Connecting circuits to sub-panel

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  #1  
Old 12-15-10, 12:13 AM
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Connecting circuits to sub-panel

I have a new sub-panel in my home connected to the main service panel
with a 4-conductor wire. The sub-panel has a neutral bar and an equipment
grounding bar.

For a typical circuit on the main panel, the neutral and ground are both connected
to the ground bar. For a typical 3-wire circuit in the sub-panel do I connect the white to the neutral
bar and the bare copper ground to the equipment grounding bar? Or can I just
connect them willy-nilly wherever they fit (to whichever bar is closer). The problem
I see is that one bar is on the left side of the panel and the other is on the
right. A wire coming in from the top of the panel would have to be split up
with the bare ground going to one side of the panel and the white to the other.

Since both bars are electrically connected at the main service panel, why would
it matter which one I use?

Thanks

Victor
 
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Old 12-15-10, 04:16 AM
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When wiring in the sub-panel you MUST keep the equipment ground conductors (green or bare) connected to ONLY the grounding bus and the neutral conductors (white or grey) connected ONLY to the neutral bus.

The reason is to prevent any normal current flow on the equipment grounding conductors. The equipment grounding conductor ONLY has current flow under fault conditions and then only long enough to trip the circuit breaker supplying the faulted circuit.

And to answer the unasked question...no, electricity does NOT follow the "path of least resistance" but follows ALL paths back to the source. Intermixing the neutral and equipment grounding conductors will cause the electricity that "normally" flows on the neutral conductor to divide and flow on both the neutral and grounding conductors. Since the equipment grounding conductor(s) are also connected to any metallic part of the circuit, including the appliance and any conduit or metal junction boxes, it WILL energize these parts if the neutral and grounding conductors are not kept separate.
 
  #3  
Old 12-15-10, 07:26 AM
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Thanks Furd. That really helps. I think I understand except for one thing. Why is it different for the main panel? The
neutral and ground conductors are all connected to the same ground bars.
 
  #4  
Old 12-15-10, 09:06 AM
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You have to understand that the purpose of the equipment grounding conductor (the bare or green wire) is to provide a low impedance (impedance is AC resistance) path for fault currents to return to the source of the electricity. That source, for all practical purposes, is the main (more properly called Service) panel in your house.

Consider this: A microwave oven has a three-prong plug and three-conductor power cord. The round prong is connected to a green wire in the cord and inside the cabinet of the oven the green wire is solidly attached to the metal cabinet. Under normal conditions this "equipment ground" does absolutely nothing. Now consider that for some reason the "hot" (black) wire in the cord makes accidental contact with the metal cabinet. Since the neutral and the equipment ground are "bonded" in the Service panel there is now a "short circuit" (circuit with minimal impedance) from the branch circuit breaker, to the receptacle, to the microwave oven, through the "fault" to the cabinet, through the equipment ground conductor back to the neutral bus in the Service panel and the current flow (amperage) will be high enough to trip the circuit breaker at almost the same instant as the fault inside the microwave cabinet occurs.

Now let's add your left hand touching the microwave oven cabinet at the same time your right hand is touching the kitchen sink faucet but just before the fault occurs in the microwave oven. There is no current flow through either your body or the equipment ground. The fault occurs and now the electricity, which is always seeking its source, has TWO paths to follow. The first path is through a very low impedance (the grounding conductor) and the second path is through your body and the metallic plumbing in the house back to the neutral bus in the service panel. Since the human body has a fairly high impedance to electrical flow the majority of the flow will be through the grounding conductor and almost instantaneously trip the branch circuit breaker. You may not even feel a tingle. This is how the equipment grounding conductor works with a circuit originating from a Service panel.

Let's now look at a slightly scenario where the receptacle circuit originates in a sub-panel where the neutral and equipment grounding buses are bonded together and to the enclosure and also have separate neutral and equipment grounding conductors (along with the "hot" conductors) between the sub-panel and the Service panel. Under normal operation (no faults) the circuit is from the sub-panel branch circuit breaker, to the receptacle, through the microwave oven, back to the sub-panel neutral bus. Seems like a normal circuit, doesn't it? BUT, the sub-panel is NOT the "source" of the electricity, merely a "way station" along the way from the source (Service panel) through the "load" (appliance) and back to the source. So the true path of the electricity is from the Service, to the sub, to the load, back to the sub and finally back to the Service panel. Except that on the way back from the sub-panel to the Service panel there are now TWO TRACKS (neutral and equipment ground) that the returning electrons can take. We already know that the electricity is going to take both paths and the current flow is going to be inversely proportional to the impedance. The problem arises because the "second path" (the grounding conductor) is "bonded" to many metallic parts throughout the house and THAT means that those parts will now become part of the current carrying system under otherwise normal operation of the appliances. The consequences of this can be anything from a mild tingle when taking a shower to death.
 
  #5  
Old 12-15-10, 09:53 AM
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I will refer you to this sticky that shows the problem with current flowing on the grounding conductor and other parts that should not be.

http://forum.doityourself.com/electr...-drawings.html
 
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