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.
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.
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.
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 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: