Conduit Fill Calculator

Three tables do all the work

Every conduit fill check uses three tables in Chapter 9 of the NEC. Table 4 gives the inside area of each conduit type and trade size. Table 5 gives the area of each wire, insulation included. Table 1 says how much of the conduit the wires may fill.

The inside area depends on the conduit type as well as the trade size, because the walls are different thicknesses. 1/2" EMT, rigid and PVC are three different numbers in Table 4, so pick the right type before you trust the answer.

Worked example: 12 AWG THHN in 1/2" EMT

The decimal is dropped because of Note 7 to the Chapter 9 tables: when the wires are all the same size, round the count up to the next whole number only when the decimal is 0.8 or more. 8 AWG THHN in 3/4" EMT works out to 5.83, so 6 are allowed, even though 6 of them fill 41.2%.

Mixed sizes, grounds and nipples

1. Look up each wire in Table 5. THHN, THWN and THWN-2 share one row.
2. Multiply each area by how many wires of that size there are, and add them up.
3. Count every wire, the ground included, to pick the limit: 40% for three or more.
4. Compare the total with that share of the conduit’s Table 4 area.

• Grounds count. Note 3 includes equipment grounding and bonding conductors in the fill. Use the Table 5 area for an insulated one and the Table 8 area for a bare one.
• Short nipples get 60%. Under Note 4, a nipple no longer than 24" between boxes, cabinets and similar enclosures may be filled to 60%, and the 310.15(C)(1) adjustment factors don’t apply to it.
• Insulation changes the answer. The numbers here are for THHN/THWN-2. XHHW-2, common on feeders, is thicker than THHN in the small sizes, so fewer fit; use its own row in Table 5.
• Annex C has it worked out. Its tables list the most wires of one size and insulation for each conduit type and trade size, calculated from Tables 1, 4 and 5 with Note 7. They only cover wires that are all the same size.

How many THHN wires fit

From 1/2" to 2", IMC has the largest inside of the five and PVC Schedule 80, with the thickest wall, the smallest. The ground counts toward every number in these tables.

Fill is only one of the checks

Fill only says the wires fit. When a conduit carries more than three current-carrying conductors, their ampacity has to be adjusted under 310.15(C)(1) (the 2020 and 2023 numbering): to 80% for 4 to 6, 70% for 7 to 9 and 50% for 10 to 20. Up to 9 wires of 12 AWG fit in 1/2" EMT, but if 7 or more of them carry current, each is good for only 70% of its ampacity.

An informational note to Table 1 also warns about jamming: when three wires are pulled in, they can jam if the conduit’s inside diameter is 2.8 to 3.2 times the wire’s outside diameter. It is mostly a concern with three larger single conductors, and the fill percentage won’t show it.

• Using EMT areas for rigid or PVC, which have different insides.
• Leaving the ground wire out of the count.
• Using the THHN area for XHHW, TW or another insulation.
• Rounding a count such as 9.5 up to 10. Only a decimal of 0.8 or more rounds up.
• Treating 40% as a target. A full conduit with several bends is a hard pull, and going up a size makes it easier.