What Size Conduit for 12/2 Romex?

For a single 12/2 NM-B cable through conduit, 1/2-inch EMT is sufficient. For multi-cable runs, step up.

Cable dimensions

12/2 NM-B with ground typically measures:

• Major axis: ~0.42 inch
• Minor axis: ~0.20 inch
• Effective area (worst-case): 0.42 × 0.20 = 0.084 in²

Use 0.085 in² for fill calculations.

Sizing options

Thermal / ampacity

12 AWG NM-B is rated 20A at 60°C, the standard residential 20A branch protection.

With multiple cables in conduit longer than 24 inches, NEC 310.15(C)(1) derates based on current-carrying conductor count. Two 12/2 cables = 4 conductors total. If all 4 carry current (typical for two 20A branches): 4 CCCs → × 0.80 → 20A × 0.80 = 16A — below the 20A breaker rating.

Bundling 20A branches in conduit longer than 24" violates the ampacity rule unless you derate to 16A breakers — defeating the purpose of using 12 AWG.

Why this matters

Many DIYers run multiple 12/2 NM cables through conduit for "neatness" without realizing the derate hit. Either:

1. Use a single 12/2 per conduit run
2. Switch to individual 12 AWG THHN conductors (90°C × 0.80 = 24A — survives derate at 20A protection)
3. Limit conduit length to ≤24 inches (nipple rule waives derating)

Common 12/2 circuits

• Kitchen counter receptacles (≥2 required per NEC 210.11(C)(1))
• Bathroom receptacles (NEC 210.11(C)(3))
• Laundry receptacle (NEC 210.11(C)(2))
• Garage and outdoor GFCI receptacles
• Workshop branches

Better alternative

For multi-circuit raceway runs, individual 12 AWG THHN gives both better ampacity and easier pull. See how many 12 AWG THHN fit in 1/2-inch EMT for the THHN math.

Quick reference

• Single 12/2 sleeve: 1/2-inch EMT
• Two 12/2 cables: 1-inch EMT (and beware ampacity derating on long runs)
• Three+ cables: 1-1/4 inch EMT, consider switching to individual conductors