Electricity flows along a continuous wire circuit from a power plant to a power-drawing device such as a lamp or appliance and back again. The amount of current flowing past a given point in a wire is measured in amperes or amps. The pressure that forces current through a wire is measured in volts. The use of electricity is measured in watts, which are equal to volts multiplied by amperes.
Power enters most houses by means of three wires that pass through a meter to a distribution panel. Two hot wires carry 120 volts each; the third wire is neutral. One hot wire and the neutral wire form a 120-volt circuit needed for light fixtures and small appliances . In a typical circuit, a hot wire, wrapped in black (sometimes red or blue)insulation, delivers current at 120 volts to a series of receptacles, light fixtures, and switches.
A white or gray-insulated neutral wire carries current at zero volts back to the distribution panel. Because the neutral wire must provide an uninterrupted path, it is connected to receptacles, but not to switches, fuses, or circuit breakers, which can cut the flow of current. In some circuits, a white wire may be hot; in that case, its ends should be painted or taped black.
Each circuit must have a grounding system to provide a safe path to earth for abnormal current flow. In a well-wired house , a continuous green-insulated or bare copper wire connects every receptacle, switch, and junction box in a circuit to the neutral bus bar in the distribution panel. A main ground wire connects the bus bar to an in-ground water pipe or to a copper rod driven into the soil. White wires are also grounded through the bus bar to keep them neutral(at zero volts).