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Knowing the basic electric wire types is essential to almost any electrical project around the house. When you’re installing new wiring, for example, choosing the right wire or power cable is half the battle. And when you’re examining existing wiring in your home, identifying the wire type can tell you a lot about the circuit the wiring belongs to—for example, when you open a junction box and need to determine which wires go where. Wiring for modern homes is quite standard, and most homes built after the mid-1960s have similar types of wiring. Any new electrical installation requires new wiring that conforms to local building codes.
Here are some common types of home electrical wire.
Wiring Terminology
It helps to understand a few basic terms used to describe wiring. An electrical wire is a type of conductor, which is a material that conducts electricity. In the case of household wiring, the conductor itself is usually copper or aluminum (or copper-sheathed aluminum) and is either a solid metal conductor or stranded wire. Most wires in a home are insulated, meaning they are wrapped in a nonconductive plastic coating. One notable exception is ground wires, which are typically solid copper and are either insulated with green sheathing or uninsulated (bare).
The most common type of wiring in modern homes is in the form of nonmetallic (NM) cable, which consists of two or more individual wires wrapped inside a protective plastic sheathing. NM cable usually contains one or more “hot” (current-carrying) wires, a neutral wire, and a ground wire.
As an alternative to NM cable, individual wires can be installed inside of a rigid or flexible metal or plastic tubing called conduit. Conduit is typically used where the wiring will be exposed and not hidden inside walls, floors, or ceilings.
These larger wires in your home are carrying 120- to 240-volt circuit voltage, often referred to as line voltage, and they can be very dangerous to touch. There are also several wires in your home that carry much lesser amounts of “low-voltage” current. These are less dangerous, and with some, the voltage carried is so low that there is virtually no chance of shock. However, until you know exactly what kind of wires you are dealing with, it’s best to treat them all as dangerous.
NM Cable
Often called “Romex” after one popular brand name, NM cable is a type of circuit wiring designed for interior use in dry locations. Most NM cables have a flattened tubular shape and run invisibly through the walls, ceiling, and floor cavities of your home. Almost all of the wiring in outlets and light fixtures a modern home is NM cable. The most common sizes and their amperage (amp) ratings are:
14-gauge (15-amp circuits)
12-gauge (20-amp circuits)
10-gauge (30-amp circuits)
8-gauge (40-amp circuits)
6-gauge (55-amp circuits)
NM cable is now sold with a color-coded outer jacket to indicate its wire gauge:
White sheathing indicates NM cable with 14-gauge conductors.
Yellow sheathing indicates NM cable with 12-gauge conductors.
Orange sheathing indicates NM cable with 10-gauge conductors.
Black-sheathed cable is used for both 6- and 8-gauge wire.
Gray sheathing is not used for NM cable but is reserved for underground (UF) cable or service entrance cable (SE or SER).
NM cable is dangerous to handle while the circuit conductors are carrying voltage.
UF Cable
Underground Feeder (UF) is a type of nonmetallic cable designed for wet locations and direct burial in the ground. It is commonly used for supplying outdoor fixtures, such as lampposts. Like standard NM cable, UF contains insulated hot and neutral wires, plus a bare ground wire. But while sheathing on NM cable is a separate plastic wrap, UF cable sheathing is solid plastic that surrounds each wire. UF cable is normally sold with gray outer sheathing.
UF cable is also used for major circuit wiring, and it carries a dangerous amount of voltage as long as the circuits are turned on.
THHN/THWN Wire
THHN and THWN are codes for the two most common types of insulated single core wire used inside the conduit. Unlike NM cable, in which two or more individual insulated conductors are bundled inside a plastic sheathing, THHN and THWN wires are single conductors, each with its color-coded insulation. Instead of being protected by NM cable sheathing, these wires are protected by tubular metal or plastic conduit.
Conduit is often used in unfinished areas, such as basements and garages, and for short exposed runs inside the home, such as wiring connections for garbage disposers and hot water heaters. The letters indicate specific properties of the wire insulation:
T: Thermoplastic
H: Heat-resistant; HH means highly heat-resistant
W: Rated for wet locations
N: Nylon-coated, for added protection
THHN and THWN wires have colored sheathings that are generally used to identify their function in a circuit:
Hot wires: Black, red, orange
Neutral wires: White, brown
Ground wires: Green, yellow-green
THHN and THWN wires are circuit wires that should never be handled when the circuits are turned on.
Low-Voltage Wire
Low-voltage wiring is used for circuits typically requiring 50 volts or less. Several common types are landscape lighting wire, sprinkler system connections, bell wire (for doorbells), speaker system wires, and thermostat wires. Wire sizes range from about 22 gauge to 12 gauge. Low-voltage wires typically are insulated and may be contained in cable sheathing or combined in twisted pairs, similar to lamp cord wire and flat cable. It must be used only for low-voltage applications. These are typically very small wires that are much different from standard circuit wiring.
Serious shocks rarely occur with low-voltage wires, but it is still always best to turn off devices before working with them.
Phone and Data Wire
Telephone and data wiring are low-voltage wires used for “landline” telephones and internet hookups. Telephone cable may contain four or eight wires. Category 5 (Cat 5) cable, the most common type of household data wiring, contains eight wires wrapped together in four pairs. It can be used for both phone and data transmission and offers greater capacity and quality than standard phone wire.
Although data wiring does carry a small amount of voltage, anything under 30 volts is generally regarded as safe (a household circuit carries about 120-volts of power).1 However, there is always a danger of data wiring coming into contact with household wiring, so you should treat it with caution and avoid touching bare electric cable.
Coaxial Cable
Coaxial cable is beginning to grow less common, thanks to the use of other forms of data wiring, such as HDMI, for television data transmission. Coaxial cable is a round jacketed cable that features an inner conductor surrounded by a tubular insulating layer, surrounded by a tubular conducting shield made of braided multi-core wire. It can be identified by the threaded connectors that are used to make unions and device hookups.
Coaxial cable was once the standard for connecting televisions to antenna or cable service delivery and is still often used to connect satellite dishes or to bring subscription television service to an in-home distribution point. It typically has black or white insulation and is perfectly round in shape, making it easy to distinguish from NM electrical circuit cables.
The minuscule amount of voltage carried by coaxial cable signals makes it very unlikely to cause shock of any type—provided the cables are not in contact with another source of current.
