How many 3 AWG THHN conductors fit in 1-1/2 inch EMT?

Eight. The 40 percent fill area for 1-1/2 inch EMT is 0.814 square inches and 3 AWG THHN is 0.0973 square inches each, so eight conductors at 0.778 square inches fit and nine do not.

Does the number change for XHHW-2?

Yes. XHHW-2 in 3 AWG is 0.1032 square inches per conductor, which limits 1-1/2 inch EMT to seven conductors instead of eight.

Do I have to derate if I pull eight conductors?

Yes, if more than three are current carrying. NEC 310.15(C)(1) applies 80 percent for four through six and 70 percent for seven through nine current carrying conductors in the same raceway.

How many 3 AWG copper wires fit in 1-1/2 inch EMT?

Eight 3 AWG THHN copper conductors fit in 1-1/2 inch EMT. That is the raw conduit fill answer from NEC 2023 Chapter 9. In practice the number of current carrying conductors you can actually use is usually lower, because ampacity adjustment under 310.15(C)(1) kicks in long before you reach eight.

The Chapter 9 math

Conduit fill for more than two conductors is limited to 40 percent of the internal cross sectional area by Chapter 9 Table 1. Table 4 gives 1-1/2 inch EMT a total internal area of 2.036 square inches, so the 40 percent column is 0.814 square inches. Table 5 gives 3 AWG THHN an approximate area of 0.0973 square inches.

Divide 0.814 by 0.0973 and you get 8.37. Chapter 9 Note 7 says you round down whenever the result comes out to 0.8 or above only for the specific case of a decimal of 0.8 or larger with one conductor, so in normal practice you drop the fraction. Eight conductors occupy 0.778 square inches, which fits. Nine would occupy 0.876 square inches and exceeds the limit.

If the conductors are XHHW-2 instead of THHN the area per conductor is larger at 0.1032 square inches, and the answer drops to seven. Always run the numbers with the actual insulation type printed on the wire, not a generic assumption.

Why the answer is usually six or fewer in the field

Table 310.16 gives 3 AWG copper THHN an ampacity of 100 amps at 90 degrees C and 85 amps at 75 degrees C, which is the column you terminate on for most equipment under 110.14(C)(1). Once you have more than three current carrying conductors in the raceway, 310.15(C)(1) applies an adjustment factor: 80 percent for four through six, 70 percent for seven through nine, 50 percent for ten through twenty.

Two three wire 120/208 volt circuits gives you six current carrying conductors, so the 90 degree ampacity of 100 amps adjusts to 80 amps, then gets limited by the 75 degree termination rating. Push to eight current carrying conductors and you are at 70 percent, or 70 amps, which is below what most people are running 3 AWG for in the first place. Neutrals in a balanced three phase four wire wye system are not counted under 310.15(E), which sometimes buys back a step.

Field takeaway

Eight 3 AWG THHN conductors is the fill limit for 1-1/2 inch EMT. Before you pull eight, check the adjustment factor and the terminal temperature rating, because derating is what usually forces you up to 2 inch EMT rather than the fill table.