Figure 1. Wiring inside the walls.
I grew up in a house under construction. Every day for the first 18 years of my life, I looked at the wiring in the unfinished walls. It was nice to have the wires. They were fat, black and rubber-coated. Because they ran horizontally through the 2x4s, they made a convenient place to hang things - towels in the bathroom, school awards and prizes in the kitchen, crayon drawings in the kids' bedrooms.

My dad was an electrician, but he never explained what was going on with the wires. Maybe that's because he understood it so well he couldn't imagine it needed explaining. Or maybe he thought, “You take this wire from here and put it there - what's to explain?” Anyway, looking at the wiring in the walls every day completely mislead me about electricity.

If you could see inside your walls, you'd see what looks like one wire connecting wall outlets to switches to ceiling lights (see Figure 1).

What you're seeing isn't a wire - it's a cable. Inside the cable are at least two wires. In the 1950s when my dad wired our house, there were just two wires in the cable. Grounding appeared in the 1960s and added another wire. In the drawings here, we'll pretend we're back in the '50s and, for simplicity, do without grounding.

Figure 2. Electricity can flow through the outlet and, at the same time, around it. Grounding wires are omitted for drawing clarity. Note: This drawing shows continuous wires. In modern wiring, for safety, a wire nut connection is required in the white neutral wires.

Wiring Wall Outlets

When you look at a drawing like this one, it gives no clue that at the same time electricity can go through the first outlet, it can also go around that outlet to the second. The next drawing (Figure 2) shows what the wires inside the cable look like with the outside cable covering stripped away.

The Complete Circuit

On the left side of the drawing, the cable is connected to the service panel (as it is shown in Figure 1). The service panel is ultimately connected to the power plant. So we can think of the service panel as both the power supply and the switch for the circuit with the first outlet. In this case, think of the outlet as a load, or potential load. (Read my November 2005 PM column.)

Notice that the two wires in the cable are black and white. Black is “hot.” It carries to the load the electricity that's full of energy. White is neutral. It carries back to the power supply the electricity after the load has used the energy.

The complete circuit for the first outlet is the power plant as the power supply, the fuse or circuit breaker in the service panel as the switch, and the outlet as the potential load. Until we plug a lamp or appliance into the outlet, there's no real load, and no electricity flows through the circuit. When we plug in a lamp, the light bulb is the load. Then the outlet is simply the connecting point where the light bulb load joins the circuit.

Figure 3. Cable and outlets with grounding wire (black and white wires omitted for clarity).

Terminal Screws

If you look at the outlet without its cover, there are four terminal screws. As you look at an actual outlet in your wall, don't touch any wires or terminal screws, because some of them can hurt you.

“Terminal” means ending. The terminal screws are where the wires end, or connect. On the left side are two silver terminal screws. These are for the neutral (white) wires. On the right are two brass screws. These are for the hot (black) wires. The screws on each side of the outlet are connected to each other by a brass strip: brass screw to brass screw, and silver screw to silver screw.

A two-wire cable from the service panel at the left brings a black and a white wire to the first, or left, outlet. The white is attached to the top silver terminal. The black is attached to the top brass terminal.

A second two-wire cable connects this first outlet to a second outlet. From the second cable, the white wire connects to the bottom silver terminal of the first outlet. The black wire from the second cable connects to the first outlet's bottom brass terminal.

Finally, the wires at the other end of the second cable attach to the second outlet. The white wire goes to the top silver terminal. The black wire goes to the top brass terminal.

Electrically, two things can happen at the same time. If there's a lamp or appliance plugged into the first outlet, electricity comes in the black hot wire, goes through the lamp or appliance that's plugged in, and leaves through the white neutral wire back to the service panel.

At the same time, whether or not there's a load plugged in to the first outlet, electricity can move on to the second outlet. Electricity moves from the service panel to the top black terminal of the first outlet, and along a connector between the two screws on the same side to the second terminal of the first outlet. It then moves on to the second outlet.

If there's a load plugged into the second outlet, electricity goes from the black wire, through the load, through the white wire back to the bottom silver terminal of the first outlet, through a connector onto the top silver terminal of the first outlet, and back to the service panel.

Figure 4. Black (hot), white (neutral) and copper or green (grounding) wires together.
Outlets are often called duplex outlets because they are in pairs, like duplex apartment units. The number of duplex outlets is limited only by the amp rating of the circuit. (For circuit amp ratings, see my December 2005 PM column.)

If the wiring was done in the 1960s or later, the cable also contains a third wire, which is a grounding wire of bare copper or insulated green. It is connected to a green screw on each outlet, or to a grounding screw in the outlet box. Here is an illustration (Figure 3) of how the grounding wiring would look without the black and white wires.

And here is a drawing of the black, white and grounding wiring all together (Figure 4). This is how the wiring might look if you looked inside an outlet wired within the last 40 years.

When you look at any wiring, it might look like a bowl of wire spaghetti. But stop and think about what you now know. Then you can begin to make sense of what the electrician does.