Subpanel bonding (3-phase mains) question

Mike Cook

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Nov 6, 2009, 2:02:25 AM11/6/09

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Is it never allowed (US National Electrical Code) to bond the ground bus bar
to the neutral bus bar?

I opened a subpanel and found that both the ground bus bar and cabinet were
bonded to the neutral bus bar.

Is this allowed under any circumstance?

This is a "Y" 208/120 configuration utilizing 5 conductors in an industrial
building.

Thanks.

jk

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Nov 7, 2009, 2:17:38 AM11/7/09

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Mike Cook <mc...@NOTyahoo.com> wrote:

Yes, ther are a number of situations where this is allowed/required
Service Entrance
Separately Derived System

jk

PeterD

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Nov 6, 2009, 8:08:38 AM11/6/09

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On Thu, 5 Nov 2009 23:02:25 -0800, Mike Cook <mc...@NOTyahoo.com>
wrote:

When was it installed, and have you read the revelant sections of the
NEC?

bud--

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Nov 6, 2009, 9:46:28 AM11/6/09

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Sounds good. An example is a panel fed from a transformer.

If it is a panel fed from another panel, the neutral bar must be isolated.

A better newsgroup is probably alt.engineering.electrical.

--
bud--

Steve Lusardi

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Nov 7, 2009, 8:50:57 AM11/7/09

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Yes, but only once. The rule is the safety earth is bonded to neutral at source....the distribution transformer. It is never
advisable to make this bond too far away from the transformer.because it introduces a reactance issue for high frequency noise. In
point of fact, in Europe the equalization bus must not exceed 1 meter in length. If another bond exists, ground loops are
introduced, which is another story altogether.Please also note that the same earth reference at the transformer must, at the same
point, be earthed through a ground rod or earth plane ground rod array.
Steve

"Mike Cook" <mc...@NOTyahoo.com> wrote in message news:0001HW.C7190A01...@news.eternal-september.org...

notme

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Nov 7, 2009, 12:56:36 PM11/7/09

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Rich.

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Nov 7, 2009, 1:42:59 PM11/7/09

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"notme" <no...@notme.org> wrote in message
news:0001HW.C71AF4D4...@news.eternal-september.org...

No, according to the NEC it is not allowed in a subpanel at all.

Ryan Evans

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Nov 7, 2009, 6:10:55 PM11/7/09

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"notme" <no...@notme.org> wrote in message
news:0001HW.C71AF4D4...@news.eternal-september.org...

No. As long as it is a sub-panel.
Not to be confused with a seperately-derived system(after a transformer).

RE

Tom Horne

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Nov 8, 2009, 2:13:41 PM11/8/09

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With the exception of feeders to separate buildings under previous
editions of the US NEC it is not permissible.
--
Tom Horne

Mike Cook

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Nov 9, 2009, 1:45:29 AM11/9/09

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> With the exception of feeders to separate buildings under previous
> editions of the US NEC it is not permissible.
> --
> Tom Horne

Single building, not large (2000 sq ft?) one service entrance, one main
panel, 3 or 4 subpanels.

It looks like this bond needs to be broken...

Thanks,
Mike (OP)

Proteus IIV

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Nov 9, 2009, 6:08:06 PM11/9/09

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On Nov 7, 12:56 pm, notme <no...@notme.org> wrote:

TROLL DROPPINGS

TOM LAY OFF
YOU NEVER GOT THIS RIGHT TO BEGIN WITH

I AM PROTEUS

Paul Keinanen

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Nov 9, 2009, 7:01:20 PM11/9/09

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On Thu, 5 Nov 2009 23:02:25 -0800, Mike Cook <mc...@NOTyahoo.com>
wrote:

>Is it never allowed (US National Electrical Code) to bond the ground bus bar

At least in the rest of the world (=non-US), this is a typical TN-C-S
wiring case, in which the utility company 4 wire TN-C (3L+N) is
delivered to building, in which this is separated to a 5 wire TN-S
(3L+N+PE) system with separate neutral and ground.

This separation is done exactly *once*, typically at the main
entrance, after that, neutral and ground should be kept strictly
separated.

Paul

daestrom

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Nov 9, 2009, 9:08:59 PM11/9/09

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When you say 'sub-panel', it may be confusing. A step-down transformer
fed from the service entrance, that in turn feeds 208/120 to a panel is
not a sub-panel. In that case the first panel off the transformer is
not a usual 'sub-panel' but a fed from a separately derived source (the
transformer).

You mentioned 208/120 in an 'industrial building', so I think the
building service is not 208/120 directly but may be a higher voltage and
you have a step-down transformer inside the building.

In that case, the ground/neutral *should* be bonded in the first panel
after the transformer.

Now, any other panels fed from the first panel after the step-down
transformer are true 'sub-panels' and thus should not have the ground
and neutral bonded.

So just to be clear here, are you talking about a true 'sub-panel', or
the first panel downstream of a step-down transformer?

daestrom

Mike Cook

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Nov 10, 2009, 2:35:47 PM11/10/09

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> At least in the rest of the world (=non-US), this is a typical TN-C-S
> wiring case, in which the utility company 4 wire TN-C (3L+N) is
> delivered to building, in which this is separated to a 5 wire TN-S
> (3L+N+PE) system with separate neutral and ground.
>
> This separation is done exactly *once*, typically at the main
> entrance, after that, neutral and ground should be kept strictly
> separated.
>
> Paul

Correct.

The power is delivered as 3-phase plus neutral to the building. From this --
directly, without transformer -- we have 120 "single phase" branch circuits
in the panel along with 3-wire (3 phase) circuits.

The panel I am describing (with the N & G bond) is not the main panel, so I
presume should not be bonded together.

Thanks,
Mike (OP)