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I have checked How Stuff Work, About.com, Ask.com, Google and even asked an electrical engineer and found no satisfying answer to the following questions:
For household AC current, why do we need 2 live wires, a neutral and a ground, total 4-wire connection? We get 3 wires from the street transformer where the Secondary coming towards home has a center tap that is grounded. The phase difference from either one end to the middle tap of the Secondary is 110 volts RMS, and end-to-end 220 volts RMS. We use one of the single phases for the light bulbs and toasters (110V) etc. and we use both phases for the Central Air Conditioner (220V).
If you hookup a light bulb only with the live wire and use the ground instead of the neutral wire, it still lights up. I know power companies use the planet's ground as a wire to return electrons.
The center tap for the 2-phase Secondary which is coming to the home is grounded in the USA, is it not?
Since the neutral wire and the ground wire both are used for returning the electrons, why don't we just use the live wire and the ground wire and no neutral wire for single phase appliances?
Why do we need the separate neutral wire for a single phase 120 volt light bulb?

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You don't need it. Older homes don't have it. But, it is better to have it than not.

http://en.wikipedia.org/wiki/Ground_and_neutral

The ground wire is there to provide a chassis ground for your equipment in case a wiring fault or failure causes the hot or neutral wire to contact the chasis. They cannot simply use 2 wires and connect the neutral to the chasis because not all plugs are polarized and there's the risk of the recepticle being wired wrong. It is also necessary for GFCI operation. I'm sure there are other reasons as well, but the main ones are for protection.

KuoH

Free1As1A1Bird said:   Why do we need the separate neutral wire for a single phase 120 volt light bulb?
 

Thanks for the explanation

I don't know the reason, but I suspect the requirement is related to 240VAC appliances often having 120VAC circuits inside them, meaning there will be a greater load between one line and neutral than between the other line and neutral, causing neutral to deviate in voltage from ground.  Fortunately the deviation is almost never more than a few volts, rather than a dangerous voltage.  

The neutral is a current carrying conductor. On all circuits of 30 amp or less, the ground is required to be the same size as the circuit conductor. On larger circuits the ground is downsized. The neutral is seldom downsized.

Some local codes require the same size neutral in the service entrance as the two hots, some allow it to be downsized slightly as per the NEC. If you used the ground wire instead, the ground from the ground rod or local grounding system would have to be the same size as that the neutral would have been.

In the beginning there was no "grounded conductor" (neutral). this meant the only danger of electrocution was between the two phases, not to ground. So people would throw wires across the supply lines and steel electricity. Now that the earth is part of the whole electrical system (established by grounding) you can't just throw wires around without danger of "grounding" one of them.

Some 240 volt systems also require a neutral. The most common is the cloths dryer. It takes a 240 volt circuit because it has a 240 volt heating element, but the motor is 120 volt and therefor requires a neutral instead of just a ground. There are many many dryers still running on circuits that are only 2 wire with ground. If the ground is ever lost the motor won't run and there is potential for serious shock if you touch the dryer. There is also potential for back feeding voltage into the grounding system if the incoming system ground is lost. Been there done that, lost ground at service entrance and dryer back feed into the ground and cost a new tv. This wouldn't have happened if the neutral had also been grounded, but the NEC requires the grounds and neutrals to be separated on sub fed panels. All mobile homes have "floated" grounds.



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