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A three-phase motor is more compact and less costly than a single-phase motor of the same voltage class and rating, and single-phase AC motors above 10 hp (7.5 kW) are uncommon. A three-phase induction motor has a simple design, inherently high starting torque and high efficiency. Costs can be prohibitive for transmission lines, but high capacity, long distance super grid transmission network costs could be recovered with modest usage fees. Measuring instruments can be digital or analog. Most groups of houses are fed from a three-phase street transformer so that individual premises with above-average demand can be fed with a second or third phase connection. They are used when three-phase power is not available or cost is not justifiable. Zigzag transformers may make the equivalent of six-phase full-wave rectification, twelve pulses per cycle, and this method is occasionally employed to reduce the cost of the filtering components, while improving the quality of the resulting DC. The output of such rectifiers is much smoother than rectified single phase and, unlike single-phase, does not drop to zero between pulses. Such rectifiers may be used for battery charging, electrolysis processes such as aluminium production and the electric arc furnace used in steelmaking, and for operation of DC motors.

Rectifiers may use a three-phase source to produce a six-pulse DC output. Distributing single-phase loads among the phases of a three-phase system balances the load and makes most economical use of conductors and transformers. Inductive and capacitive loads will cause current to either lag or lead the voltage. A current as low as 30mA can cause ventricular fibrillation, so RCDs (residual current devices) that detect earth leakage faults are designed to trip and shut off power when a current of 30 mA flows (e.g. if you touch a power cable). Consequently, such harmonic currents add in the neutral in a wye system (or in the grounded (zigzag) transformer in a delta system), which can cause the neutral current to exceed the phase current. If the loads are evenly distributed on all three phases, the sum of the returning currents in the neutral wire is approximately zero. Capacitors and autotransformers can be used to approximate a three-phase system in a static phase converter, but the voltage and phase angle of the additional phase may only be useful for certain loads. When properly designed, these rotary converters can allow satisfactory operation of a three-phase motor on a single-phase source.

Some railway locomotives use a single-phase source to drive three-phase motors fed through an electronic drive. Individual heating units are often connected between phase and neutral to allow for connection to a single-phase circuit if three-phase is not available. This motor-generator combination can provide a frequency changer function as well as phase conversion, but requires two machines with all their expenses and losses. The motor-generator method can also form an uninterruptible power supply when used in conjunction with a large flywheel and a battery-powered DC motor; such a combination will deliver nearly constant power compared to the temporary frequency drop experienced with a standby generator set gives until the standby generator kicks in. Such converters may also allow the frequency to be varied, allowing speed control. Phase converters are used when three-phase equipment needs to be operated on a single-phase power source. Variable-frequency drives and digital phase converters use power electronic devices to synthesize a balanced three-phase supply from single-phase input power. Gas-discharge lamps and devices that utilize rectifier-capacitor front-end such as switch-mode power supplies, computers, office equipment and such produce third-order harmonics that are in-phase on all the supply phases.

Sometimes the utility company will upgrade the transformer if more homes are connected and voltage drops excessively. Any unbalanced phase loading on the secondary side of the transformer will use the transformer capacity inefficiently. Except in a high-leg delta system and a corner-grounded delta system, single-phase loads may be connected across any two phases, or a load can be connected from phase to neutral. Similarly, an unbalanced load may occur if one line is consistently closest to the ground and operates at a lower impedance. Maybe one of the phases is centre tapped? It can be reduced by evenly spreading line frequency operated light sources across the three phases so that the illuminated area is lit from all three phases. Line frequency flicker in light is detrimental to high-speed cameras used in sports event broadcasting for slow-motion replays. Early types were available fused in both poles; later types fused in the line only and provided an earth pin.

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