Knowing the basic wire types is essential to almost any electrical project around the house. When you're installing new wiring, choosing the right wire or cable is half the battle. And when you’re examining existing wiring in your home, the wiring type can tell you a lot about the circuit the wiring belongs to—for example, when you open up 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.
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It helps to understand a few basic terms used to describe wiring. An electrical wire is a type of conductor, a material that conducts electricity. In the case of household wiring, the conductor itself is usually copper or aluminum, and either solid 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 insulation or are 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 wiring will be exposed and not hidden inside walls, floors, or ceilings.
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Commonly called “Romex,” after the popular brand name, NM cable is designed for interior use in dry locations. Almost all of the wiring in 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)
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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 completely surrounds each wire.
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THHN and THWN are codes for the two most common types of insulated wire used inside 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:
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- T: thermoplastic
- H: heat-resistant; HH means highly heat-resistant
- W: rated for wet locations
- N: nylon-coated, for added protection
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Low-voltage wiring is used for circuits typically requiring 50 volts or less. Three common types are landscape lighting wire, bell wire (for doorbells), and thermostat wire. 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 pairs, similar to lamp cord wire. It must be used only for low-voltage applications.
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Phone and Data Wire
Telephone and data wiring are low-voltage wires used for “land line” telephones and for internet hookups. Telephone cable may contain four or eight wires. Category 5, or 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.