 0.6474 sq in, or 0.6474 / 2.036 = 31.8% of the inside area. The limit for exactly two conductors is 31%, so they do not fit; 2 in EMT at 0.6474 / 3.356 = 19.3% does. Add a third 4/0 and the limit rises to 40%, so three of them (0.9711 sq in, 47.7%) also need 2 in.

Quick reference

How conduit fill is calculated

The NEC sizes raceways by area. Chapter 9 Table 4 lists the inside area of every conduit type and trade size, along with the area at 40% fill; Table 5 lists the area of each conductor size and insulation type, insulation included. Add up the conductor areas, divide by the inside area of the conduit, and compare with the Table 1 limit: 53% for a single conductor, 31% for two and 40% for three or more. Every wire counts, including the neutral and the equipment grounding conductor.

When all the conductors are the same size, Annex C of the NEC tabulates the answer directly, and this calculator reproduces those numbers for THHN in EMT and Schedule 40 PVC, including the Note 7 rule that lets a result of 0.8 or more round up. Annex C is built on the 40% column even for two conductors; this calculator applies the stricter 31% from Table 1 in that case, which occasionally moves two large conductors up one conduit size.

Beyond the fill percentage

A conduit that is legal at 40% can still be miserable to pull if it has several 90 degree bends, so many electricians plan for 30 to 35% on long runs and keep the total bends between pull points at or under 360 degrees, which is the NEC maximum. Mixed conductor sizes are handled the same way: add the individual Table 5 areas and compare with the 40% area. Other insulation types such as THW or XHHW are thicker than THHN and reduce the count.

Fill is only one limit. Bundling more than three current-carrying conductors in a raceway forces an ampacity derate, and a conduit on a hot roof needs a temperature correction too. PVC also needs expansion fittings on long exposed runs. Have a licensed electrician confirm the raceway, conductor count and ampacity against the NEC edition and amendments adopted where you live.

Key facts

• NEC Chapter 9 Table 1 limits conduit fill to 53% of the inside area for one conductor, 31% for two and 40% for three or more.
• The limit for two conductors is stricter than for three because two conductors lie side by side and can jam in the bends; three or more pack more evenly.
• 12 AWG THHN has an area of 0.0133 sq in including insulation, so 3/4 in EMT (0.533 sq in inside) holds 16 of them and 1/2 in EMT holds 9.
• Ampacity must be derated when more than three current-carrying conductors share a raceway: 80% for 4 to 6, 70% for 7 to 9, and 50% for 10 to 20 (NEC Table 310.15(C)(1)).
• Note 4 to Table 1 allows a nipple of 24 in or less between enclosures to be filled to 60%, and the conductors in it are not subject to the bundling derate.

Frequently asked questions

Sources and references

• •NFPA 70, National Electrical Code (NFPA)
• •Re3 Conduit Fill Calculator (Southwire)
• •Code Hunter: Calculations, IAEI Magazine (IAEI)