What Size Conduit for 12/2 Romex?
For 12/2 NM-B in conduit, 1/2-inch EMT fits a single cable comfortably (~28% fill). For multi-cable runs, step up to 3/4-inch or 1-inch.
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
| Configuration | Cable count | Total area | Fill rule | Min conduit |
|---|---|---|---|---|
| Single 12/2 sleeve | 1 | 0.085 in² | 53% | 1/2-inch EMT (0.304 in² → 28.0%) |
| Two 12/2 cables | 2 | 0.170 in² | 31% | 1-inch EMT (0.864 in² → 19.7%) |
| Three 12/2 cables | 3 | 0.255 in² | 40% | 1-inch EMT (0.864 in² → 29.5%) |
| Four 12/2 cables | 4 | 0.340 in² | 40% | 1-1/4 inch EMT (1.496 in² → 22.7%) |
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:
- Use a single 12/2 per conduit run
- Switch to individual 12 AWG THHN conductors (90°C × 0.80 = 24A — survives derate at 20A protection)
- 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