Sub Panel Quandry


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Old 07-02-04, 08:35 PM
L
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Sub Panel Quandry

Okay, now I don't know if I am coming or going. A local electrician says since the neutral conductors and ground conductors in the main service panel are bonded together that the sub panel can be as well, at least he came across that way. Everyone here is saying not to bond the neutral conductor to the sub panel. I'm a tad confused right about now. Seems to me a straight forward and common answer exists. I just want to make sure the project is safe and to code.

Thanks

Leslie
 
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Old 07-02-04, 08:52 PM
J
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First of all, the term "subpanel" has no official definition, so there is always some room for misunderstanding when using that word.

Furthermore, the term "ground conductor" is also ambiguous, so there is also possibility misunderstanding there too. I will use the term "grounding conductor" to be clear, by which I mean the "equipment grounding conductor". This term has nothing to do with any connection to dirt or plumbing.

Within a given building, neutral conductors and grounding conductors may only be bonded in one place. So if the main and subpanels to which you refer are in the same building, then the answer is very clear-cut. The neutral and grounding conductors may only be bonded in the panel with the main disconnect.

If the subpanel is in a different building, then it depends on how it is wired. If there is a grounding wire coming from the other building (from one panel to another), then the neutral and grounding must be kept separate in the downstream panel (the subpanel).

If the subpanel is in a different building, but there is no grounding wire between the buildings (only allowed in certain narrow conditions that I will not bother to describe), then the neutral and grounding must be bonded in both buildings. Some people don't even call this a subpanel.

I hope this is clear. Grounding and bonding are confusing subjects.
 
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Old 07-02-04, 10:52 PM
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neutral and ground

If you you were to attach the neutral wire (called the grounded wire) to the grounding wire (the one protecting the system) in the subpanel, then any current flowing on the neutral due to an imbalance of load on the two opposing hot wires (this is the normal job of the neutral wire) from the loads served by that subpanel, will flow back to the main panel via both the feeder neutral wire and the feeder grounding wire. That is what the neutral wire should be doing alone. The grounding wire should never have any service current flowing on it. Within any single building, or within any group of buildings fed with a common grounding wire, there should be exactly one point of bonding the neutral wire to the ground, and that point should be along the path back to the source of power and where the grounding wire is attached to earth electrodes (not out on some branch circuit). That would generally be the service entrance. This ensures that neutral currents cannot flow on the grounding wire, and that no circuit can introduce leakage or fault voltage onto the grounding wire that spreads into the rest of the system.

NEC definitions:
Grounded Conductor. A system or circuit conductor that is intentionally grounded.
Grounding Conductor. A conductor used to connect equipment or the grounded circuit of a wiring system to a grounding electrode or electrodes.
 
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Old 07-03-04, 04:31 AM
Spark Chaser
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If in John Nelson'e example - that there is no equipment ground between the Main and Sub Panel and you bond the neutral and the equipment ground in the sub panel - Then you need to drive a ground rod and supply an equipment ground to the sub panel - A 5/8 x 8 ft ground rod is acceptable in the area where I live - May vary where you are located - Need to check this out.
 
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Old 07-03-04, 07:06 AM
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grounding the outhouse

Originally Posted by Spark Chaser
If in John Nelson'e example - that there is no equipment ground between the Main and Sub Panel and you bond the neutral and the equipment ground in the sub panel - Then you need to drive a ground rod and supply an equipment ground to the sub panel - A 5/8 x 8 ft ground rod is acceptable in the area where I live - May vary where you are located - Need to check this out.
In addition to the requirement to bond the neutral to the newly derived ground in the separate building to which no grounding wire is fed, no other metallic feeds may be provided. This include metal pipes (water, gas, steam), telephone, cable TV, ethernet, etc. The reason is that these metal paths would then be carrying some of that neutral current due to the two points of bonding (if they are not grounded, then you end up having possible high voltage between them and the electrical system and/or ground). So it would be best to run a separate grounding wire (or use PVC water pipes, fiber optics, transformers, etc).
 
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Old 07-03-04, 08:25 AM
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If in John Nelson'e example - that there is no equipment ground between the Main and Sub Panel and you bond the neutral and the equipment ground in the sub panel - Then you need to drive a ground rod and supply an equipment ground to the sub panel - A 5/8 x 8 ft ground rod is acceptable in the area where I live - May vary where you are located - Need to check this out.
The requirement for a grounding rod in a detached structure is independent of whether or not you ran an equipment grounding conductor between buildings. The connection to a grounding rod is called an grounding electrode conductor, not a equipment ground.
 
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Old 07-03-04, 03:53 PM
Spark Chaser
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You may be correct in the technical sense - But where I come from we use different terminlogy I guess - I am sure my peers would understand my meaning.- 35 Years Industrial experience / 4 year indentured apprenticeship- Very Little House wiring - Sorry for mis speaking.
 
 

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