> 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 > >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.
> When was it installed, and have you read the revelant sections of the > NEC?
If it's a subpanel off the main, it's not supposed to have the ground and neutral connected. They can only be bonded together at the Service Entrance.
Unless it's a separately derived ground system, or some other exception in Article 250. Ask yourself: What happens to the ground at the subpanel if the neutral to the building opens. If you can answer that without the possibility of electrocuting anyone who touches a grounded conduit, you're probably good to go. (pass inspection, etc)
FYI - This is a MAJOR problem at cell sites (antenna transmission facilities). They ground everything past the service entrance (for lightning protection). I guess they figure there's more risk in not doing so.(?)
Google: Mike Post NEC. Usually some good references there on this topic.
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
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 >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.
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.
notme wrote: > 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.
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?
> 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.
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