House Wiring 101

Anatomy of the Service Panel

• Behind the door of your service panel is a series of switches called circuit breakers. When you unscrew and remove the cover, you can see that each breaker snaps alternately onto one of two brass bus bars. These bars carry live electricity from the power company and are always energized and dangerous, even when you turn off the main power disconnect. Besides the brass bars, there is a silver one for the white neutral wire of each circuit and a ground bar for the bare ground wire. The ground bar must be connected to ground -- usually via a metal bar pounded into the earth somewhere near the panel.

Residential Circuitry

• Each of the hot bus bars is at a voltage of 120 volts relative to ground, and the two bars are at a voltage of 240 volts relative to each other. Most residential circuits connect to one or the other of these bars via a black wire connected to the breaker and to the neutral bar via a white wire, making the voltage in the circuit 120 volts. More powerful circuits for large appliances connect to both bus bars to provide a working voltage of 240 volts. They have two hot wires -- one black and one red -- and each wire connects to a separate breaker on a paired set.

Wire and Breaker Sizes

• The 120-volt breakers are typically rated for 10, 15 and 20 amps while 240-volt ones are usually rated for 30, 50 or 60 amps. The wire size you need for each circuit depends on the breaker size. The thinest wire for residential use is 14 gauge, and it is suitable for 120-volt ,10- and 15-amp breakers, but you need 12-gauge wire for 120-volt, 20-amp circuits. If you're wiring a 240-volt circuit, you need 10-gauge wire for 30-amp circuits and 8-gauge for 50-amp ones. The 220-volt circuits are usually dedicated, meaning that they power only one appliance, such as a dryer or water heater.

Running Wire and Making Connections

• One of the most difficult jobs in residential wiring is running the wiring itself. Since it must be hidden, do it before the walls are covered, although you can fish wires through existing walls with retractable metal coil called fish tape. Making connections to outlets, switches and lights is straightforward as long as you remember the basic color standard. Red and black wires always connect to brass terminals, white wires to silver ones and ground wires to green ones. Always make your connections with the breaker powering the circuit switched off, or make the connections before you hook up the breaker.