SUB feed

618electrician said:
WHAT IS THE DANGER ?
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When a conductor carries current there is a voltage drop across it. Properly selected conductor for the load will be fairly minimal but still is a voltage drop on it as no conductor has a zero resistance.

The conductor that starts out at the supply end as being at earth potential will have a voltage to earth at the load end that is equal to whatever voltage is dropped across the length of the conductor. That voltage will exist between anything that is at earth potential that you may be able to touch at same time as anything connected to the equipment grounding conductor in your wiring system.

We do have same problem with the service neutral as well as the neutral conductors of many POCO primary distribution lines that are all bonded together and grounded at nearly every pole or other POCO structure and and customer service equipment, but that is another discussion as well as beyond the scope of the NEC when it comes to us non utility electricians being able to do much about it.
 
618electrician said:
WHAT IS THE DANGER ?
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Please turn off caps lock.

Simply speaking, a 3-wire feeder is like a service, except a service neutral is well-reestablished as the zero-volts-to-earth point despite voltage drop on the service neutral, by the electrode system.

The load end of a sub-panel feeder's neutral doesn't have such a method of establishing itself as the zero-volts point, so any voltage drop along the neutral can energize normally-bonded surfaces.

Thus, while the neutral may be dragged away from zero volts, we want that voltage isolated from the equipment grounding system. Voltage between the neutral and the equipment ground is normal.
 
LarryFine said:
Please turn off caps lock.

Simply speaking, a 3-wire feeder is like a service, except a service neutral is well-reestablished as the zero-volts-to-earth point despite voltage drop on the service neutral, by the electrode system.

The load end of a sub-panel feeder's neutral doesn't have such a method of establishing itself as the zero-volts point, so any voltage drop along the neutral can energize normally-bonded surfaces.

Thus, while the neutral may be dragged away from zero volts, we want that voltage isolated from the equipment grounding system. Voltage between the neutral and the equipment ground is normal.
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what danger of electrical shock is it to me ?????
 
618electrician said:
what danger of electrical shock is it to me ?????
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To you personally? I guess it depends on whether you're the electrician or the customer.

Voltage difference between the normally-grounded metal parts and the earth around you.

Touch a grounded (to the EGC) object while touching a grounded (not to the EGC) surface.

Example: Touching an outside A/C unit, or a refrigerator in the garage, while barefoot.
 
Also, as the voltage on the neutral increases, or if the neutral becomes open, all the equipment grounds connected to the open side of the neutral will become live relative to the equipment grounds connected to the main.
 
Ok - let's walk thru an undesirable practical example of what Infinity, Kwired and Larry are talking about. And it does not include any broken connection scenarios, just normal electrical operation with an improperly bonded subpanel.

Suppose you have a subpanel with a bonded neutral to EGC connection. It's a good distance from the main panel. And let's say you have a reasonable load (say 30 to 40 amps) on one leg (L1 to N).

One of these loads is a washing machine.

The voltage at the main is 120V (L1 to N).

The voltage at the subpanel is 117V (L1 to N). 3 volts is dropped along the feeder to the sub due to the current carried.

Let's also just estimate that the voltage drop is split unequally - the Hot (L1) loses 2 volts and the combination of the Neutral and EGC loses 1 volt. (The Neutral and EGC lose less because they are paralleled.) These numbers are just examples, but are not WAGs.

So, because the EGC is carrying current, the "Ground" at the subpanel is 1 volt higher than the ground at the main.

And because you plugged in a washing machine, whose frame is now connected to the EGC potential at the sub, the voltage between the Maytag's frame and the nearby slop sink's hot or cold faucet (if plumbed with metal piping) is 1 volt.

Would that kill you if you touched them both at the same time? Absolutely not (might tingle... maybe).

Would it be totally safe? Also, absolutely not. You could easily create a nice little spark with just 1 volt if the washing machine frame (bonded at the sub) bounced against a metal pipe (that was bonded back at the main).

The whole point of an EGC (besides obviously carrying fault current) is to create a single equipotential point for all conductive objects to tie to. The EGC can only do that if it carries no current. Any current flowing on an EGC means voltage drop on it, and one end would have a different voltage than the other. Which is exactly what you don't want. You want any and all conductive thing(s) in a building to be "grounded" and have zero potential to each other and to the single point of bonding at the first disconnect.

Hope this helps.
 
MD Automation said:
Ok - let's walk thru an undesirable practical example of what Infinity, Kwired and Larry are talking about. And it does not include any broken connection scenarios, just normal electrical operation with an improperly bonded subpanel.

Suppose you have a subpanel with a bonded neutral to EGC connection. It's a good distance from the main panel. And let's say you have a reasonable load (say 30 to 40 amps) on one leg (L1 to N).

One of these loads is a washing machine.

The voltage at the main is 120V (L1 to N).

The voltage at the subpanel is 117V (L1 to N). 3 volts is dropped along the feeder to the sub due to the current carried.

Let's also just estimate that the voltage drop is split unequally - the Hot (L1) loses 2 volts and the combination of the Neutral and EGC loses 1 volt. (The Neutral and EGC lose less because they are paralleled.) These numbers are just examples, but are not WAGs.

So, because the EGC is carrying current, the "Ground" at the subpanel is 1 volt higher than the ground at the main.

And because you plugged in a washing machine, whose frame is now connected to the EGC potential at the sub, the voltage between the Maytag's frame and the nearby slop sink's hot or cold faucet (if plumbed with metal piping) is 1 volt.

Would that kill you if you touched them both at the same time? Absolutely not (might tingle... maybe).

Would it be totally safe? Also, absolutely not. You could easily create a nice little spark with just 1 volt if the washing machine frame (bonded at the sub) bounced against a metal pipe (that was bonded back at the main).

The whole point of an EGC (besides obviously carrying fault current) is to create a single equipotential point for all conductive objects to tie to. The EGC can only do that if it carries no current. Any current flowing on an EGC means voltage drop on it, and one end would have a different voltage than the other. Which is exactly what you don't want. You want any and all conductive thing(s) in a building to be "grounded" and have zero potential to each other and to the single point of bonding at the first disconnect.

Hope this helps.
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THE same thing would happen ' with sub feed or not . the sub feed has nothing to do with voltage drop . 0 volts from EGG TO A NOTHER IS NEXT TO IMPOSSICLE in the real world !
 
The same thing can and does happen on the utility's side of the service. The difference is that the neutral at the service is re-established as the zero-volts point, which does not occur with a feeder.
 
LarryFine said:
The same thing can and does happen on the utility's side of the service. The difference is that the neutral at the service is re-established as the zero-volts point, which does not occur with a feeder.
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You have a grounding electrode connected at the service, but this zero re-establishment is really only in close proximity to that grounding electrode, usually is very close proximity in my experiences.
 
kwired said:
You have a grounding electrode connected at the service, but this zero re-establishment is really only in close proximity to that grounding electrode, usually is very close proximity in my experiences.
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While we often call it that, what we are really doing, because there is neutral to earth voltage on most neutrals, is raising a small area of earth around the electrode to the voltage on the neutral.
 
kwired said:
You have a grounding electrode connected at the service, but this zero re-establishment is really only in close proximity to that grounding electrode, usually is very close proximity in my experiences.
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Understood and agreed. I wasn't saying that it is necessarily absolutely at zero volts, merely that it is nonetheless considered to be the 0v reference for the premises.
 
If you lose the neutral the neutral current will be on the ground wire which may be bare and it could overheat the building surface it is attached to and start a fire.
 
Eddie702 said:
If you lose the neutral the neutral current will be on the ground wire which may be bare and it could overheat the building surface it is attached to and start a fire.
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If you are talking about a grounding electrode conductor, such heating is very very unlikely. The grounding electrode will not permit that amount of current to flow.
 
618electrician said:
SO why have a GEC that has max of #6 copper ?
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For a ground rod as Don stated very little current will flow on the GEC through the earth. For a metal water pipe electrode with a city water system it's likely that most if not all of the neutral current can return on the GEC.
 
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