How Many Mixed 12 AWG and 10 AWG THHN Wires Fit in 1/2 Inch EMT?

Conduit fill charts only work when every conductor is the same size. The moment you mix 12 AWG and 10 AWG THHN in the same pipe, the printed chart is useless and you have to do the area math from Chapter 9. In 1/2 inch EMT the working answer is four 12 AWG plus three 10 AWG. Add a fourth 10 AWG and the pipe is over fill.

The two tables that do the work

Start with NEC 2023 Chapter 9, Table 4. Half inch EMT has a total internal cross sectional area of 0.304 square inches. Table 1 sets the allowable fill percentage by conductor count: 53 percent for one conductor, 31 percent for two, and 40 percent for three or more. A normal branch circuit pull has three or more, so the usable area is 0.304 times 0.40, which equals 0.122 square inches.

Now go to Chapter 9, Table 5 for the approximate area of each conductor including insulation. THHN and THWN 12 AWG is 0.0133 square inches. THHN 10 AWG is 0.0211 square inches. Multiply and add. Four 12 AWG is 0.0532. Three 10 AWG is 0.0633. The sum is 0.1165 square inches, which sits under the 0.122 limit with room to spare. Swap that third 10 AWG for a fourth and the total becomes 0.1376, which is over the limit and fails.

A few other combinations worth memorizing for half inch EMT with THHN: nine 12 AWG is 0.1197 and passes, ten 12 AWG is 0.133 and fails. Five 10 AWG is 0.1055 and passes, six is 0.1266 and fails. Two 10 AWG plus five 12 AWG is 0.1087 and passes.

Fill is only half the problem

Passing Table 1 does not mean the conductors keep their book ampacity. Section 310.15(C)(1) requires adjustment once more than three current carrying conductors share a raceway for more than 24 inches. Four to six current carrying conductors derate to 80 percent, seven to nine to 70 percent, and ten to twenty to 50 percent. In the four 12 AWG plus three 10 AWG example, if all seven are current carrying, 12 AWG THHN at the 90 degree C value of 30 amps derates to 21 amps and 10 AWG at 40 amps derates to 28 amps. You then apply the 240.4(D) small conductor limits, which cap 12 AWG at 20 amps and 10 AWG at 30 amps, so both still support their normal breaker sizes. Push to ten current carrying conductors and the 50 percent factor drops 12 AWG THHN to 15 amps, which kills a 20 amp circuit.

Grounding conductors and grounded conductors carrying only unbalanced current do not count as current carrying under 310.15(E), so a shared equipment ground and a neutral on a multiwire branch circuit can help the derating count even though they still count for fill. Count everything for area, count only the current carriers for derating, and check both before you pull.