Determining what wiring to use for a particular outlet takes more than just picking a wire gauge. A lot depends on where the outlet is located, the type of outlet in question, the anticipated load on the outlet and local electrical codes. Understanding these variables takes the guess work out of selecting the proper wiring for safe outlet installation and use.
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Wire Gauge Basics
Residential electrical wire, commonly known as "ROMEX®," comes in a variety of gauges based on load capacity. In residential wiring, the most common wire gauges are 14, 12 and 10. Each gauge is intended for a different circuit amp. For example, 14-gauge wire is intended for 15-amp breakers, 12-gauge for 20-amp breakers and 10-gauge for 30-amp breakers or higher. Appliances such as stoves and dryers typically use a larger 6-gauge wire connected to a double-pole breaker for appropriate outlets.
Inside the Wire Sheathing
A secondary number displayed on ROMEX® wire packaging and stamped on wiring sheathing indicates the number of insulated wires inside. ROMEX® wire stamped with "10/2," for example, is 10-gauge wire containing a white and a black insulated wire, along with a bare copper wire for grounding. Alternatively, "10/3" contains a white, black and red insulated wire, along with a bare copper ground wire. Each type of wire features both indoor and outdoor versions, with varying types of sheathing to suit its purpose.
Standard 120-volt residential outlets typically fall into one of three categories: two-slot ungrounded receptacles, grounded receptacles and ground fault circuit interrupter receptacles. Two-slot receptacles are obsolete, but still found in older homes. Grounded receptacles are the minimum standard today, featuring a third slot for grounding appliances and other electrical items. Typical grounded receptacles are rated for 15 amps, but grounded receptacles with a horizontal top slot are rated for 20 amps. GFCI receptacles are used in areas near a water supply, such as bathrooms, kitchens and basements. These outlets use standard ROMEX® wire in 10, 12 or 14-gauge.
Stoves, hot water heaters and clothes dryers need more electricity than standard electrical appliances. Most are double-pole, meaning they require a full 240-volt line to operate. As such, these appliances need outlets and associated wiring rated for 240 volts. These special outlets typically require a much higher gauge wire than an outlet used for a toaster oven or lamp. Common wire gauges for such outlets include 10 or 6 gauge.
Determining Proper Wire Needed
To determine the right wire for a particular outlet, three key pieces of information are necessary: applicable electrical codes, outlet type and load. Electrical codes dictate minimum standards required for all outlet types and serve as a guide for selecting the right wire and outlet for a particular location. Once the outlet type is selected, the choice in wiring may be predetermined, as with large appliances and 240-volt outlets. If not predetermined, the anticipated load on the circuit to which an outlet is added, combined with outlet type will dictate required wire gauge.