The real power supply in a building is a matter of human perspective.

I love to talk about the fact that everything in a circuit has to be a power supply, a switch or a load. They're connected together with wires. There must be at least one of each. And I just can't get enough of the fact that every last thing in a circuit is one of these, and nothing else.

But what's going on when some things seem to be more than one of these functions? Is a wall outlet a load or a power supply? Is a service panel the power supply, or a bunch of switches?

First, here's a quick review of what the three parts of a circuit do:
  • A power supply is where the electricity comes from. In a flashlight, the power supply is the battery. In a house it's ... well, is it the wall outlet, or the service panel, or the utility company's power plant, or all three? We'll see.

  • A switch turns the electricity on and off, whether it's the flashlight switch or a room's wall switch.

  • A load is the consumer of electricity, such as a light bulb or the motor in your electric drill.
Now back to the question of what is the function of an outlet or a service panel. The answer is: It depends on your perspective. Yep, you may have thought electricity is a hard and fast science. But a person's perception is a factor. Here's why.

Is a wall outlet a load or a power supply?

A wall outlet is in two different circuits (see Figure 1). It's the exact same wall outlet, but its function is different depending on the circuit you're looking at.

Look at the circuit on the left side of Figure 1. The power plant is the power supply, the service panel is the switch, and the outlet is the potential load. I don't mean “potential” here as in “potential electricity.” What I mean is, the outlet is where a load, such as a lamp, can be plugged in. So in this circuit, the outlet is a “load waiting to happen.”

On the right side of Figure 1, the circuit consists of the same wall outlet, the lamp switch and the lamp bulb. In this circuit, the outlet represents the power supply.

So, the part that a wall outlet plays in a circuit depends on which circuit you're looking at. Your perception is the key to the whole thing. Of course, electricity doesn't care what we call any of it. But correctly naming the function makes a difference in our understanding of what's going on.

What the real power supply in a building is also is a matter of human perspective.

The wall outlet is the power supply when we plug in a lamp. The lamp contains both the load (the light bulb) and the switch, and we need only the power supply to complete the circuit.

Or we can take one step back and look at the service panel as the power supply. That's where the electricity enters the building.

Yet, the ultimate power supply, where electricity is made, is the utility company's power plant.

Switches In The Service Panel

Although we often think of the service panel as the power supply, it is actually made up of switches. The purpose of these switches is safety. A building is wired with a number of circuits. Each has a switch in the service panel to turn it off for service disconnect or if an unsafe condition arises. The switch opens (turns off) if the circuit is overloaded or shorted.

The switches in the service panel may be fuses or circuit breakers (see Figure 2).

In buildings wired before the 1960s, the service panel is a fuse box. A fuse is a one-time, disposable switch. It's like a paper cup - use it once and replace it.

The maximum amperage (A) of the circuit is marked on the fuse box, and the fuse should match. If you overload the circuit by trying to get too much electricity out of it, or if there's a short in the circuit, the fuse “blows.” That means the thin piece of metal inside the fuse burns or breaks. The resulting opening turns off the circuit.

When you look at a blown fuse, you can see the reason the fuse failed. If it's blackened, there was a tiny explosion in the fuse because of a short in the circuit. A short is a massive overload. If the fuse is burned through, the action was slower because of an overload in the circuit.

Either way, the fuse has to be replaced for the circuit to work again. It should be replaced with a fuse of the amperage that's marked on the fuse box.

It may be tempting to replace the fuse with one of a higher amperage in order not to have it blow again. That's a bad idea. The fuse should be the weakest part of the circuit so that it burns, or “blows,” before anything else. It's better to have a burned fuse than a sizzling wire deep inside the wall.

The reason that putting a penny or other piece of metal in a fuse socket works (and should never be used) is that the metal is a path for electricity. It closes the switch. It defeats the safety factor; that's the reason for having a fuse in the first place. The heavier metal won't break if the demand on the circuit is too great. The circuit can overheat, and there can be a fire.

There are often two 60A main fuses. If there are “odd” things going on with the building's electricity - such as half of the lights being dim, or the dryer drum turning, but with no heat - a simple solution may be to replace both of the 60A fuses, even though one looks all right.

More recently wired buildings have circuit breakers instead of fuses. Each circuit breaker is a switch that stays closed unless the circuit is overloaded or shorted. When there's an overload or short, the circuit breaker switch opens. That keeps electricity from flowing into that circuit until someone resets, or closes, the circuit breaker. Circuit breakers are often labeled, such as “kitchen and family room,” or “upstairs bath and bedrooms.”

What if the circuit breaker keeps opening? Then something's wrong in the circuit. Perhaps there's a short in the wiring, or in something that's plugged into the circuit. For example, when I'm teaching a wiring class, if someone mis-wires a training board, it can open the circuit breaker. Then we don't have any electricity until we find the breaker box and close the opened circuit breaker.

Another situation that will open the circuit breaker is too much load, or amperage, plugged into the circuit. Each appliance or light has an amp rating. If it's a 15A circuit and I plug in appliances and lights that exceed that, the circuit breaker will open.

But wait a minute! Light bulbs don't have amp ratings. Tune in next month for a simple solution.

Thanks to Dale Watterson of Madison Gas and Electric for being my line voltage shepherd.