What Size Conduit for 6/3 Romex?

6/3 NM-B (commonly called Romex 6/3, or "with-ground" 6-gauge three-conductor cable) is most commonly run without conduit through dwellings. But when conduit is required — physical protection, code transitions, or pulling through walls — NEC Chapter 9 governs fill.

When does 6/3 Romex go in conduit?

• Sleeve protection where the cable exits a wall and runs exposed (NEC 334.15(B))
• Transition between framing assemblies (e.g., through a non-fire-rated wall)
• Risers up from a basement to feed a sub-panel above grade
• Outdoor protection for the short run from the structure to outdoor equipment

For long parallel runs in raceway, NEC 334.10/334.12 push installers toward individual THHN conductors rather than bundled NM cable. NM in conduit is discouraged for thermal reasons — the cable's own jacket traps heat that the conduit also traps.

NM-B cable as "one conductor" in Chapter 9 fill

NEC Chapter 9, Note 9 (and Note 5) instruct that flat or oval cable in conduit be treated as a single conductor with cross-sectional area equal to the major axis squared × π/4, or use manufacturer data.

For 6/3 NM-B (with ground):

• Major axis: ~0.65 inch
• Minor axis: ~0.32 inch
• Effective area (oval treatment): π × (0.65/2) × (0.32/2) = 0.163 in²
• Effective area (worst-case square treatment): 0.65 × 0.32 = 0.208 in²

Use 0.205 in² as a conservative estimate.

Conduit sizing

In practice, conservative installers default to 1-inch EMT for a single 6/3 sleeve — easier to pull, more room for bends, and inspectors prefer the margin.

Thermal considerations

Per NEC 310.15(C)(1) Note 4, NM cable conductors inside conduit must use the 60°C column of Table 310.16. 6 AWG NM-B is rated 55A at 60°C — typically used on a 60A breaker. Adjustment factors still apply if the run is over 24 inches.

Better alternative — individual conductors

For runs longer than a few feet, pulling individual 6 AWG THHN conductors instead of NM-B gives you:

• 75°C column (65A for 6 AWG vs 55A at 60°C)
• Easier pull (smaller bundle)
• Easier reroute if changes are needed

Use the conduit fill calculator to compare 6/3 NM-B vs 6 AWG THHN raceway sizing.

Quick reference

• Single 6/3 sleeve: 3/4-inch EMT minimum; 1-inch typical
• Two 6/3 cables: 1-1/4 inch EMT
• Long runs: consider individual THHN conductors instead of NM-B
• Outdoor or wet locations: UF cable or THWN-2 conductors required (not NM-B)