Prior to 1996, electric dryers were supplied by a dedicated circuit that had three conductors: two hots and a neutral. There was no ground slot on the outlet, and dryer cords had no ground wire or ground prong. The ground for the appliance itself was connected to the neutral wires. This system worked pretty well and is still in use in many homes today. But in 1996 the National Electrical Code (NEC) began requiring 4-conductor outlets with a separate ground.
To be code-compliant, all newly installed outlets for dryers must be compatible with 4-prong, grounded dryer cords.
4-Prong Dryer Outlet Wiring
A 4-prong dryer outlet is wired as a 120/240-volt circuit. The 120-volt service is for the dryer's timers, sensors, and other electronics, while the 240-volt service supplies the heating elements. The NEC requires that dryers have a dedicated circuit with a minimum of 30 amps. This calls for a 30-amp, double-pole breaker and 10 AWG wire. In a standard installation with nonmetallic (NM) cable, 10-3 NM-B (with ground) cable is run from the service panel to a recessed outlet. If the outlet is surface-mounted, such as on a concrete or block wall, the circuit includes cable or insulated (THHN/THWN) wires inside nonmetallic or EMT conduit. The outlet may be a special surface-mount receptacle or may be a standard dryer receptacle installed in a surface-mounted box.
Wiring the 4-Prong Dryer Outlet
With the standard wiring configuration, the black hot wire is connected to one of the two brass hot terminals on the outlet, and the red hot wire connects to the other brass terminal. The hot terminals are interchangeable. The white neutral wire is connected to the silver neutral terminal on the outlet.
The ground wire connects to the ground terminal. If the outlet is mounted in a metal electrical box, the circuit ground wire is connected to two pigtails -- one for the outlet's ground terminal and one for the ground screw on the box.
Wiring the Circuit Breaker
The 30-amp breaker is wired with the standard double-pole configuration: the ground wire connects to the panel's ground bus bar. The white neutral wire connects to the panel's neutral bus bar. The red and black hot wires connect to the two brass terminals on the breaker. Note: Installing and wiring a breaker in a service panel is a job for a licensed electrician. Even if you install the circuit cable, box, and outlet, have an electrician make the final connections at the service panel.
Installation Tips for Dryer Outlets
- Position the outlet box in a convenient location for both the dryer cord and the dryer vent. Whenever possible, dryers should be near an exterior wall to minimize the length of the vent run.
- Local codes typically do not specify an installation height for dryer outlets, but it's helpful to be able to reach the plug without moving the dryer all the way out from the wall. A height of about 30 - 36 inches from the floor often keeps the plug accessible while hiding it from view behind the dryer, depending on the height of the dryer.
- Orient the outlet so the L-shaped (neutral) slot is at the bottom. This ensures the cord will extend downward from the outlet.