What Size Conduit for 10-2 Romex (NM-B)?
Smallest EMT, PVC Schedule 40, RMC and flex trade size for 1, 2, or 3 runs of 10-2 Romex (NM-B). Used for 30a water heaters and 240v a/c disconnects.
SMALLEST CONDUIT BY CABLE COUNT
Each 10-2 Romex (NM-B) is sized as a single conductor with diameter = major OD (0.470"). Equivalent area = π·d²/4 = 0.1735 in².
| Conduit family | 1 cable 53% fill | 2 cables 31% fill | 3 cables 40% fill |
|---|---|---|---|
| EMT (Article 358) | 3/4" 32.6% fill | 1-1/4" 23.2% fill | 1-1/4" 34.8% fill |
| RMC (Article 344) | 3/4" 31.6% fill | 1-1/4" 22.7% fill | 1-1/4" 34.1% fill |
| PVC-40 (Article 352) | 3/4" 34.2% fill | 1-1/4" 23.9% fill | 1-1/4" 35.8% fill |
| FMC (Article 348) | 3/4" 32.6% fill | 1-1/4" 27.2% fill | 1-1/2" 28.0% fill |
Source: NFPA 70 (2023) Chapter 9 Table 1 Note 9 + Table 4 conduit interior areas. Cable OD reference: Southwire / Cerrowire spec sheets — verify ±5% with the cable manufacturer.
WHY EACH CABLE COUNTS AS ONE CONDUCTOR
NEC Chapter 9, Table 1, Note 9 instructs that any multi-conductor cable (Romex, MC, UF-B, SO cord, etc.) be treated as a single conductor for the fill calculation. For cables whose cross-section is elliptical — which all jacketed building cables are — the area is computed as a circle whose diameter equals the major axis of the ellipse.
Why elliptical cables get the major-axis treatment
In a real pull, the cable can rotate inside the raceway. The major axis is what limits how much room it actually consumes. Using the major-axis as a circle diameter gives a conservative (slightly oversized) area that protects against tight rotations during the pull.
How NM-B in a sleeve is treated by the code
NEC 334.15(B) explicitly permits NM-B cable to pass through a raceway used as a sleeve, provided the conduit protects the cable end-to-end against the physical damage hazard. The conduit is sized using the fill rules above; the cable retains its NM-B identity inside the sleeve and is not converted into THHN.
Sleeve vs. raceway wiring method
NM-B is not a raceway-wiring conductor — you do not pull individual NM-B conductors through long conduit runs as if they were THHN. If your run benefits from being fully in conduit (length, exposure, geometry), pull individual THHN conductors of the same gauge and use NEC Annex C tables for capacity.