What is a Romex?

Understanding Romex

Romex is a trade name for nonmetallic-sheathed cable. Many people shorten this name to NM cable. The cable has insulated copper conductors inside a plastic outer jacket. Some cable types also contain a bare or insulated equipment grounding conductor.

Related glossary terms: Branch Circuit, BX Cable, Conduit.

Common markings include 14/2, 12/2, and 12/3. The first number identifies the conductor size. The second number identifies the insulated conductors. Many cable descriptions list the grounding conductor separately. A 12/2 cable usually contains two insulated conductors and a ground. You must confirm the exact construction from the jacket markings.

How Romex Works, Is Measured, or Is Used?

NM cablecarries electric current through insulated conductors. It runs from a panelboard to outlets, switches, lights, or appliances. One conductor commonly carries ungrounded, or hot, current. Another conductor returns current as the neutral. The grounding conductor provides a low-resistance path for fault current. This path lets.circuit breaker trip when a fault occurs.

Electricians select cable by conductor size, circuit rating, and conductor count. They also consider temperature rating and installation location. The jacket identifies the cable type and voltage rating. Cable must have support and protection from physical damage. It must also pass through approved openings. Electricians must secure it at electrical boxes. These steps follow.National Electrical Code.

Romex is commonly installed inside framed walls and ceilings. It also works in other dry areas. It shouldn't be placed where water or sunlight can damage the jacket. Crushing, sharp edges, or repeated movement can also cause damage. Exposed exterior walls and wet locations may need conduit. Places subject to impact may need another wiring method.

Why Romex Matters?

Cable selection affects safety, circuit capacity, and inspection results. A cable with conductors that are too small can overheat. This can happen under the circuit's rated load. A cable with the wrong grounding arrangement can also fail requirements. The wrong jacket rating can create the same problem.

The cable jacket keeps the conductors together. It also provides basic protection during installation. Its design can reduce labor compared with individual wires in conduit. This benefit matters in typical wood-frame construction. That convenience doesn't remove the need for correct wire gauge. You still need correct box fill, splicing, support, and overcurrent protection.

When Romex Matters Most?

Romex selection becomes important during new construction and remodeling. It also matters during whole-home rewiring. You need to match the cable to the circuit breaker and expected load. You must also consider building conditions and local inspection rules. Kitchens, bathrooms, garages, and laundry areas often have extra requirements. Outdoor circuits may have them too. These requirements can include ground-fault circuit interrupter protection or cable placement.

Inspect the jacket before installation and after rough-in work. Cuts, crushed sections, and exposed conductors can create serious defects. Loose staples can also create defects. Cables passing through unprotected metal can cause problems. For an existing circuit, compare the cable marking with the breaker rating. Check the grounding conductor, box connections, and device terminals. Do this before changing equipment.