Transmission and Distribution

A power plant generates electricity at approximately 25,000 volts. Transmission lines can carry higher voltages than the plant generates – up to 765,000 V - so a transformer steps up the voltage for the transmission lines. Higher voltages mean less current for the same amount of power, so wires can be smaller and cost less, but higher voltages need more insulation, which is more costly and limits the voltages carried by transmission lines. Electricity travels the transmission lines to a substation, at which point it is stepped down to between 4kV – 13kV. The electricity then travels on distribution lines atop wooden or concrete poles, or is run underground through special conduits. The electricity is stepped down again when it reaches a structure to between 120 – 240 volts. A cable composed typically of three wires – two live and one ground – carries the power to the structure through a meter box. This schematic shows the direction of electricity flow. Substations connect transmission lines to distribution lines. In a simplified AC circuit, power is generated, goes to the sending transformer, where it is stepped-up, then goes through the transmission lines to the receiving transformer, where it is stepped-down in voltage for use.