numerous nm cables running into an electrical box

NM Cable

Originally contributed by • last updated 2/22/2021

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In most cases, the electrical wiring found inside our homes is done by professionals, requiring very little knowledge or expertise on the part of the homeowner to have it maintained or repaired. However, in some cases, such as if you were looking to wire up a new light fixture or GFCI plug, there’s a good chance you could come across terms such as Romex brand or NM cable.

Definition

The term NM cable (non-metallic sheathed cable) refers to a type of electrical wire that is typically used in residential and light commercial electrical systems. These wires are often incorrectly referred to by the brand name Romex, Romex wire, or 14/2 Romex (a trademarked brand of the Southwire Company) in the same way tissue paper is commonly called Kleenex. NM cable generally consists of two or three insulated copper conductor wires, an uninsulated ground wire, and a thick outer thermoplastic sheathing. With most modern versions, the color of the sheathing indicates the wire gauge, with one of the most common, 14 AWG wire, typically being white.

What Are The Different Types Of NM Cables?

Not all cables and wires are created equal and it’s important to know which ones should be used when it comes to both indoor and outdoor wiring. There are a few different types of NM cables that are used for different purposes around the house and outside. Here are a few key tips to make sure you choose the right one:

  • NM cable is commonly described as two-wire or three-wire cable, depending on how many insulated conductor wires are inside the thermoplastic sheathing. (It’s important to note that this does not include the uninsulated ground wire which is also found in the sheathing.) For example, 14/2 wire has two 14 gauge conductors and one uninsulated ground wire inside the sheathing, making three wires in total. Whereas, a 12/3 has three 12 gauge wires and one ground, making 4 wires in total.
  • If you’re wiring up a connection outside, there is a special NM cable that’s designed to be buried underground known as underground feeder cable (UF-B). This type of cable comes with extra layers of protection to prevent it from sustaining damage. It’s what you’ll need if you want to run power to a shed or garage.