Neutral Isolation in Subpanels, History of

[jim k]

In my area, there are hundreds ? perhaps even thousands ? of subpanels that are supplied by three-wire feeders and that have their equipment grounding conductors terminated in the neutral terminal bars. It?s only in installations that were done in the last 10 years or so that I see 4-wire feeds and isolated neutrals.

When I?ve asked about this, the ECs always say, ?Ground-neutral separation at sub-panels didn?t used to be required. It?s a new thing.? But they can never give me chapter & verse.

As I read 250.24(A)(5) in the 2005 NEC it seems pretty clear on the point. But so does 250-61 in the 1993 edition. In fact, that section seems to have changed very little since the 1947 edition (the oldest one I own):
The grounded circuit conductor on the load side of the service disconnecting means shall not be used for grounding equipment, cable armor, or metal raceways except as provided in paragraph c of section 2557; and in section 2560.

I know that article 250 was drastically reorganized in 1999, but the substance of this particular requirement doesn?t seem to have changed.

Is it common in other parts of the country to find older subpanels with no ground ? neutral separation?

If so, why? Hasn?t it always been prohibited? Is there a NEC section or exception that I?m missing regarding this?

- Jim Katen, Oregon

 

[physis]

Re: Neutral Isolation in Subpanels, History of

I haven't been in this business long enough to know much of the history of the trade.

But I do know that code violations are relatively typical.

One of the two reasons that I advocate for an easier to use and understand NEC.

 

[bphgravity]

Re: Neutral Isolation in Subpanels, History of

Not only is the code hard to understand as Sam has pointed out, but the theory is too. Grounding and bonding is a particularly tough concept to grasp in whole. Most electricians ground and bond things the way they were showed and told as apprentices, which in many cases was completely wrong. It becomes a repeating history. The phrase, "that's the way I've always done it" or "the inspector has never failed me before" are common sayings among the grounding and bonding challenged.

To make things worse, a bonded "subpanel" still works. It will perform properly under normal operation and faults will be cleared just as effectively as properly bonded panels. The danger is not evident unless you understand how electricity works. And still, that imminent danger only expresses itself on the load side of the main disconnect and apparently is not a problem when repeatedly done on the line side of the main.

 

[allenwayne]

Re: Neutral Isolation in Subpanels, History of

Scenario : Last year i went to wire a church in the cherokee nation for my church.Great time 32 guys 4 days done up ad running.
When I got home no tv`s,dvd`s,internet main tripped.found out the 3/2 home i am renting is supplied from a main 200 ft away 3 wire feed no ground rod bonded neutrals and grounds in the panel,so where was the fault path ? through $1,400 of equuiptment.It blew the phone companies d mark 100 ft away

 

[don_resqcapt19]

Re: Neutral Isolation in Subpanels, History of

Quick fix was a ground bar seperate grouds install groung rod and # 4

Where is the fault clearing path???
Don

 

[jwelectric]

Re: Neutral Isolation in Subpanels, History of

Originally posted by don_resqcapt19:

Quick fix was a ground bar seperate grouds install groung rod and # 4

Where is the fault clearing path???
Don

Well let?s hope it is not through the rod.

by Jim K
When I?ve asked about this, the ECs always say, ?Ground-neutral separation at sub-panels didn?t used to be required. It?s a new thing.? But they can never give me chapter & verse.

They don?t know any better. When the ?sub panel? is bonded with the grounded (neutral) conductor then the enclosure becomes ?live or hot if you will.?
The same happens to a range or dryer that is wired with a three wire.


[ August 15, 2005, 08:53 AM: Message edited by: jwelectric ]

 

[roger]

Re: Neutral Isolation in Subpanels, History of

Allen, your fix is more dangerous than before you did anything.

Roger

 

[peter d]

Re: Neutral Isolation in Subpanels, History of

Originally posted by allenwayne:
When I got home no tv`s,dvd`s,internet main tripped.found out the 3/2 home i am renting is supplied from a main 200 ft away 3 wire feed no ground rod bonded neutrals and grounds in the panel,so where was the fault path ? through $1,400 of equuiptment.It blew the phone companies d mark 100 ft away [/QB]

Allen, with this statement it really calls into question if you own have any knowledge of the NEC and proper grounding and bonding arrangements whatsoever.

Under the old arrangement, if the "grounds" and "neutrals" were bonded together (albeit illegally), you still have a fault clearing path via the grounded/neutral conductor.

I don't understand how this equipment got cooked. It sounds like a lighting strike or open neutral situation to me.

By isolating the grounds without a connection to the grounded conductor, you have indeed created an extremely dangerous situation.

[ August 15, 2005, 12:53 PM: Message edited by: peter d ]

 

[milwaukeesteve]

Re: Neutral Isolation in Subpanels, History of

If you were to bond the grounded and the grounding conductors somewhere beyond the main, 2 things happen (or could happen):
(1) your neutral conductor becomes your ground (not bad but not for the purpose)
(2) the equipment ground now becomes a current carrying conductor If this is a metal raceway, that raceway is now a current carrying conductor. Do you want to touch that and the plumbing pipe at the same time?

Now if you bond the "neutral" and the "ground" together (the grounded and the grounding) in the panel, that raceway can see upwards of 20,30,50, 80 100... Amps, depending on panel loads. Do you really want to touch that and a copper cold water line? Would you want a customer?

That is why the grounds should be kept seperate from the neutrals.
The "ground" is your grounding conductor and takes fault current back to the transformer.
The "neutral" is your grounded conductor and has nothing to do with equipment grounds. It is a current carrying conductor that is grounded so that it can return the return current back to transformer, not the fault current.

edited: errored on my bold

[ August 16, 2005, 04:23 PM: Message edited by: milwaukeesteve ]

 

[roger]

Re: Neutral Isolation in Subpanels, History of

Allen, what happens if some one gets hurt because of your fix.

This is a rental home and the work should have been performed under a licensed contractor. I know you work for an EC, but was their competency license involved?

If you had been on the clock, your fix would have been under the license held by your employer and there might have been some one to catch your mistake.

Roger

 

[George Stolz]

Re: Neutral Isolation in Subpanels, History of

Originally posted by allenwayne:
It blew the phone companies d mark 100 ft away

I'm hoping we're just misunderstanding you.

 

[roger]

Re: Neutral Isolation in Subpanels, History of

George, there's no misunderstanding Allens post, where the misunderstanding comes in, is Allens misunderstanding of grounding.


Roger

 

[jimwalker]

Re: Neutral Isolation in Subpanels, History of

I am hoping Allen just simply wrote this wrong.If not there is indeed a serious hazard now.Allen where are your neutrals and grounding conductors connected together ?

[ August 17, 2005, 06:14 AM: Message edited by: jimwalker ]

 

[allenwayne]

Re: Neutral Isolation in Subpanels, History of

I should have read before I posted What I meant was that there was a 3 wire feed /grounding conductor.All neutrals and grounding conductors tied together in sub panel. The dmark on opposite side of building had no ground .
So i added a ground bar seperated grounded and grounding conductors in panel,removed the bond screw from the neutral bar and sank a ground rod for the d mark a Quick fix