Single phase vs three phase

What's the difference between single phase and three phase?

Electricity is either connected at 230 or 240 volts (single-phase, which accounts for the majority of domestic situations), or 400 and 415 Volts (three-phase). The latter is better suited to providing for powerful appliances and fixed plant, and is more commonly used by industrial and larger commercial users.

Single-phase comes to the home with two wires: active and neutral. The neutral wire is connected to earth (water pipe, earth stake, etc.) at the switchboard.

Three-phase has four wires: three actives (called phases) and one neutral. The neutral wire is earthed at the switchboard

When is three phase more appropriate?

1. Big electric motors (usually more than 2 kilowatt) need three-phase power. This includes large workshop equipment.

2. Large domestic installations sometimes have three-phase because it distributes the total load in a way that ensures that the current in each phase is lower.

For example: Imagine the total electrical load is 24 kilowatts (24,000 watts - that's a lot for a domestic installation). For a normal, single-phase power supply at 240 volts, the maximum current would be 100 amps. The current in amps multiplied by voltage in volts gives power in watts (Power = voltage x current).

If a three-phase supply is available, then the 24,000 watts are divided by 3, meaning that 8000 watts is being used per phase. Now the current per phase is also down to a third of what it would be with a single phase supply (about 30 amps per phase, rather than 100). Putting that in perspective, ten 100 watt lighting fixtures represent 1 kilowatt of power, which equates to a bit under 40 amps.

A word of caution though: connection fees for three-phase are higher, and there are fixed annual charges as well for three-phase so don’t contemplate it for a new home unless you really need it.

Rural connections and SWER

Depending on your locality you may be connected to a SWER line. These are used in many country areas. Single wire, earth return (SWER) delivers single phase power. It’s an economical way of distributing power, because only one transmission line (active) is needed. There is no neutral - instead the earth is employed as the ‘return’ conductor.

If three-phase motors have to be used, a single-phase to three-phase power converter has to be installed by the electricity consumer