What Size Conduit for 12/2 Wire?

12/2 wire pulled as individual THHN conductors (two #12 plus one #12 ground) needs 1/2 inch EMT, or 1/2 inch Schedule 40 PVC, at the NEC 40% fill limit. The 3 conductors total 0.0399 square inches (NEC Chapter 9 Table 5); with three or more conductors the raceway may be filled to 40%, so the smallest conduit whose 40% area covers 0.0399 sq in is 1/2 inch EMT.

1/2 inch EMT. Its internal area at 40% fill exceeds the 0.0399 sq in that 12/2 (two #12 plus one #12 ground) occupies. Going one trade size down would exceed 40% and violate NEC Chapter 9 Table 1. If you are running RMC use 1/2 inch; IMC and PVC each have their own internal area, shown in the table above.

Yes. Every conductor in the raceway counts for fill, including the equipment grounding conductor (NEC Chapter 9 Table 1). That is why 12/2 is figured here as two #12 plus one #12 ground: the 3 conductors together, not just the current-carrying ones. Leaving the ground out of the count is the most common conduit-fill mistake.

If you sleeve an intact 12/2 NM cable (Romex) rather than pulling individual THHN, the fill is figured differently: a single cable is treated at the 53% one-conductor limit using the cable's actual cross-sectional area (NEC Chapter 9, Note 9), which usually needs a larger conduit than individual THHN. Most conduit runs use individual THHN conductors, which is what the 1/2 inch answer above assumes.