Electric utility companies generate three-phase power for transmission and distribution to customers. Commercial and industrial customers, along with some farms, make up the majority of three-phase customers. With the exception of high-rise apartment buildings, most residential customers have single-phase current. Some residential customers use phase converters if they require three-phase current for operating large motors on home shop equipment.
Single Phase 120 Volt Wiring
Most single-phase branch circuits in the United States use phase-to-neutral wiring. This type of branch circuit wiring supplies current in the range of 110 to 125 volts. Phase-to-neutral circuits have two wires for carrying current; the hot wire and the neutral wire. The hot wire carries current from the circuit breaker to the outlet or appliance, and the neutral wire returns the current to the service panel. National electrical code calls for white insulation on neutral wires, and any color except green or white on hot wires.
Single-Phase 240 Volt Wiring
Single-phase 240 volt circuits have two hot wires. Each hot wire supplies current to the appliance, and each hot wire also returns current to the service panel for the other wire. The hot wires connect to a double circuit breaker in the service panel. Electric hot water heaters and central air conditioning units use this type of phase-to-phase wiring.
The addition of a neutral wire to the circuit allows for electronic controls and motors to use 120 volts, while heating elements use 240 volts. Electric ranges and dryers use this type of circuit.
Use any color wire except white or green for the hot wires. Electrical codes reserve white for neutral wires, and green for ground wires.
Three-Phase with Neutral
High-rise apartment buildings often have a three-phase supply, but individual customers only receive two of the three phases. One phase of the supply combined with a neutral supplies 120 volts; two phases combined provide 208 volts.
This type of three-phase wiring is no different than similar, single-phase circuits, but the use of high-voltage appliances designed for 208 volts is recommended over the more common 240 volt appliances.
Common wire colors for three-phase are black, red and blue for phases one, two and three respectively. Use white for neutral and green or bare for ground wiring.
Three-Phase Motor Wiring
Electric three-phase induction motors require all three phases of electric current and use phase-to-phase wiring connections without a neutral wire. The three phases combine to form an electric current nearly as efficient as a DC supply. Three-phase motor circuits use triple circuit breakers and one wire for each of the three phases of electric current.
This type of three-phase wiring requires close attention to the phase connections. On a single voltage motor, phase 1 connects to the motor lead labeled T1, phase 2 to T2 and phase 3 to T3. Reversing any two leads will reverse the motor direction.