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.

120-Volt Receptacles

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.

240-Volt Receptacles

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.