There is a lot of misconceptions in electricity about series circuits and parallel circuits. But if circuits wired up in series have an infinite variety of uses in electronics, it doesn't work when wiring outlets in a home. Let's illustrate with simple sets of mini-lights from 20+ years ago, with 50 or 20 mini-lights, where if one bulb was pulled out or burnt out, the whole string went out. That's because the current flow goes from the outlet to one side of the first bulb and out the other side of the same bulb, then through one side of the second bulb and out of the other side and so on until it gets at the other end and return to the other side of the outlet. By pulling out one bulb, it cuts off the path for the current to go through to complete the circuit, just like turning off a switch. Also, another point worth mentioning is the fact that the replacement bulbs for mini-lights come in 2.2V for sets of 50, 3.6V for sets of 30, and 5.5V for sets of 20 lights. That is because in a series circuit, the voltage is divided up evenly between each and every light bulb on that string, and if we do the math, 50 lights x 2.2 Volts = 110V, or 20 lights x 5.5 Volts = 110 Volts. So two outlets wired up in series would need for an appliance must be first plugged in, second turned on, for the power to run through but at only half the voltage which in most cases couldn't work (a 100 Watts light bulb couldn't deliver more than 50 Watts of power!)
Wiring outlets has to be done in parallel, since that way, should one outlet fail or a bulb inserted in one light fixture burn out, the rest of the fixtures and appliances hooked up to that same line will keep working. Parallel wiring is used in homes because of the reliable self-containment of each outlet or light fixture that allows current to flow past them even if they fail than to have the whole circuit interrupted by one bad outlet.
Parallel Wiring of Outlets
Current flows through a circuit from the main circuit panel to each receptacle in its path. Home circuit panels are usually fitted with 15-amp or 20-amp circuits. Each of these can provide power in parallel to six or more outlets. To do this, the hot, neutral, and ground wire from the circuit panel will extend to the first outlet in the chain and on to the next and the next until the end of the circuit. Each outlet is a circuit interrupter but provides power independently of the others.
Step 1 - Turn off Power to the Circuit
When working on any sort of household electrical project, always be sure to turn off the power at the main circuit breaker dedicated to the circuit you're working on.
Step 2 - Wire the Outlets in the Middle of the Circuit
Every outlet until the last is considered in the middle of the circuit. Run the Romex up in through the bottom of the first outlet box and remove 6 inches of the outer insulation to separate the three wires inside. There will be a black, white, and bare or green wire. Strip 1 inch of insulation off of the black and white wires and green if present. There should be five screws on the outlet itself: two silver, two brass, and one green. Attach the white wire to the lower silver screw and the black wire to the lower of the brass screws.
Now feed enough Romex through the top hole in the outlet box and extend it to the next outlet, bringing it through the bottom hole in the second outlet box. Leave 6 inches extending into the first box and 6 inches coming into the second box. Go back to the first outlet and strip the wires in the same way. Connect the second white wire to the upper silver screw and the black to the upper brass screw. Connect a small piece of (bare or green) ground wire to the green screw on the first outlet and pigtail the other end of that plus the two ground wires coming into and leading out of the first box. You pigtail them by twisting the three wires together and screwing a wire nut over the end.
Step 3 - Connect the Other Outlets in the Middle
Complete each of the outlets in the middle of the circuit in the same manner. Lead into each outlet box from the bottom and exit through the top to the next outlet. Make a pigtail with the ground wires in each box, connecting a small length of bare or green wire from each pigtail to the green screw on the outlet. This will require that you have an extra length of wire for the ground connections.
Step 4 - Complete the End of the Circuit
When you reach the last outlet, you will only enter into the outlet box with the Romex. The circuit will go no farther. Strip the Romex in the box, strip the individual hot and neutral wires, connect the white to the upper silver screw and the black to the upper brass screw. Pigtail the ground wire from the Romex with a length of ground wire connected to the green screw.
With the last outlet, the circuit is wired in parallel. Check each of your connections again, but don’t affix the outlets to the wall just yet.
Turn the power back on to the circuit and check each outlet for power. Once you are satisfied they are all working properly, fasten them to their respective boxes and cover them with their faceplate. That's all there is to wiring outlets in parallel.