Type NM stands for nonmetallic sheathed. The cable consists of a factory assembly of two or more insulated conductors within an overall nonmetallic jacket. Additionally, there is an equipment-grounding conductor that is bare or has green insulation. Type NM is identified by the size of the conductors and number of them. For example, the very commonly used Type NM 12-2 cable contains two 12 American Wire Gauge (AWG) insulated conductors. In this numbering system the bare or green equipment-grounding conductor is not counted. Hence it is known as NM 12-2 w/ground.
Closely related cable types are Type NMC, which has a corrosion-resistant outer covering, and Type NMS, which contains, in addition to the usual power conductors, communications (signaling) wires. When the letter B is appended, as in NM-B cable, it indicates that the conductor insulation is rated for 90 degrees C.
In the United States and elsewhere that it is applicable, all non-utility and non-mine electrical installations must comply with the National Electrical Code (NEC). Additionally, there is the Canadian Electrical Code (CEC).
In NEC Chapter Three, specific articles pertain to the various types of wire that are recognized. Type NM is covered in Article 334. This section should be used as a guide for installing Type NM cable. Like each of the cable articles, it contains sections titled Uses Permitted and Uses Not Permitted. Broadly speaking, Type NM is permitted in one- and two-family dwellings and their attached or detached garages and storage buildings. Type NM is not to be used for services, outdoors, underground, embedded in concrete, or as data or fire alarm cable (in centrally-controlled fire alarm systems). In limited circumstances, Type NM can be used in multi-family dwellings such as apartment houses and in commercial buildings. This depends upon the type of construction, as outlined in Informational Annex E at the back of the Code book.
A very common mistake that is made is using Type NM for the wiring in commercial garages. There may be gray areas, but the basic idea is that NM should never be used where there may be vapors in the air from flammable liquids such as gasoline, or where flammable gases are used as fuels. A garage is sometimes built as part of a residential property where it eventually becomes a backyard facility. These operations, even if they are not for profit, where engines and gas tanks are removed and fuel systems are opened, are sure to contain at times vapors from flammable liquids. Acetylene and arc welders add to the mix. These buildings should not be wired in Type NM cable.
Article 334 also contains installation requirements for Type NM cable and they should be carefully followed. Minimum bending radius, securing and supporting intervals including distances from boxes (different for metal and plastic) and protection from physical damage must be observed.
As in any wiring, sizing out the cable, i.e. conductor ampacity, is based on the amount of current that the conductors will have to carry. This in turn derives from the over-current protection (breaker or fuse) rating. Also, ambient temperature and bundling of current-carrying conductors enter the picture. All of this is outlined in NEC Chapter Three.
All of these issues are covered in electricians’ licensing exams. In most jurisdictions, homeowners are permitted to do electrical work in their own homes. However, for fire and shock protection Code mandates should be followed faithfully.
By Guest Columnist David Herres