What is Romex Cable?

Understanding Romex Cable

Romex Cable is the common trade name for nonmetallic-sheathed cable, often marked NM-B. A plastic outer jacket holds several insulated conductors and a grounding conductor in one assembly. The cable is factory-made. Installers can route a complete circuit cable. They don't need to pull separate wires through conduit.

Related glossary terms: Wire Gauge, Residential Electrical Code, Electrical Grounding.

Most residential NM-B cable contains a hot conductor and a neutral conductor. It also contains a bare or insulated equipment grounding conductor. A two-wire cable with ground commonly serves a 120-volt circuit. A cable with two hot conductors, a neutral, and a ground serves certain 120/240-volt circuits. The exact cable type must match the location, circuit design, and applicable electrical code.

How Romex Cable Works, Is Measured, or Is Used?

Electricians identify cable by conductor size, conductor count, and rating. Wire gauge describes conductor diameter. A smaller gauge number generally indicates a larger conductor. Cable markings also identify voltage rating and temperature rating. They show the presence of a grounding conductor. A common residential cable marking might read 14/2 with ground or 12/2 with ground. The correct selection depends on the planned load and overcurrent protection.

During installation, the cable runs through holes in framing. It connects to a breaker panel, switches, receptacles, lighting outlets, or equipment. The cable must remain protected from nails, screws, crushing, sharp edges, excessive heat, and moisture. Installers secure the cable near boxes. They leave enough conductor length for safe terminations. They also protect openings with approved fittings or nail plates. This protection applies when physical damage could occur.

• Check the cable jacket for its printed type and size markings.
• Match the conductor size to the circuit breaker and calculated load.
• Keep NM-B cable in dry, protected spaces unless a listed assembly allows another location.
• Maintain the required box fill and grounding connections at every outlet box.

Why Romex Cable Matters?

Correct cable selection affects overcurrent protection, voltage performance, and fire safety. A breaker protects circuit conductors from excessive current. A breaker that's too large for the cable can allow dangerous overheating. A cable that's too small for the load may also produce excessive voltage drop. This risk is higher on long runs or equipment with high starting current.

NM-B cable can reduce installation time because several conductors share one jacket. That benefit doesn't remove the need for careful planning. Connections still require approved boxes and properly sized devices. They also need secure terminations and a continuous equipment grounding path. Local inspection requirements may also affect cable routing, box placement, and protection methods.

When Romex Cable Matters Most?

Romex Cable becomes especially important during new construction, room additions, basement finishing, and residential rewiring. Existing wiring may have damaged insulation or poor connections. It may also lack an equipment grounding conductor. A remodel can expose cable to damp areas or mechanical damage. Insulation conditions may also make the original wiring method unsuitable.

Before choosing cable, review the circuit load, breaker rating, installation location, and equipment instructions. Wet or exposed areas may require a different wiring method, such as conduit. They may also require a cable listed for that environment. Work that changes branch circuits may require an electrical permit and inspection under local rules. Confirm the installation method before walls are closed.