Blurb continued...
As stated before, the technically incorrect but easiest way to think about house wiring is that the power comes from the generating station on the red and black wires and goes back to the generating station on the white wire. You can use that same analogy when it comes to the individual circuits in your house as well. The power comes from the electrical panel on the black or red wires and goes back to the panel on the white wire. Again, it doesn't, but that way of thinking about it will at least help you better understand the reasons behind doing certain things certain ways.
For example, there is a standard wiring convention when wiring 110 volt electrical outlets. You will notice that the screw connections on one side of the duplex receptacle will be chrome plated, whereas those on the other side will be bare brass. The 120 volt wiring convention is to always connect the light coloured wire to the light coloured screw, and the dark coloured wire to the dark coloured screw. So, if you are mounting a duplex receptacle in an electrical box, you would connect the black or red pigtail to either of the bare brass screws on one side of the duplex receptacle and the white pigtail to either of the chrome plated screws on the other side of the duplex receptacle. The same convention holds true for wiring the male and female ends of extension cords; the dark coloured insulation always goes to the dark screw and the light coloured insulation always goes to the light coloured screw. You'll notice that on switches, both screws will be identical because the switch should always be on the black or red supply side of the circuit, not the white return side of the circuit.
The reason for this convention is safety. If homeowners, electricians and appliance manufacturers all follow this wiring convention, that will ensure that the on/off switch for 110 VAC appliances will always be on the power supply wire (that is, the red or black wire). And, this is important from a safety perspective because it ensures that the on/off switch will shut off power from coming into the appliance. In the time before electrical plugs and receptacles were "polarized", both prongs on a cord plug and both slots of an electrical receptacle were the same size. So, you could plug a 110 VAC appliance into a receptacle with the plug either right side up or upside down. The appliance would work equally well either way. However, in one of these positions the on/off switch to the appliance would be on the power supply wire and would shut off the power going into the appliance. In the other position, the on/off switch would be on the white neutral wire and would shut off the power getting out of the appliance. So, if there were a short circuit in the appliance, having the switch on the power supply wire would ensure that there would be no power in the appliance when the appliances on/off switch was in the off position. But, if the plug was upside down, then the on/off switch would be on the white wire, then there would ALWAYS be power in the appliance as long as it was plugged in. So, you could still get a shock from the appliance even if it was turned off. That's cuz the switch is now only preventing power from leaving the appliance via the white wire. The power can still come into the appliance via the black or red power supply wire and leave the appliance via some other route, like through YOUR BODY!!! Thus, if you happened to be touching a faucet while you were touching a toaster with a short circuit in it, you could end up getting 110 volts at 15 amps across your heart even if the toaster was turned off at the time, and that could potentially kill you. Thus, by sticking to this dark wire to dark screw wiring convention, we always end up with the appliance on/off switch on the power supply line, and that ensures that appliances can't give you a shock if they're turned off.
There is also a standard wiring convention used when wiring the plugs, receptacles and terminal blocks of 220 volt appliances, all of which will have provision for connecting THREE wires as well as a ground wire. Normally the ground wire terminal will be easy to identify because it will be grounded to the electrical box by an electrical conductor, and the remaining three connection points for the red, white and black wires will be arranged in a row. The wiring convention for
220 volt appliances is that the white neutral wire is ALWAYS connected to the middle terminal in that row of three connection sites, and the red and black wires are connected on either side of it. It doesn't matter which side you connect the red or the black wires to, as long as the white is in the middle and the red and black are on the outside, you're good to go. If your stove or dryer doesn't come with a cord and you want to connect one to the terminal block of the appliance, the same rule applies, namely "white in the middle, black and red on the outside". If you're wanting to wire a receptacle for a stove or electric dryer, then again, the same rule applies. First identify the ground wire terminal, and the remaining three connection points will be for the red, white and black wires and they should be arranged in a recognizable "row". Always connect the white in the middle of those three sites and the red and black on either side of the white.
Every dryer cord will have 4 prongs sticking out of it. The straight ones are for the red and black wires, the "L" shaped one is for the white wire and the round one is for the ground wire. Range cords also have 4 prongs, but they will use 3 straight prongs for the red, black and white wires and a round one for the ground wire. Configuring the plug and receptacle differently (with an "L" shaped prong instead of a straight one) is done so you can't stick a 30 amp dryer plug into a 50 amp range receptacle or vice versa.
The heating elements in both electric dryers and stoves require 220 volt power, but you still need to run the white wire to the stove or dryer. The reason why is that there will be circuits within the stove or dryer that require only 110 volt power. For example, the electric motor that turns the dryer drum or the light bulb inside an oven will both require 110 volt AC power, not 220 volt power. So, in an electric stove the heating elements will be connected between the red and black wires because they need 220 volts, but the electrical outlets provided for convenience on the stove console will be connected between the white wire and either the red or black power wires, because the convenience outlet is intended to provide only 110 volts to power 110 volt appliances. And, this is also why you can have TWO convenience outlets on a stove instead of just one. One of those convenience outlets will be powered by the black wire, and the other one by the red wire. Since the main black and red wires going to a stove are fused at 50 amps each in the electrical panel, any circuit between the red and white OR black and white wires inside the stove will give you a 110 volt 50 AMP CIRCUIT which probably won't stop pumping out the electricity if there's a short anywhere in that circuit, and 50 amps going through wiring rated at 15 amps is a great way to start a fire. That's why for the electric outlets provided for convenience on range cooktops, there will be a 15 amp fuse right in the range somewhere that fuses each convenience outlet down to
15 amps. If you have two cooktop plugs, one will be driven by the red wire and one by the black wire, and each will have a separate 15 amp fuse on it.
Also, if you stop to think about it, if the black supply wire is feeding a 110 AC voltage sine wave into the white "return" wire and the red supply wire is also feeding an equal but opposite voltage into that same white "return" wire, then theoretically, there should be no voltage in the white wire since the two sine voltage waves would cancel out. Similarily, the resultant current sine waves from the red and black wires would cancel each other out when they both meet at the white "return" wire. If the world was perfect and all electrical loads were purely resistive, like light bulbs, toasters, electric ranges and coffee makers, then the voltage and current sine waves from the two power supply wires would indeed cancel each other out, and there would be theoretically be ZERO voltage and ZERO current in the white wire. However, in the real world there are electric motors and television sets and computer monitors, all of which have some "impedance". In an electric motor, for example, the magnetic fields created by the electric motor windings impeded the flow of current through those same motor windings, so the motor windings themselves cause the current sine wave coming out of the motor to lag behind the applied voltage sine wave. Also, television sets and those old CRT style computer monitors have huge capacitors in them. In a capacitor, the current OUT of the capacitor is highest when the CHANGE in voltage is highest, and that occurs when the voltage sine wave passes through the point of ZERO voltage. Thus, capacitors cause the current sine wave coming out of those computer monitors and TV sets to actually preceed the applied voltage sine wave. So, even though the red and black wires carry equal and opposite 110 AC voltage sine waves, the impedance of "reactive" loads like electric motors and TV sets cause timing differences in the resulting current sine waves coming out of those loads. So, the current and voltage sine waves generally DON'T cancel out in the white wire, and there can be significant voltages and currents in the white wire as a result. So, to be safe, treat every wire as having dangerous voltage in it.