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Electrical Wiring

3 wire vs. 2 wire 120 V. wiring understanding

16 years ago

I have an old house with 2 wire feeds to all outlets. I am putting in a new line for an air conditioner: 3 wire (Romex black, white and bare ground) to a circuit breaker in a separate box.

I understand how to do this and am having the electrical inspector certify the work. What I donÂt understand is how the 3 wire system works, i.e. the "third prong" on he a/c plug is the round one and it is grounded within the air conditioner and that ground side (the bare copper in the Romex) is grounded to the outlet box and the outlet box is grounded thru the Romex back to the service panel breaker box. Does this method vs. the two wire system that has no bare ground conductor protect the situation by blowing the breaker if EITHER the white or black shorts to ground?? I am just curious how this works and why it is better vs. the two wire old system. Thanks for your help.

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Comments (2)

DavidR
16 years ago
The ground provides a solid, reliable path to the grounded point in the electrical system. If there should be leakage or short circuit between the air-con's wiring and its frame, the breaker will trip rather than causing a shock to you or someone else. Otherwise, the electricity would return to the system ground through the person's body.
brickeyee
16 years ago
If the hot touches either the white (groundED) conductor or the green/bare (groundING) conductor a short will be created and the breaker (or fuse) will open the circuit.
The white wire carries the return current from the black wire, and WILL develop a small voltage caused by the resistance of the wire itself.
If the appliance frame was connected to this wire, it would have this voltage present. If the white wire connection became open, the frame would rise to a full 120 V.
For these reasons the groundING (AKA 'safety ground') was added. It NEVER carries current except when a fault occurs.
This means the equipment frame stays at a very solid ZERO volts.
Since most portable lamps do not have a ground connection (2 prong plugs) one blade of the fixture connection is made larger. This is called a 'polarized' plug. It ensures that the screw shell of the lamp is on the white conductor and not connected to the 120 V hot side. It is much easier to come i contact with the screw shell of the bulb socket that the hot side. The hot side is all the way at the bottom of the screw i socket, while the shell is relatively exposed.
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