Single and Three-Phase Supply

The electricity from the sub-stations are stepped down through local transformers and supplied to nearby buildings as single-phase or three-phase supply (50 cycles per second at 220 volts between the phase and the neutral). Then the supply is 230 volts between a phase and a neutral and it is  volts between the phases. In a single-phase supply, there shall be one live-phase wire and a neutral. In a three-phase supply, there shall be three live phases and a neutral.

When the electrical power load is small and the total current drawn is less than 30 amp, a single-phase may be sufficient. That is only for lighting and use of fans a single-phase may be enough. However, when the load is heavy, as when use of many equipment such as air conditioners, cooling ranges, etc., it is necessary to draw current from three live phases and a neutral. The electric current metre for a three-phase supply is different from that of single-phase. It is a condition that all equipment used in residences must be 220–230 volts.

Instead of underground cable, if supply is drawn from overhead insulated cable special care has to be taken. In this case it is to be first run down the walls with insulated cables and then turned up at the entry point of the building to prevent rainwater running along the cable into the electric board.