What Size Conduit for 10/3 Romex?
For 10/3 NM-B in conduit, use 3/4-inch EMT minimum. Cable's effective area is ~0.12 in², fitting comfortably with single-cable headroom.
For a single 10/3 NM-B cable sleeve or transition, 3/4-inch EMT is the practical minimum. For multi-cable raceway runs, 1-inch or 1-1/4 inch.
Cable dimensions
10/3 NM-B with ground typically measures:
- Major axis: ~0.50 inch
- Minor axis: ~0.24 inch
- Effective area (worst-case): 0.50 × 0.24 = 0.120 in²
Use 0.12 in² for fill calculations.
Sizing options
| Configuration | Cable count | Total area | Fill rule | Min conduit |
|---|---|---|---|---|
| Single 10/3 sleeve | 1 | 0.12 in² | 53% | 1/2-inch EMT (0.304 in² → 39.5%) — fits but tight |
| Single 10/3 recommended | 1 | 0.12 in² | 53% | 3/4-inch EMT (0.533 in² → 22.5%) |
| Two 10/3 cables | 2 | 0.24 in² | 31% | 1-inch EMT (0.864 in² → 27.8%) |
| Three 10/3 cables | 3 | 0.36 in² | 40% | 1-1/4 inch EMT (1.496 in² → 24.1%) |
Why 3/4-inch over 1/2-inch
While 1/2-inch passes the fill math, the practical considerations push to 3/4-inch:
- NEC 314.16(B)(4) calls for box volume calculations that 1/2-inch fittings often complicate
- Pulling tension on Romex through small EMT is significant — jacket can nick on burrs
- 3/4-inch is the standard knockout on most appliance disconnects (dryer outlets, AC disconnects)
Thermal / ampacity
10 AWG NM-B is rated 30A at 60°C (the cable's terminal temp rating). Inside conduit, NEC 334.80 keeps it at the 60°C column. 30A protection is the standard residential dryer breaker.
If multiple cables share the same conduit longer than 24 inches, NEC 310.15(C)(1) derates based on the count of current-carrying conductors:
- Two 10/3 cables = 6 conductors total. Hots + neutrals = 4 CCCs (if neutrals carry current). With 4–6 CCCs: × 0.80 → 30A × 0.80 = 24A — below the 30A breaker rating.
This is why bundling multiple dryer-class NM cables in one conduit is uncommon — derating kills the ampacity.
Better alternative
For multi-circuit raceway runs, pull individual 10 AWG THHN conductors. THHN's 90°C ampacity (40A) survives the 0.80 derate at 32A — comfortably above 30A protection.
Use the conduit fill calculator to compare 10/3 NM-B bundling vs separate THHN conductors.
Quick reference
- Single 10/3 sleeve: 3/4-inch EMT (recommended), 1/2-inch (math passes but tight)
- Two 10/3 cables: 1-inch EMT
- Three 10/3 cables: 1-1/4 inch EMT — and consider derating impact
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This page's figures are transcribed from NEC 2023 (NFPA 70) Chapter 9, Tables 1, 4 and 5, and checked by a version-controlled automated test suite — they are not signed off by a licensed professional. WireFillChart is an independent reference, not affiliated with or endorsed by NFPA. Always verify against the code edition adopted by your local authority having jurisdiction (AHJ).