ECN Forum Forums Code Related Discussion NEC & other Code issues Transformer and grounded conductors

I was finishing up a job last week and this came up. The job was to install a 225A 208/120 panel and a 75KVA 480Y to 208Y/120 transformer . there were already 3 of these transformers located in the utility room and I was adding a forth. The installation was such that the new transformer was going to supply an existing panel and then that transformer would feed the new panel. Well the day of the plant shutdown, to switch it all around, was last Wens. and that's when I noticed the original installation had 5 wires on the primary. Four #2 and one #4. There was the #4 for the bond and three #2 for the 480 plus, and this is what I do not understand, a #2 grounded conductor from the primary on the same lug as The #4/0 that is the grounded conductor of the secondary going to the panel. I immediately called one of our state inspectors (Maine) and we discussed the situation and the way we left it is the new transformer, not having a grounded conductor from the primary, is to stay that way. The disconnected transformer is to be connected to the new panel with this primary grounded conductor wire disconnected and the other two transformers will have the grounded conductors, from the primary, removed during the next shutdown. Some added iformation is that when I checked the grounded conductors on the two still operating transformers I get 0 amps load on the 4/0 and about 10A on the #2 on one transformer and the other has 3A on the 4/0 and 5A on the #2. It should be noted the demand for 120V is small on all of these transformers and the highest reading on any secondary wire was about 15A at the time. It appears the worst case scenario would be the loss of a 4/0 grounded conductor would place the entire demand a #2 grounded conductor of another transformer. Can it be explained why anyone would wire a transformer this way and how, in laymen terms, can I best explain to the customer why I want to remove the primary grounded conductor on all of their other transformers. I hope this reads right.

gto6t7
It might have been a Y-Y Autotransformer used for supplying power from a three-phase,four-wire system. This system is connected and uses the same neutral(grounded conductor).
Check existing name plate. That if you can still read it. Autotransformer shall not be used for branch circuits. Good luck

You have my interest on this one!!
Could you reply back to this thread with where the 4th conductor on the primary side was connected through the Transformer and what it's voltage to ground is?

Also, what is the type of transformers are they [Autotransformers, zig-zags or a common Isolated type], along with the grounding electrode systems.

Was the primary fed from a typical 3 phase 480 volt system, or is it on an Emergency power system.

Any information is helpful.

Looking forward to your reply.

Scott

Sorry about the last post, hit the wrong button!

Thanks scott. The transformer has the 4th wire connected under the same lug as the grounded conductor for the secondary (as if it were an autotransformer but it is not.) The voltage to ground is 0 volts as the lug is also bolted to the bond going to the frame. The trnsformer is an isolated type. The supply to the transformer is a normal 480/277 3P 4wire system. The original plan for the plant appears to be it was to have 480V on all equipment with 277V lighting and the 208/120 was to supply mostly 120V needs. The way things have evolved is that most of the equipment they have installed needs 208 3P thus the addition of this transformer.

More info on the transformers. They are delta/wye. I looked back in my other post and noticed I never stated this fact.

[This message has been edited by gto6t7 (edited 12-20-2000).]

You can not bring the grounded conductor from the 480 volt supply and connect it to the bonded XO terminal of the transformer. This would be a rebonding of the 480 volt grounded conductor and is a code violation. You must bring an equipment grounding conductor from the 480 volt source to the transformer and this conductor will be indirectly connected to the secondary grounded conductor via the main bonding jumper.
Don(resqcapt19)

Ya, it looks like someone screwed up with the connections. The Grounded Conductor on the Primary side [the 480Y277 side] should not have been run to the transformer to begin with, let alone being re-grounded in the transformer enclosure.

Don hit the nail on the head with the code violation, too.
What this will do is cause objectional current to flow on the conduits for the 480Y277 system. The current drawn for L-N loads will try to flow on both the conduit and in the Grounded Conductor. This happens even if the bond is no where near any L-N loads. They just need to be flowing that direction, or through that certain subpanel [or gear section], for this to happen.
Are the Conduits for subfeeders and the transformers by chance hotter than average? Do they spark when a coupling is opened ? Are you noticing any Locknuts that appear to be discolored, have a tarnished spot around the KOs where the connector enters, or even some Ash like deposits just "showing up"?
Those are classic cases I have seen where a Grounded Conductor was mistakenly re-grounded somewhere on the local system.

Give your self, plus your Inspector a pat on the back for spotting this! It can be a small problem, or a 911 problem!

Keep up with the sharp eye!

Scott.

This is the very reason I don't do transformers ,if I need one I call what I hope is an expert and has a lisc. You guys lost and confused me about half way thru the first post

For those in the groupe that have emaied me to see how this is going I have gotten permission from Bob Hulse an engineer with Cutler Hammer to post his reply he sent me regarding this problem. There are two post, the initial reply plus a second post when I asked permission to post his reply and he added some further info. I might add again the transformers in question are delta/wye.


The situation you described, where the X0 bushing of the 480/208Y120
transformer is both bonded to ground and connected to the 480 volt system
neutral is a common mistake which I have seen in many older commercial
building installations. Generally this situation is discovered after an
upgrade of the 480 volt system requires the installation of ground fault
protection which then exhibits nuisance tripping. Though I do no have time
at this point to research and quote all the code articles that apply, I can
tell you with confidence that correct engineering practice dictates the
system to be configured as described in the following paragraphs:

The 480 volt system neutral should be grounded at, and only at, the service
entrance point; i.e. the bonding jumper in the service entrance switchgear
or transformer. Since the neutral conductor carries current under unbalanced
load conditions, it will be at a potential above ground (caused by the IxZ
drop along the conductor) at any given point beyond the bonding jumper;
which is why it is an insulated conductor. Any additional connections
between the neutral and ground is effectively a ground fault on the system
and will result in the ground conductor/system becoming a parallel path for
neutral current. Under severe single phase loading this can result in high
step and touch potentials to be present at any point in the system where the
integrity of the bonding conductor is compromised; i.e. a loose or corroded
connection or conduit fitting where the conduit serves as the bonding
conductor.

At the 480/208Y120 transformer secondary, the X0 bushing constitutes a
separately derived neutral and should not be connected to the 480 volt
system neutral, but must be bonded to the system ground conductor
(non-current carrying). No other connection between neutral and ground
should be made past this point on the 208 volt system for the same reasons
as mentioned above.

I hope this is a help to you. Please feel free to contact me if you have
additional questions or wish to discuss this further.

Regards,
Bob Hulse
Advisory Engineer
Cutler-Hammer Engineering Service and Systems Division
Winthrop Maine


Dave,
I finally got a chance to visit the site and review the replies, Scott35 had
a very good answer. In the interest of completeness of the discussion, I
would offer the additional comment dealing with the transformer primary
connection.

If the primary is a delta then only the three phase
conductors are required even though the source panel may be 4 wire. If the
transformer primary winding is a wye, the source neutral conductor is also
required to be connected to H0. Omission of the H0 connection on a Y-Y
transformer will adversely effect transformer performance under unbalanced
load, and effectively result in some level of impedance grounding of the
secondary, even with X0 bonded.

You may post my previous reply, and the comment above if you wish, with my
name and affiliation attached. I hope you found this helpful in your
situation, and if I can clear this issue up for others it is a bonus.

Happy New Year,
Bob

Thanks everyone for the input and I also would like to wish all a happy new year

Dave T.