Q. Residential wiring and safety considerations?

Residential electric power service commonly consists of a three-wire ac system supplied by the local power company. A distribution transformer with a primary and two secondaries formed with a center tap on the secondary side, as illustrated in Figure, is located on a utility pole, from which three wires originate. Two of them are known as "hot" wires, while the third is called a neutral wire, which is connected to earth ground. In accordance with standard insulation color codes, "hot" wires are denoted by either B (black) or R (red), neutral by W (white), and ground (or uninsulated wire) by G (green). The functional difference between neutral and ground wires will be brought out later when considering circuit wiring.

Domestic loads consisting of 120-V appliances and lighting, divided nearly equally between the two 120-V (rms) secondaries, are connected from hot wires to neutral. Appliances such as electric ranges and water heaters are supplied with 240-V (rms) power from the series-connected secondaries, as shown in Figure.

Minimizing the power loss in the lines (known as I²R loss) is important from the viewpoint of efficiency and reducing the amount of heat generated in the wiring for safety considerations. Since the power loss in the lines is directly related to the current required by the load, a lower line loss will be incurred with the 240-V wiring in delivering the necessary power to a load. For the lower voltage case, however, the size of the wires is increased, thereby reducing the wire resistance, in an effort to minimize line losses.