Having a Bad Day (Wiring)

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Old 10-08-06, 03:25 PM
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Having a Bad Day (Wiring)

So, I'm standing on a 5 gallon plastic paint bucket, checking on some lite fixture wiring, when suddenly, the lid cracks open, I fall into the bucket of paint, then the bucket and I fall over on the floor. There I was, laying in a puddle of green paint. I was hurting too much to laugh, or, to clean up the mess. Lesson learned: Use a step ladder next time!!
My question is: I hooked up a lite, with two three way switches. It worked great. Then I went to hook up a ceiling fan that was fed(with power)from the lite. I tested the wires to make sure I had power. Multi-meter from black to white wire, 120 volts. But then, I also found out I had 120 volts when I tested from black to ground. I figured that white and ground were touching somewhere, so I undid ALL the wiring from the switches and the lite.I tested the cable coming from the breaker box to the lite, and I still had 120v from black to ground. But, I unhooked the cable from the breaker box, checked for continuity between ground and white, and there was none. Why am I getting 120v reading from black to ground when the cable is hooked up to the breaker box? It is a sub panel, and the ground and neutral busses are not bonded. Thanks in advance.
 
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Old 10-08-06, 03:40 PM
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You are getting 120 volts between the hot wire and the ground wire because the ground wire and the neutral wire are connected together. They are supposed to be. If they weren't, the ground wire would be just about useless.

In a normal main panel, the ground and neutral are connected together at the main panel. In a sub-panel the ground and neutral are still connected together at the main panel.

Follow your wiring. If you have a four wire feed to this sub-panel then the ground and neutral are isolated from each opther at the sub-panel. However, when you follow the wires back to the main panel, the ground and neutral are connected to the same place.
 
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Old 10-08-06, 04:22 PM
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Thanks Bob. I knew all that, but I figured that when electricity returned through the neutral wire, back to the main panel, and then back to the source, that the ground wire would not be energized with 120v, because the path of least resistance would be back to the source, and not back to the outlet, since the ground wire at the outlet isn't hooked to anything that is drawing power through it. Maybe that doesn't make sense, but when u r an electical rookie, sometimes electrical theory, or traits, if u will, don't always make sense.
So if I use a multi-meter on any outlet, and touch one end to hot, and the other end to ground, I will always get a 120v reading? So, if I were to put one finger on the hot bus in a panel, and the other finger on the outside of the dryer, lets say, I would get a shock, if the dryer had a ground wire screwed to it's exterior? Sorry to sound dumb, just trying to understand. Thanks. . . .so I unhookedd all my wiring for nothing, except maybe, a good lesson learned. . .
 
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Old 10-08-06, 05:25 PM
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Originally Posted by Willg54
the ground wire would not be energized with 120v, because the path of least resistance would be back to the source, and not back to the outlet, since the ground wire at the outlet isn't hooked to anything that is drawing power through it

So if I use a multi-meter on any outlet, and touch one end to hot, and the other end to ground, I will always get a 120v reading?

So, if I were to put one finger on the hot bus in a panel, and the other finger on the outside of the dryer, lets say, I would get a shock, if the dryer had a ground wire screwed to it's exterior?
1. Path of least resistance is thru your meter. It is completing the circuit thru the movement. Although I don't think path of least resistance is really a rule. I think it's R1 & R2.

2. Yes unless you have a typically commercial system such as 480 or 600V.

3. Yes, you should get a shock.
 
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Old 10-08-06, 05:47 PM
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Let's look at another example.

Say we have a single receptacle on a circuit. In a new house this might be the 20 amp circuit that serves the laundry, or it might be a circuit that serves a built in microwave oven.

That single 20 amp circuit is wired with 12-2 w/ground NM cable. This 12-2 cable has three wires inside. Two are insulated, a white and a black, and one is bare copper.

The proper way to wire is, of course, that the black wire is connected to the circuit breaker, the white wire is connected to the neutral buss and the bare wire is connected to the ground buss. (Again, in your main panel the ground and neutral busses are the same place.)

Aside from the fact that the bare wire has no direct insulation, what is the difference between the white wire and the bare wire? The answer is nothing, until you turn on the washer or the mircrowave or whatever is connected to the circuit and start drawing power.

When you start drawing power, the white neutral wire carries current. If your load draws 12 amps, then the 12 amps flows on the black wire, through the microwave, and on the white wire. No current (during normal operation) flows on the bare wire.

The bare wire serves only as a safety mechanism*. Specifically, it serves as a path back to the source in the event that somehow the metal shell of the washer becomes energized, or metal exterior of the microwave becomes energized. Current will flow and it will be enough current to trip the circuit breaker.

So in your testing, you read 120 volts between the hot wire and the bare wire because the bare wire completes the circuit back to the power company. Your meter can't tell the difference between the insulated white wire and the bare wire.


*In some cases the bare ground wire is also used as a ground reference. This is mainly for computers and other electronics. The bare ground wire is also used by some surge supressors to dissipate the surge. But again, under normal operation the ground wire does not carry current.
 
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Old 10-08-06, 05:47 PM
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1. Path of least resistance is thru your meter. It is completing the circuit thru the movement. Although I don't think path of least resistance is really a rule. I think it's R1 & R2.

. Voltage is simply the measurement of the difference of potential between two points. That is why when you measure from one hot wire to another hot wire (of the same phase) you get a 0 reading but obviously there is voltage (when compared to ground).

For there to be a difference of potential, there must be resistance between the two points. (an open circuit has "infinate" resistance between the two points)




"So, if I were to put one finger on the hot bus in a panel, and the other finger on the outside of the dryer, lets say, I would get a shock, if the dryer had a ground wire screwed to it's exterior? "

Presuming of course the ground wire is complete to the panel ground.
 
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