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
Re: Transformer and grounded conductors#76135 12/18/0010:30 PM12/18/0010:30 PM
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.
Re: Transformer and grounded conductors#76137 12/20/0007:47 AM12/20/0007:47 AM
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)
Re: Transformer and grounded conductors#76139 12/20/0009:01 PM12/20/0009:01 PM
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 " 35 " Thompson Just Say NO To Green Eggs And Ham!
Re: Transformer and grounded conductors#76140 12/22/0009:16 PM12/22/0009:16 PM
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