Sub Panel Question

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Old 10-28-09, 05:54 AM
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Sub Panel Question

-I have a question in referance to the sticky note drawings on sub panels.

-The three wire feeder (detached building)drawing states that the neutral and ground ARE bonded as per NEC 250.32(B)(2).

-The four wire feeder(detached building)drawing states that the neutral and ground DO NOT be bonded to ground as per NEC 250.24(A)(5)

-For the three wire feeder would keeping the neutural and ground seperate. One bar for neutral and one bar for ground with required ground rod.(neutral not bonded to ground )Would this be a safe practice?

-I understand 2008 NEC does not allow 3 wire feeders.This is a general question for my own educational purposes.
 
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Old 10-28-09, 06:28 AM
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Originally Posted by 63zerb View Post
-I have a question in referance to the sticky note drawings on sub panels.

-The three wire feeder (detached building)drawing states that the neutral and ground ARE bonded as per NEC 250.32(B)(2).

( Not any more )

-The four wire feeder(detached building)drawing states that the neutral and ground DO NOT be bonded to ground as per NEC 250.24(A)(5)

( Correct )

-For the three wire feeder would keeping the neutural and ground seperate. One bar for neutral and one bar for ground with required ground rod.(neutral not bonded to ground )Would this be a safe practice?

( You can't do it because you would need 4 wires - 2 hots, 1 neutral and a ground. You would also need a ground rod with this system that would hook to your ground bar in your sub panel.)


-I understand 2008 NEC does not allow 3 wire feeders.This is a general question for my own educational purposes.


Hope this helps that I have wrote after each question.
Jim
 
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Old 10-28-09, 07:30 AM
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-The three wire feeder (detached building)drawing states that the neutral and ground ARE bonded as per NEC 250.32(B)(2).
ok. and they should be if you use a 3 wire feed, which is still legal in some areas (although I recommend against them regardless)

-The four wire feeder(detached building)drawing states that the neutral and ground DO NOT be bonded to ground as per NEC 250.24(A)(5)
the neutral is not bonded. The EGC and GEC are bonded at the service in the detached building. The verbiage states that the neutral is not bonded to ground. That does not mean the incoming grounding conductor is not bonded to the grounding electrode system, which it needs to be.

-For the three wire feeder would keeping the neutural and ground seperate. One bar for neutral and one bar for ground with required ground rod.(neutral not bonded to ground )Would this be a safe practice?
3 wire; neutral is bonded to EGC/GEC with a main bonding jumper.

I have been informed that it is not proper to allow the neutral bar to be bonded to the panel (i.e. isolated neutral bar must be used) as this allows the panel tub to be used as a current path, which is not acceptable. I have never seen this enforced and while it makes theoretical sense, I, PERSONALLY, do not see a problem with it. Regardless, it still needs to be bonded to the ground bar/ EGC/GEC in a 3 wire feed and I would recommend following the code, regardless of what I believe.

-I understand 2008 NEC does not allow 3 wire feeders.
correct but 08 code is not in effect in all areas.
 
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Old 10-28-09, 12:17 PM
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Originally Posted by 63zerb View Post
-I have a question in referance to the sticky note drawings on sub panels.

-The three wire feeder (detached building)drawing states that the neutral and ground ARE bonded as per NEC 250.32(B)(2).

-The four wire feeder(detached building)drawing states that the neutral and ground DO NOT be bonded to ground as per NEC 250.24(A)(5)

-For the three wire feeder would keeping the neutural and ground seperate. One bar for neutral and one bar for ground with required ground rod.(neutral not bonded to ground )Would this be a safe practice?

-I understand 2008 NEC does not allow 3 wire feeders.This is a general question for my own educational purposes.
I believe what your asking is why will separating neutral and ground at the sub-panel being served with a 3 wire feeder be unsafe. I've attached a modified drawing of the one on the sticky drawing that would illustrate what your asking. All current whether it is fault current or neutral current or phase current (240 volt circuits) in your homes electrical system must have a low impedance/resistance path back to the transformer. When speaking of fault current, like you would in a ground fault, without that low impedance path back to the transformer to complete the fault circuit your circuit breakers would not have enough current flow through them to make them open and clear the fault. If you were to use earth as the only path to the transformer for fault current your breakers would not open. All metal in the fault path will become energized. This would be the situation if we did not bond ground to the neutral in a 3 wire feeder. To show this I've attached another drawing that shows the "effective fault path" current must use in order to get to the transformer with little impedance so that enough current flows through the circuit breaker to open it.
You will notice that if we do not bond the neutral and ground on a 3 wire feeder to a detached buildings sub-panel then there is no way for fault current to use the feeder neutral to get back to the main panel and then to the service neutral to return to the transformer. It is forced to use earth as shown in the diagram becuase we have taken away the low impedance path by not bonding the equipment ground to the neutral.
What you might be thinking is that earth is used for the path for current and that is why we have ground rods. this is incorrect ground rods and other electrodes connected to a homes electrical system make up what is called the grounding electrode system (GEC). This is for property protection from lightning and other forms of huge voltage and current events.

The equipment grounding system which is bonded metal and of course the ground wire ran in your homes electrical cables is used to intentionally construct the low impedance fault path back to the transformer. This path is required for your breakers to open a fault if one occurs. So it is for human safety and earth does not enter into its proper operation.

If you look at the diagrams I think it will become clear the differences. Thing to remember is given a low impedance path back to the transformer to complete the circuit almost all current will use it to get to the transformer even if another high impedance path (like earth) exists.



 
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Old 10-28-09, 12:39 PM
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There are four necessary conditions in the "exception" that permits using the Neutral for conducting fault-currents between the seperate stucture and the Service --- (1) an existing installation-- (2) there is no metallic path between the structures --- (3) the Feeder Conductors do not include an Equiptment Grounding Conductor--- (4) there is no GFI protection on the Feeder Conductors.
 
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Old 10-29-09, 05:03 AM
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Thank You

Thank You to all for the input....Bruto Thank You for the detailed explanation on the proper installation for the equipment,grounding electrode system and feeder.
 
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Old 10-29-09, 09:26 AM
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Originally Posted by 63zerb View Post
Thank You to all for the input....Bruto Thank You for the detailed explanation on the proper installation for the equipment,grounding electrode system and feeder.
Your welcome, one of the neat things about DIY forums is you have a resource of many people who when taken cumulatively usually cover all the necessary bases and correct any wrong information.

I'm also glad that PATTBAA pointed out the requirements you must meet to have a 3 wire feeder serving a detached structure.... This is exactly what I mean by knowledgable people adding information to a thread so the poster has all the necessary information to be able to understand the 'why' and then that leads to the correct decision.
 
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