Subpanel bonding (3-phase mains) question

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

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.

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

jk

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

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.

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

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

RE

bar

re

al

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)

bar

re

al

TROLL DROPPINGS

TOM LAY OFF YOU NEVER GOT THIS RIGHT TO BEGIN WITH

I AM PROTEUS

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

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

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)