Isolation of neutral at Sub Panels

[eagle]

Can I get a simple explantion on the requirements and or need to isolate at the SUB. This seems to be a topic of alot of confusion here in Austin. I'm hearing .. YES .. NO ... Not sure. I have been under the opinion from some basic code classes that it is required at all subs. Also a still at a loss in understanding how continuity is avoided at the sub when it's present at the main. I understand jumpers needs to be off and the neutral bars should float .. HELP

 

[ryan_618]

Re: Isolation of neutral at Sub Panels

Article 250.24 (A)(5) addresses this. Also, take a minute and browse through this site and you will probably find over 100 topics that address this very issue. Hope this helps,

 

[eagle]

Re: Isolation of neutral at Sub Panels

Thanks for the response and link to the code section. Will take the advise and look around some more.

Regards

Glen

 

[charlie b]

Re: Isolation of neutral at Sub Panels

Current will follow every available path back to its source. For a sub-panel (and any of its branch circuits), the main panel can be viewed the source. If you tie the neutral bar and the ground bar at the sub, then neutral current (going back to the main) will have two available paths. The first path is via the neutral conductor (connecting the main and the sub). The second path is via the equipment grounding conductor (EGC). The second path would include the conduit itself, if the conduit is used as the EGC. What this means is that every metal case between the sub and back to the main would be carrying current. If you touch the outside of the panel, you would receive a shock. That is why you don?t tie the neutral to the ground anywhere other than at the main.

It is OK (in fact, it is required) to tie the neutral to the ground at the main itself. As I mentioned above, that point is ?the source.? If you touch the external metal case of the main panel, you are not at risk of a shock. That is because the neutral current that is returning to the source will not go backwards from the source, via the main bonding jumper to the ground bus, onto the external case, and into your hands. That direction would be away from the source, and current seeks a path towards the source.

Does this answer your question?

 

[eagle]

Re: Isolation of neutral at Sub Panels

I'm almost there ... still need to clairfy

Question 1 : when testing for continuity at the sub .. I should find NO continuity between the branch ciruit neutral and ground bar .. YES ... NO ?

Question 2 : If NO.. how is that avoided on sub panels that donot have the new type of stand off bars and they directly contact the panel frame ?

 

[Guest]

Re: Isolation of neutral at Sub Panels

Here is one forum thread that gets into 3-wire vs. 4-wire feeders:
http://www.mikeholt.com/cgi-bin/codeforum/ultimatebb.cgi?ubb=get_topic&f=3&t=000378#000001

It's still a work in progress.

 

[charlie b]

Re: Isolation of neutral at Sub Panels

Originally posted by eagle: Question 1 : when testing for continuity at the sub .. I should find NO continuity between the branch circuit neutral and ground bar??

The branch circuit neutral runs back to the main neutral, which is connected (via the main bonding jumper) to the main ground, which runs forward to the sub-panel ground bus. Therefore, there will be continuity between the branch circuit neutral and ground bar. Should you find NO continuity? No. (Better wording of question: ?Should you find continuity?? That answer would be ?Yes.?) The rest of your question will have to be addressed by someone else.

 

[bennie]

Re: Isolation of neutral at Sub Panels

eagle: You have noticed that my opinion is not well accepted

However I stand by my interpretation for the following reasons; The absence of the term "sub-panel" is glaringly apparent in all editions of the NEC.

The NEC does not contain a section that states "the neutral and ground shall be separated in any panels". If so please tell me.

This procedure is the creative method by someone, to stop current flow on conduits.

Section 250.24(5). Does not apply to a panel, the intent is to insure the equipment ground and neutral are separated at the utilization equipment.

Preventing objectionable current from appearing on conduit or other conductors, can be done by any of the methods as described in 250.6(B). When metal conduit is used as an equipment ground, there is no choice but to separate the neutral and ground in the panel.

The reason that the term "sub-panel" does not, and never will appear in the NEC, is due to the technical incorrectness of the term.

When you have six panels, with mains, grouped on a wall with a 30 ft. long gutter, supplying the service conductors to each one, do you consider the panels to be service panels? Of course they are.

Now should the panels be replaced with main circuits breakers and feeding six panels in apartments, does this change the logic of the electrical application?

The service mains have to be grouped, there is nothing wrong with separating the panels.

I understand that some individuals remove the neutral to ground jumper ribbon in transformers to prevent parallel current when the neutral/ground is bonded in the first switch.

This is a dangerous method, and should never be done. This is the same thinking when it was decided to run a fourth wire to a place that only needed three.

 

[eagle]

Re: Isolation of neutral at Sub Panels

Thanks to all for the continued input. I am working off illustrated NEC 2000. If off to to buy the full blown version as illustrated does not give me the sections I need to relate to your responses.

PS .. if what Charlie B says is correct .. then why do I find 50% of the newer sub panels with NO continuity between the branch N & G bars. How can that be ? Granted they are the newer style that has the plastic? stand off bars

 

[pierre]

Re: Isolation of neutral at Sub Panels

Eagle

Generally speaking, you WILL have continuity between the grounded conductor and the equipment grounding conductor anywhere in the dwelling. Remember Charlie mentioned that the electrons (current) always want to go back to the source, and do not want to flow in the direction away from the source.That is why we can separate the EGC from the GC away from the first point of disconnect (service).

Pierre

 

[pierre]

Re: Isolation of neutral at Sub Panels

If you are finding no continuity between the GC and the EGC, something is not right. It may be your tester or testing method.
Or the installation is not proper to start with.

Pierre

Bennie
Are you saying that the neutral to ground connection at a panel downstream of the main service (or first panel if you will) will be a safe installation?

 

[don_resqcapt19]

Re: Isolation of neutral at Sub Panels

Eagle,
250.142(B) very clearly prohibits the connection of the grounding conductors and grounded conductors in panels other than the service disconnect.

(B) Load-Side Equipment. Except as permitted in 250.30(A)(1) and 250.32(B), a grounded circuit conductor shall not be used for grounding non?current-carrying metal parts of equipment on the load side of the service disconnecting means or on the load side of a separately derived system disconnecting means or the overcurrent devices for a separately derived system not having a main disconnecting means.

Don

 

[charlie b]

Re: Isolation of neutral at Sub Panels

Originally posted by eagle:?why do I find 50% of the newer sub panels with NO continuity between the branch N & G bars?

Do you mean there is no continuity when you just take the new panel out of the box, and have not yet installed it? Or do you mean in the installed configuration? If the later, I agree with Pierre: something is not right.

 

[bennie]

Re: Isolation of neutral at Sub Panels

I am saying that a panel fed in such a manner that no conductor is in parallel with the neutral load, there is no danger.

250.24.A.5. A grounding connection shall not be made to any "grounded" circuit conductor on the load side of the service disconnecting means except as otherwise permitted in this article.

There is no grounded circuit conductors, on the load side, for the neutral to be connected. The panel is not grounded (connected to earth).

Review the definition of "grounded". Grounded means connected to earth. The equipment ground conductors do not become grounded until connected.

The intent of the section is; to prevent direct multi-point earth connections.

 

[iwire]

Re: Isolation of neutral at Sub Panels

Bennie we can not control what happens after we leave the job, there is a very good possibility that a parallel path will happen after we leave.

Lets say a dishwasher is wired up to the main panel and a washing machine is wired up to a panel fed from the main panel, instant parallel path. (Stainless steel braided washing machine hoses are the norm around here)

There is just so many ways that a parallel path could be developed that to me it makes much more sense for us to wire in a way that it can not happen.

JMO, Bob

 

[Guest]

Re: Isolation of neutral at Sub Panels

Originally posted by eagle:
<snip>why do I find 50% of the newer sub panels with NO continuity between the branch N & G bars. <snip>

Are you doing your continuity test while the panels are powered or are you doing your continuity test while the panels are unpowered?

 

[iwire]

Re: Isolation of neutral at Sub Panels

Originally posted by awwt:
Are you doing your continuity test while the panels are powered or are you doing your continuity test while the panels are unpowered?

Now I'm confused.

 

[don_resqcapt19]

Re: Isolation of neutral at Sub Panels

Bennie,
If a panel, other than the service, panel is fed with only three wires, then the grounded conductor feeding the panel is being used as the equipment grounding conductor for everything served by that panel. That is a violaiton of 250.142(B).
Don

 

[ryan_618]

Re: Isolation of neutral at Sub Panels

Originally posted by iwire:

Originally posted by awwt:
Are you doing your continuity test while the panels are powered or are you doing your continuity test while the panels are unpowered?

Now I'm confused.

Well, maybe there are people who like to buy new meters every time they test!

 

[Ed MacLaren]

Re: Isolation of neutral at Sub Panels

ryan and iwire,

I'm sure awwt must have meant wired or unwired, not powered.

I think the point he was making to the thread originator, eagle, is that there will not be continuity before the panel is wired, because the neutral bar supposed to be insulated from the enclosure, until the bonding jumper (or green screw) is installed.