Houses are generally wired in
circuits for a couple of reasons. Think of the series circuits on old Christmas tree lights. If one light bulb doesn't work, none of the lights will come on, because all the electricity has to flow through each light bulb in sequence. A broken filament in one bulb creates an
and the electricity can't flow.
Another problem with series wiring is that as we extend the circuit, adding more lights, each light we add makes the other lights dimmer. That's because we're increasing the total linear resistance in the circuit. The voltage is fixed, so as the resistance increases, the current flow must decrease.
Neither of these are desirable situations and, therefore, our houses are wired in
. Electricity has several paths it can follow from the energy source to ground. Even with several light fixtures controlled by one switch, the light fixtures are in parallel. If one light bulb burns out, electricity still flows through the other bulbs.
The other feature of parallel circuits is that adding another light or resistor of any kind will not cause the others that are already working to get dimmer or draw less current. If you think of a simple circuit with a 60-watt light bulb, a 120-volt power supply seeing a 60-watt light bulb will have a resultant current of 1 /2 amp (I=P/V=60/120=1/2). Any place in this circuit where we measure the current, we have 1 /2 amp flowing. If we add a second 60-watt light bulb in parallel, the circuit has a second branch. In each leg of the branch, the current flow would be 1/2 amp. Before the branch splits, and after it comes back together, the current would be 1 amp. However, when the second light is added, the first light still sees the 1/2 amp current flow and does not change in brightness. If this seems like magic to you, you'll just have to accept that this is the way electricity works. Incidentally, you can extend this picture. If you put a third branch in with another 60-watt light bulb, it too, would draw 1/2 amp, and the total current draw in the common parts of the circuit would be 1 1/2 amps. There are three
paths, each carrying 1/2 amp.
You can see that if you put in thirty 60-watt light bulbs, you are going to draw 15- amps (I=P/V=30x60/120=15). Fifteen amps flowing through a conventional household wire is close to the point where you'll blow the fuse or trip the breaker. This is the threshold of an
situation. A general design limitation is to restrict a 15-amp circuit to 80% of its rated capacity. This limits the circuit to 12 amps, maximum.