Romex NMB Wire

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WesBell Romex® wire or NM-B cable is comprised of THHN wires and a PVC jacket. There is also an additional bare copper ground wire which can be slightly smaller than the rest of the insulated conductors. It's a nonmetallic sheathed cable used only indoors for wiring to outlets and switches throughout homes and buildings.


ROMEX® is a branded product designed and manufactured by Southwire Company. Southwire has done a great job marketing the brand because most electrical contractors and electricians know and understand NMB wire as ROMEX®, just like most individuals know and understand adhesive bandages as Band-Aids®. Numerous manufacturers design and make NMB cable very similar to ROMEX® but none carry the brand name. However, the fit, form and function are quite similar to standard home and building applications.

NM-B Wire Construction

NM is an acronym for Non-Metallic compared to MC, or Metal Clad cable. Non-metallic cables are used indoors to connect electrical current from the electrical box to outlets, switches and lights. You’ll see it running along the walls and ceiling of your basement, and it runs up through the walls to each outlet and light switch in your home. Inside the cable are 2 or 3 THHN wires and a solid bare copper ground, usually 1 hot and 1 neutral or 2 hot and 1 neutral. A light wall PVC jacket covers the conductors and ground for an easy installation compared to running each wire individually.

The THHN insulation is thermoplastic with a nylon coating over it. The easiest way to describe the nylon coating is to envision the nylon piece at the end of a shoe lace. Together, the thermoplastic insulation and nylon coating resist water, moisture, heat, oil and gas. However, even though THHN wire can be used outdoors and in conduit, that’s not the case for NMB cable due to the bare copper ground wire (unprotected).

NM-B Wire Approvals

THHN wire sold by WesBell is approved as ASTM B3 and B8, UL83, UL719, AA59544, RoHS, REACH. The first two acronyms are descriptions of the bare copper type. The two UL (Underwriter’s Laboratories) approvals are conducted at UL to assure end users that the wire is in fact 600 volts, 90°C, etc. AA59544 is a federal specification part number system that designates a type of electrical wire. Finally, RoHS and REACH restrict certain metals such as lead, and are approvals that make sure the metals (copper) used in electrical wire are safe.

NM-B Wire Applications

Whether your wire is NMB or the brand ROMEX®, it can only be installed within indoor applications. Using it outdoors can ruin the PVC jacket and compromise the bare copper ground wire. In order to run a cable outdoors it must be either MC (metal clad), UFB (underground feeder) cable or individual THHN wires (green insulated THHN ground instead of bare). NMB cables are used for residential electrical wiring applications in homes and buildings. Common connections run from the electrical box to outlets, light switches and ceiling fans. The bigger sizes connect more power to appliance outlets such as stoves and dryers (8 AWG and 6 AWG) because those appliances need more power to run.

NM-B Wire Manufacturers

Several manufacturers have designed their own version of NMB cable along with their own specifications and approvals. For example, we already saw that Southwire has their own branded NMB cable called ROMEX®. Southwire is the biggest copper wire manufacturer in the USA and create numerous brands such as SIMpull® and MUSTANG®.

Common manufacturers are Southwire, Encore Wire and Cerro Wire. If you need a specific approval for an installation then I suggest you either request a PDF specification sheet from your distributor or you review the manufacturer’s website for the product specification in order to be sure it’s approved.

  • Construction
  • Bare Copper
  • THHN-PVC Insulation
  • Nylon Coating
  • PVC Jacket
  • Black
  • Approvals
  • ASTM B3 and B8
  • UL83
  • UL719
  • AA59544
  • RoHS
  • Applications
  • Residential wiring
  • Indoors only
  • Stove
  • Dishwasher
  • Refridgerator
  • Washer/Dryer
  • Hot tub (indoor only)
  • New electrical panel (indoor only)