What is the use of 2 phase electrical wiring

2 phase electrical wiring is where you have 2 wires each providing the same voltage AC but out of phase with each other. There are two variants of 2 phase with different “phase angles”, ie: the amount that the phases are out of step with each other. One variant uses a 90˚ phase difference, with the 2 phases looking like this:

This 90˚ phase angle produces a smooth power delivery, the varying power from the two phases always summing to a constant total power as the peaks from one phase correspond with the zero crossings for the other. This is ideal for high power machinery powered from 2 phase as it avoids vibration and stress with power variation. It also enables motors to self-start.

However the downside is that the two phases are out of step and so a third return wire is always needed, wasting conductor material. In fact the return wire needs to be twice the cross-sectional area of each phase, so you require as much conductor again. Often two equal size wires are used for the neutral, making it a four wire system.

The other variant uses a 180˚phase difference, looking like this:

This is commonly referred to as “split phase” to distinguish it from 90˚ 2 phase. It has the advantage of no longer needing a return wire as long as current from the two phases is balanced, cancelling to zero. However as both phases now cross zero simultaneously their combined power delivery is continuously varying from maximum to zero. This makes this option unsuitable for powering industrial machinery.

So a compromise has to be made. You either use 90˚ 2 phase but at the expense of needing twice as much conductor for distribution, or you use 180˚ split-phase but at the expense of being unsuitable for industry.

It turns out that moving up to 3 phases at 120˚ spacing solves this problem. Balanced usage cancels out the return current avoiding the need for a return wire making it as conductor efficient as split-phase, whilst the sum of the powers delivered is constant. As a consequence 3 phase is therefore used almost exclusively for AC transmission and distribution.

2 phase was however used historically for distribution in North America, and hasn’t entirely been phased out. Apparently the city centre of Philadelphia is a notable exception (as of 2016 at least).

Meanwhile split-phase 120V is still widely used in North America for domestic supplies. A common neutral is also provided and most appliances and lighting use a single phase to neutral. Unfortunately 110/120V is really too low a voltage even for domestic mains supply, and compared with the 220–240V used in most of the world a 120V supply results in double the current and so four times the copper needed to deliver to the house and carry around the house, and within appliances.

This results in most outlets having only half the power available compared with typical 220–240V outlets in the rest of the world. Small high power appliances are therefore relatively uncommon in North America, such as electric kettles which are used ubiquitously in a lot of the rest of the world. So provision of split-phase to North American houses partially overcomes this issue at the expense of additional complication, giving the option of powering high power appliances by connecting across the two phases using dedicated outlets, producing 240V AC. It is also providing a route for gradual transition away from 120V split-phase, with rewired and new build housing commonly having more widely available 240V outlets.

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