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What's the Difference: Nonmetallic Sheathed Cable

14/2 with ground: NM in a white sheathing is 14 AWG, suitable for 15-amp circuits. 14/2 with ground: NM in a white sheathing is 14 AWG, suitable for 15-amp circuits.

Wire sizes

You may know it as Romex or house wire, but nonmetallic sheathed cable (NM) has become a mainstay of residential wiring since it became available almost a century ago. NM is a quick, easy way to get power where you want it and is relatively cheap compared to other methods of wiring. The construction of NM has evolved from a braided-fabric outer jacket surrounding two conductors (no ground wire) into the color-coded, plastic-jacketed, multi-conductor cable assembly we use today. Even with the emergence of color-coded jackets and the widespread availability of home-wiring “how-to” books, there is still some confusion on what size wire is required for a particular application.

Like other multi-conductor cable assemblies, NM contains several insulated conductors housed in an outer jacket in a variety of configurations. The most common gauges include 14-, 12-, or 10-AWG solid copper in two- and three-wire configurations. NM is usually labeled by the gauge of the conductors and the number of conductors in the assembly. Regardless of the manufacturer, a label includes the wire gauge separated by a slash from the number of insulated wires in the outer jacket. For instance, 12/2 signals that two insulated 12-AWG conductors are housed in the outer jacket.
Yellow-sheathed NM covers 12-AWG cable, which can handle a circuit up to 20 amps. Click to enlarge image Yellow-sheathed NM covers 12-AWG cable, which can handle a circuit up to 20 amps.
All NM now has an integrated bare ground wire that usually is excluded from the description on the package. The versions of NM you’ll most commonly need are a two-wire version for constructing basic circuits and a three-wire version that comes in handy for wiring three-way and four-way switches, switched receptacles, or a bath or ceiling fan with separate switches for the fan and light.

Because NM is generally used for wiring one- and two-family dwellings, the stock available at your local supplier likely has the most common wire gauges and configurations for residential circuits. Lighting and receptacle circuits in single-family dwellings have a maximum circuit capacity of 15 or 20 amps, which require 14-AWG or 12-AWG conductors, respectively, but most stores are likely to carry 10 AWG and even some larger gauges. The heavier 10-AWG wire is typically used for the loads that most water heaters, AC-unit compressors, and electric clothes dryers draw, and it is generally paired with a 30-amp-maximum fuse or circuit breaker.
A 30-amp circuit needs 10-AWG conductors, which are contained in orange-sheathed NM. A 30-amp circuit needs 10-AWG conductors, which are contained in orange-sheathed NM.
Using a variety of wire gauges is crucial to safely managing the heat produced in the wire by the electricity as it flows, and each gauge is paired with an established maximum capacity for a fuse or circuit breaker. This pairing is important, because a circuit attached to a breaker that is sized improperly might serve as a gigantic heating element in your walls and give your fire department something to do for a few hours. You can always use a bigger wire than the code-required minimum, but you’ll pay for it. For example, a 50-ft. roll of 10/2 costs about $75, versus $46 for a roll of 12/2 and $32 for a roll of 14/2.

Most NM these days has a color-coded outer jacket: white for 14 AWG, yellow for 12 AWG, and orange for 10 AWG. This color-coding makes identification of new wiring easy, but it can lead to some confusion in older homes if you don’t pay close attention to the existing wiring. I often find DIY-style wiring that has tapped into 12-AWG/20-amp circuits (which was encased in a white outer jacket for a period of time) with new 14-AWG/15-amp wiring.

Other mistakes made with NM are the result of using it in an application for which it is neither designed nor approved. Some of the more common are installing it outdoors, leaving it exposed to sunlight, and running it underground. For these places, UF-B is usually the best cable to use. Similar in construction and configuration to NM, UF-B is UV-resistant and can be run outdoors as well as directly buried in some applications.
Photos: Rodney Diaz
From Fine Homebuilding229 , pp. 40