NM-B Wire

Q: What does NM-B stand for?

NM-B stands for Non-Metallic sheathed cable, Type B. "NM" indicates the outer sheath is non-metallic (PVC jacket). "B" indicates the current temperature rating standard (90°C on the individual conductors, though ampacity is calculated at 60°C per NEC 334.80). It is commonly known by the brand name "Romex."

Q: Where can I use NM-B (Romex)?

NM-B is approved for dry indoor locations — inside walls, ceilings, and attics of residential and commercial buildings. It is not permitted outdoors, underground, in wet or damp locations, in conduit (in some jurisdictions), or in commercial buildings above 3 stories (NEC 334.12). It cannot be used where exposed to physical damage.

Q: What is the largest NM-B cable available?

NM-B cable is commonly manufactured in sizes from 14 AWG to 6 AWG. Larger sizes exist but are uncommon. For circuits requiring larger wire (50A and above), most electricians switch to individual THHN conductors in conduit or SER cable.

NM-B stands for Nonmetallic Sheathed Cable (Romex). Standard residential wiring (Romex). Individual conductors rated 90°C but ampacity limited to 60°C column per 334.80.

Dry rating60°C

Voltage600V

Locationsdry

Specifications

Property	Value
Full name	Nonmetallic Sheathed Cable (Romex)
Insulation material	PVC on individual conductors, PVC outer sheath
Outer covering	PVC/nylon outer jacket
Temperature rating (dry)	60°C (140°F)
Voltage rating	600V
Approved locations	dry

Ampacity by gauge

The table below shows NEC Table 310.16 ampacity for NM-B conductors. Values shown reflect the 110.14(C) termination temperature limit, which determines the real-world usable ampacity - not just the insulation rating. Click any gauge to see the full derating chain.

Gauge	Copper	Aluminum
14 AWG	See page	-
12 AWG	See page	-
10 AWG	See page	-
8 AWG	See page	-
6 AWG	See page	-
4 AWG	-	-
3 AWG	-	-
2 AWG	-	-
1 AWG	-	-
1/0 AWG	-	-
2/0 AWG	-	-
3/0 AWG	-	-
4/0 AWG	-	-

When to use NM-B wire

Standard residential wiring (Romex). Individual conductors rated 90°C but ampacity limited to 60°C column per 334.80.

Choose NM-B when:

You're wiring inside walls, ceilings, or attics of a wood-frame residential building
The cable will be concealed and protected from physical damage
You want the fastest installation for standard residential branch circuits
Local code permits NM-B (some jurisdictions require conduit even in residential)

Don't use NM-B when:

Outdoors, underground, or in wet/damp locations - NM-B has no moisture rating
In conduit for long runs - NM-B is allowed in conduit but ampacity is capped at the 60°C column per 334.80, and the cable is harder to pull than individual THHN conductors
Commercial buildings above 3 stories - NEC 334.12 prohibits it
Where exposed to physical damage - run conduit with THHN instead

Common applications for NM-B:

In-wall residential wiring - the standard cable (Romex) for branch circuits in wood-frame construction
Ceiling lighting circuits - run through attic joists and ceiling cavities between drywall
Switch and receptacle circuits - most common cable type in single-family homes
Not permitted outdoors, underground, or in wet locations - use UF-B or THHN in conduit instead

NEC code references for NM-B

The NEC defines wire insulation types in Table 310.4(1) (formerly Table 310.104(A)). Key sections that apply:

Table 310.4(1) - lists NM-B with its temperature rating, insulation material, and approved locations
Table 310.16 - ampacity values for conductors in raceway, cable, or earth (the values shown in our ampacity table above)
Section 110.14(C) - termination temperature limits that determine real-world usable ampacity
Section 310.15(C)(1) - adjustment factors for more than 3 current-carrying conductors in a raceway

Frequently asked questions about NM-B wire

What does NM-B stand for?

NM-B stands for Non-Metallic sheathed cable, Type B. "NM" indicates the outer sheath is non-metallic (PVC jacket). "B" indicates the current temperature rating standard (90°C on the individual conductors, though ampacity is calculated at 60°C per NEC 334.80). It is commonly known by the brand name "Romex."

Where can I use NM-B (Romex)?

NM-B is approved for dry indoor locations - inside walls, ceilings, and attics of residential and commercial buildings. It is not permitted outdoors, underground, in wet or damp locations, in conduit (in some jurisdictions), or in commercial buildings above 3 stories (NEC 334.12). It cannot be used where exposed to physical damage.

Why is NM-B ampacity rated at 60°C when conductors are 90°C?

NEC Section 334.80 requires NM-B ampacity to be based on the 60°C column of Table 310.16, even though the individual conductors inside are rated 90°C. This is because the cable assembly traps heat differently than individual conductors in conduit. The 90°C rating only matters for ambient temperature derating calculations per 310.15(B)(1).

What is the largest NM-B cable available?

NM-B cable is commonly manufactured in sizes from 14 AWG to 6 AWG. Larger sizes exist but are uncommon. For circuits requiring larger wire (50A and above), most electricians switch to individual THHN conductors in conduit or SER cable.

Other wire insulation types

THHN 90°C THWN 75°C THWN-2 90°C XHHW 90°C XHHW-2 90°C RHH 90°C RHW-2 90°C USE-2TW 60°C UF-B 60°C MC Cable 90°C AC Cable (BX) 90°C SER/SEU 75°C PV Wire 90°C THHW 90°C THW 75°C THW-2 90°C RHW 75°C MI 90°C UF 60°C XHH 90°C FEP 90°C SIS 90°C

Requirements vary by state. NEC edition, licensing, permits, and GFCI rules differ by jurisdiction. Check your state or pick it below for personalized info across WireRef.

Important: For reference only. Not a substitute for a licensed electrician. Electrical work can cause serious injury, death, fire, or property damage if performed incorrectly. Always hire a licensed electrician for electrical work, especially for service upgrades, panel work, and 240V circuits. Values are derived from NFPA 70 (NEC) for educational purposes. Always verify against your locally adopted NEC edition and amendments - local jurisdictions may enforce stricter requirements. WireRef provides reference information only and is not responsible for work performed based on this content. NEC® is a registered trademark of the National Fire Protection Association. Free NEC access via NFPA · OSHA Electrical Safety · Terms of use.

NEC 2023 references verified April 2026