Buying Guide: Wire and Cable

Wire

Label information, which is printed on wire coverings, provides all of the information you need to choose the right wire for your home. That information includes:

THHN or THWN – These letters represent the most common types of individual wires used in residential applications. The letters represent the following NEC requirements:

• T is for the thermoplastic insulation on the wire;
• H is for heat resistance;
• H is for high heat resistance up to 194° Fahrenheit;
• W means the wire is rated for wet locations, such as outdoors; and
• N means the wire is impervious to damage from oil or gas.
Material — Most wire will be marked “CU,” for copper, the most common conductor of residential electricity. Because electricity travels on the outside of copper wire, wires are insulated to protect against fire and shock. Older wire could be aluminum or copper-clad aluminum.
Maximum voltage rating — This will be a number such as 600, which indicates the maximum voltage the wire can carry.
Gauge – This number indicates wire size. The most common gauges are 10, 12 or 14. Larger numbers represent smaller sizes or gauges of wire. See the table below for more about wire and cable gauge.

Color

Colored insulated wires other than white or green are always hot. Black, blue and red wires are typical hot wires in residential wiring.
White insulated wires are always neutral.
Green insulated and bare copper wires are always ground wires.

Applications

Run wires in conduit for unfinished areas such as basements or attics and outdoors for devices like landscape lighting. To learn more about conduit, see our Conduit Buying Guide

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Cable

NM-B cable is the most common form of indoor residential electrical wiring. The “NM” stands for non-metallic, which refers to the flexible, generally PVC sheathing surrounding the cable. The “B” represents a heat rating of 194° Fahrenheit. This rating ensures wires can operate at certain levels without overheating, melting the insulation and creating fire and safety hazards. Inside the sheathing are at least two thermoplastic insulated wires of the same gauge, though different cables can have different gauges. As with wire, all the information you need to know about NM-B cable is printed on the sheathing.

Type — This label will most commonly read NM-B.
Gauge — This number is the gauge of the individual wires inside the cable, such as 14, 12, 10, etc.
Number of wires — This number follows gauge. For example, 14/2 indicates that there are two 14-gauge wires (a ground wire, if part of the cable, is not included in this number) within the cable.
Grounding — The word “GROUND” or the letter “G” indicates the presence of a ground wire.
Voltage rating — The most common rating for residential use is 600 volts, though the number can vary. The number indicates the maximum voltage the wire can safely carry.
UL — Indicates that the cable is safety certified and approved for use by Underwriters Laboratories.

Wire is sized by the American Wire Gauge (AWG) system. The larger the number the smaller the wire. The following table provides information about amps (the amount of current a wire can safely carry), and wattage (the rate of electrical energy used by an appliance), and common uses of different gauges of wire and cable.

NM-B applications: Run NM-B cable behind walls and ceilings, inside floor cavities and in other unexposed areas. It is not designed for running along the outside of a wall, such as in an unfinished basement. Use NM-B indoors only, in spaces that are dry and free from moisture. Exposure to water can subject the wires to corrosion. Since the insulation is rated to handle only a certain level of heat, you’ll want to avoid placing wires near heat sources such as hot water pipes or heating ducts. Also, do not bury NM-B in cement, concrete, plaster or underground.

UF cable is similar to NM cable, but is rated for in-ground and damp-area installation. In fact, UF stands for underground feeder. From the outside UF cable looks like NM-B cable, but the wires are embedded as a group in solid thermoplastic (rather than individually encased in flexible thermoplastic). Like NM cable, UF cable comes in a variety of gauges to meet all electrical code requirements and is labeled with the same information carried on NM cable plus the designation UF.

Armored cable (AC) is wire enclosed in metal sheathing. Often called by the trade name, BX, AC consists of insulated hot and neutral wires, plus a bare bonding wire, all wrapped in paper. The metal sheathing acts as the grounding conductor. AC is relatively expensive and difficult to work with, so it’s not often used in new residential construction. It can be found in older homes, however. AC is rated for indoor use only.

Metal-clad cable is similar to AC, except that the wires are wrapped in plastic rather than paper. In addition, MC cable has a green grounding wire, because its metal sheathing can’t be used as a ground. MC cable is rated for indoor use only, though, like AC, it’s not often used in residential applications. Both MC cable and AC require:

Special cutting tools, which cut the sheathing but not the wires;
Bushings, which are inserted in the ends to prevent the sharp edges of the metal from damaging the wires; and
Special electrical boxes and connectors.

Coaxial cable, usually called coax, is a metallic cable most often used to carry television signals and connect video equipment. Coax features a central wire conductor covered with a dielectric or non-conducting insulator surrounded by mesh or a metal sheath and covered by a thin plastic layer for protection. For more information on coax, see our Networking Cables Buying Guide.

Category 5e cables, also called cat-5e, are the industry standard for unshielded twisted-pair cables (UTP) for connecting phones, computer networks, home automation networks and audio/video distribution systems. Often generically called Ethernet cable, the copper cabling typically consists of 4 pairs of wire (8 total conductors) wrapped in a single jacket. For more information on cat5-e cable, see our Networking Cables Buying Guide.

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