Three-phase power basics

Three-phase power delivers electricity on three conductors whose voltages rise and fall in turn, each one a third of a cycle behind the last. The result is a steady flow of power that runs motors smoothly, needs less conductor material than single-phase for the same load, and is the standard supply for almost every commercial, institutional and industrial building in Canada.

Houses get single-phase power, which is enough for lights, appliances and a furnace. Once a building has large fans, pumps, compressors, elevators or many panels, three-phase becomes the practical choice. Facility staff do not need to do the maths, but understanding the basics makes it easier to read nameplates, spot unbalanced panels and recognise why a motor is running hot or backwards.

Three waves, one third of a cycle apart

In alternating current, the voltage swings from positive to negative and back, 60 times a second in Canada. A single-phase supply has one such wave. A three-phase supply has three, labelled A, B and C, each shifted by a third of a cycle, or 120 degrees. When one phase is at its peak, the other two are part-way through their swings. Added together, the power delivered by the three never drops to zero, unlike single-phase power, which dips to zero twice in every cycle.

Why buildings use it

• Motors run better. Three evenly spaced phases create a rotating magnetic field inside a motor, so three-phase motors start on their own and run smoothly, with less vibration than single-phase motors of the same size.
• Less copper for the same power. Three conductors carry about three times the power of a single-phase pair, with only one more wire. Feeders and switchgear are smaller for the load they serve.
• Two voltages from one system. A wye-connected supply gives a phase-to-neutral voltage for lighting and receptacles and a higher phase-to-phase voltage for larger equipment, such as 120 V and 208 V, or 347 V and 600 V.

Voltages, currents and power

In a wye system, the voltage between two phases is the phase-to-neutral voltage multiplied by the square root of three, about 1.73. That is why 120 V goes with 208 V and 347 V goes with 600 V.

For a three-phase load, power is calculated as 1.73 times the phase-to-phase voltage times the current in each phase times the power factor. As a worked example, a balanced load drawing 100 A on each phase of a 600 V supply, with a power factor of 0.9, uses about 1.73 × 600 × 100 × 0.9, or roughly 93 kW. The same load on a 208 V supply would need nearly three times the current.

Power factor describes how much of the current does useful work. Motors, especially lightly loaded ones, draw extra current to build their magnetic fields, which lowers the power factor. Some utilities bill commercial customers for low power factor, which is covered in our guide to demand charges and power factor.

Reading a three-phase nameplate

A motor or piece of equipment usually shows its rated voltage, the number of phases, the frequency, the full-load current and its power in kilowatts or horsepower. Check that the voltage matches the supply, that it says three-phase, and that the full-load current is within what the circuit was designed for. The nameplate often lists a service factor, which is how far the motor can be loaded beyond its rating for short periods, and an efficiency class. These numbers are what an electrician or engineer will ask for when sizing a circuit, a starter or a drive, so photograph nameplates during maintenance and keep them with the equipment records.

Wye and delta connections

Three-phase windings can be connected two ways. In a wye, one end of each winding meets at a common centre point, which becomes the neutral and is normally grounded. In a delta, the windings are connected end to end in a triangle with no centre point. Building services in Canada are commonly wye, which provides a neutral for single-phase loads. Many transformers have a delta primary and a wye secondary, and some industrial systems are delta throughout, which needs its own approach to grounding and ground fault detection.

Common problems facility staff meet

Unbalanced loads

Single-phase loads such as receptacles and lighting are spread across the three phases. If more of them end up on one phase, that phase carries more current, transformers and feeders heat unevenly, and voltages at the panel drift apart. Panel schedules that list load by phase, and occasional measurement of phase currents, keep this in check.

Loss of a phase

If one phase is lost, from a blown fuse, a failed connection or a utility problem, three-phase motors may keep running on two phases while drawing much higher current. They overheat quickly unless their protection trips. Motor protection relays that detect phase loss are common on important equipment for this reason.

Phase rotation

The order in which the phases peak, A then B then C, sets which way a three-phase motor turns. If two phases are swapped during utility work, a service change or a generator connection, motors run backwards. Pumps move little water, fans move air the wrong way and some equipment can be damaged. Rotation is checked with a meter before equipment is restarted after any such work.

Neutral current from harmonics

In a balanced system with ordinary loads, the currents of the three phases cancel in the neutral. Computers, LED drivers and other electronic loads produce harmonic currents that add in the neutral instead, sometimes enough to overheat it. Designers allow for this in buildings with many electronic loads.

Identifying the phases

Phase conductors in Canadian three-phase systems are commonly identified by colour, with red, black and blue for the three phases, white or grey for the neutral and green or bare copper for bonding. Older installations do not always follow the convention, so never rely on colour alone. Testing and identifying conductors is work for qualified electrical workers using proper procedures and equipment.

How ehZee can help

We design three-phase distribution for new and existing buildings, balance panel loads and help diagnose recurring problems with motors and feeders. See our power distribution design service, or read 120/208 V vs 347/600 V systems and motors, starters and VFDs.

Common questions