12/2 wire has two live conductors without counting a grounding conductor. The correct size of conduit for a 12/2 cable is 1/2 inch. The most popular types of conduit used with this size of wire are PVC and EMT. You should opt for a larger size only if planning to run more than one 12/2 cable in a single conduit.

Let's break it down depending on the type of cable. 

Size Of Conduit For 12/2 Cable Depending on Type

12/2 NM-B Romex: PVC is the most common size of conduit for NM-B Romex in residential settings. EMT can be used in commercial settings. A size 1/2 inch is perfect for a single 12/2 Romex. However, if you are running two cables at once, it is best to use a 3/4 conduit. 1/2 conduit can only fit one NM-B Romex cable. While running two or more cables through one conduit may be practical for you, always remember that the total cross-sectional area of all cables inside a conduit should not exceed a specific percentage of the conduit's internal cross-sectional area, which is 40%.

12/2 UF-B Cable: The same rule about the conduit size applies to the UF-B cable. 12/inch is excellent for one cable, but 3/4 is the right choice for two cables. UF-B does not use conduit too often because it is suitable for direct burial. When it does because of local code requirements, the common choice is PVC.

12/2 MC (Metal Clad) Cable: In a similar fashion to NM-B Romex and UF-B cable, 12/2 MC (Metal Clad) Cable uses a 1/2 inch conduit. If you wish to run multiple cables, consider using a 3/4 conduit. MC cable is not often used with conduits because it has armor. However, an MC cable can be used in a conduit for additional protection or aesthetics. For sleeve protection and protection of termination points or bends, EMT and flexible metal conduit can be used. Liquidtight Flexible Metal Conduit can be used if additional protection from moisture is required. PVC conduit is not common for MC cables as it does not provide many additional benefits compared to MC cables' own armor.

Individual 12 AWG THHN/THWN Conductors: THHN/THWN is used with conduit more often than any other type of cable. As there are no 12/2 THHN Cable, only single-conductors 12 AWG cables, you can fit more THHN cables into a 1/2 conduit. Therefore, you can comfortably fit three single-conductor THHN cables in a 1/2 EMT conduit and six single-conductor THHN cables in a PVC conduit.

If you need an individual grounding conductor with THHN live wires, you can comfortably fit two conductors plus ground into EMT and five conductors plus ground into PVC. This practically means that EMT fits one set of cables (two live wires plus one ground), and PVC conduit fits up to 2 sets of cables. RMC and IMC conduits fit the same number of THHN conductors as EMT, two plus ground.

When making a choice, remember to check the actual cross-sectional area of the cable according to manufacturer specs, as it may vary depending on the manufacturer.