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With DC wiring, there is an obvious, positive 'hot' wire and negative/ground wire. In AC wiring is there still such a concept as a single 'hot' wire?

If yes, why is this since either wire will be at +/- 120VAC?

If there is only one 'hot' wire, then why is there an additional ground wire, if the neutral wire already is tied to ground?

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In which particular part of the world? Wiring systems vary. –  John Gardeniers Nov 9 '10 at 4:15

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up vote 8 down vote accepted

Yes, there is a difference. The hot wire (black in the US) carries 120VAC to an electrical load such as your TV or a lightbulb. Most of that voltage is dissipated across the load, so that the voltage on the neutral line (white in the US) is very close to zero, but still AC. The key point is that the neutral serves as the return path for the electrical current to the service panel: current into the load has to match the current out. For example, a 60W light bulb will draw about 500mA of current, and that 500mA will be carried in on the hot wire and out on the neutral.

The ground lead, on the other hand, normally never carries current. It is there to provide additional safety. For example, if the hot wire comes loose in a device and touches a metal part, anything connected to that metal is now energized at 120VAC. If you touch any exposed metal, you could get a shock. On the other hand, if the device is grounded, when the hot wire touches the metal, the circuit breaker will trip, removing the hazard.

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This is a good answer, but to add to it: voltage is also called "potential difference" because it's actually measured as the difference between two lines. In AC circuits, earth is the –  gregmac Nov 9 '10 at 2:00
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Oops, he must have tripped a breaker. –  Dennis Williamson Nov 10 '10 at 0:50

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