Nonmetallic-Sheathed Cable (NM-B)

What it is

Type NM-B is a factory assembly of two or more insulated conductors plus a bare equipment grounding conductor, inside a nonmetallic outer jacket. It is listed to UL 719 and installed under NEC Article 334. The conductors are copper, with insulation rated 90°C. A paper wrap usually separates the conductors from the PVC jacket. It is sold in boxed coils and on reels, and is often called "Romex," which is a brand name.

How to read it

The jacket print gives the size, conductor count, type and voltage. "12/2 with ground" or "12-2 W/G" means two insulated 12 AWG conductors (black and white) plus a bare ground. "12/3 with ground" adds a red conductor. Conductor count never includes the ground. The print also shows "NM-B," the voltage rating (600 V), the maker and the listing mark.

Jacket colors (industry convention)

The colors are a voluntary manufacturer convention. They are not an NEC requirement. Always read the jacket print.

Key points

• NEC 334.10 lists permitted uses. It is common in one- and two-family dwellings and is allowed in other buildings depending on construction type. Check the article.
• NEC 334.12 lists prohibited uses, including wet or damp locations, embedding in poured concrete or masonry, and use as service-entrance cable.
• Conduit installed outdoors or underground is a wet location, so NM-B cannot be pulled into it. Use UF-B or individual wet-rated conductors.
• The 90°C insulation rating does not set the allowable ampacity. NEC 334.80 governs ampacity for NM cable. Use the code tables, not this sheet.
• Securing, supporting and protection from damage are in NEC 334.30 and 334.15.

Common mistakes

• Sizing a circuit by jacket color. Use the printed size and the code.
• Running NM-B in exterior conduit or in a basement slab.
• Counting the bare ground as a conductor and ordering 12/3 when 12/2 with ground was needed.
• Treating "Romex" as a cable type instead of a brand.

Governing standards: NEC Article 334; UL 719 (nonmetallic-sheathed cables).

Reference only. Verify against the current governing code or standard, the manufacturer's data and the project documents before use. Apps for Distributors disclaims liability arising from reliance on this reference.