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#52131 - 05/20/05 12:49 AM 3 phase transformer grounding debate  
techgod  Offline
Member
Joined: Dec 2003
Posts: 20
Burbank, Ca USA
Hi Everyone,
This week I installed a 300kVA 3 phase wye-delta 120/208-277/480 transformer in a mall to be used for about 6 months to feed a new parking garage. The transformer primary is connected to a new 600 Amp 120/208 main panel which is tapped onto the existing adjacent 2000 amp service buss. The secondary side feeds a 300 amp 277/480 fused switch, The new main panel, transformer and switch all have less than 12" between them.

I installed a new 4/0 water bond for the 600 amp main panel, ran a neutral and equipment ground to the transformer. I landed the neutrals from both the main panel and the switch to the HO buss and the equipment grounds to the transformer case. There is no bond between the HO buss and transformer case.

The inspector looked at it today and asked that the neutral and equipment ground be removed between the Main panel and the transformer, and install a second cold water bond for the transformer HO terminal and bond between the HO and the case with the equipment ground.

I understand that the inspector has asked to have this setup as a separately derived system but I opted not to go that route because:

1) The transformer is the only load on the new main panel. It will have no neutral returning to the switch with a 4 wire service.

2) The second cold water for the transformer bond will be within 10' of the main panel water bond.

3) There would be no neutral or equipment ground in the 4" seal tight between the main panel and the transformer.

From what I know both installations are correct but why would you create a separately derived system within 10' feet of the main panel?

Thanks in advance for your comments.

Mark


Mark Stetson
Stetson Electric

Work Gear for Electricians and the Trades

#52132 - 05/20/05 09:00 AM Re: 3 phase transformer grounding debate  
Yoopersup  Offline
Member
Joined: Mar 2003
Posts: 822
Michigan
Sounds like your reverse feeding a transformer right? Using the 120/208 V to feed using 480/277 . Is this transformer a Delta on 480 v side and Y on the 120/208 side? If so there'd be a HO connection on the 120/208 side but none on the 480 v. side. If thats true how are you getting the neutral on the 480 volt side??What the inspectors saying I think is feed the transformer with straight 208 volts and (ground)(required) -bond 480 v side per 250.30
Still wondering if that transformer has a neutral on the 480v side.(talked to transformer manf. He said one with neutral on 480 side would be special order.)


#52133 - 05/23/05 02:02 AM Re: 3 phase transformer grounding debate  
Scott35  Offline

Broom Pusher and
Member
Joined: Oct 2000
Posts: 2,707
Anaheim, CA. USA
I am a little corn-fused here, so please bear with me on these following questions.

<OL TYPE=1>

[*] Is this a Reverse-Connected Transformer - as in it being a 480 V Delta connection on the "H" side Terminals, and 208Y/120 V on the "X" side Terminals?,


[*] Is the "High Voltage Output" side only to be a 3Ø 3W System, or 4 Wire System?,


[*] Are you bringing a Grounded System Conductor "AKA: Neutral", to the Low Voltage Input side of the Transformer?,


[*] Are you connecting the "Output" side of the Transformer to a Grounding Electrode System?,


[*] What is meant by "HO" - is this an "H" terminal, marked with the Number Zero?
</OL>

In a nutshell, if this install was using a 480V 3Ø Delta Primary x 208Y/120 3Ø 4W Wye Secondary Isolated Transformer - connected "In Reverse", here's my 2¢:

Feed the Transformer with a 208V 3Ø Circuit (adequately sized), and include an EGC.

Connect the Ungrounded Conductors of the 208V Feeder to the "X1", "X2" and "X3" Terminals only. Do not connect anything to the "X0" Common Terminal - and if there is any jumper from X0 to the Enclosure - remove it.

Connect the EGC from the 208V Feeders to the Transformer's Enclosure/Case.

On the "Now Secondary Side", you have a choice to Ground the new SDS or not. If you do Ground the output, then pick only one "H" Terminal, and physically bond it to a GES, which has been established for that system.
Also bond the Transformer's Enclosure to it.

If you plan not to Ground the system, simply bond the Transformer's enclosure to the new GES (along with all the metallic enclosures + raceways used on that SDS).

Please reply ASAP to this message.

Scott35


Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!

#52134 - 05/23/05 04:09 AM Re: 3 phase transformer grounding debate  
e57  Offline
Member
Joined: May 2003
Posts: 2,876
S.F.,CA USA
"why would you create a separately derived system within 10' feet of the main panel?"

Isn't that what you did? It should be treated as such.

I'm maybe a little lost in how you have this set up. Is this an ungrounded delta secondary? This statement confuses me too.
"I landed the neutrals from both the main panel and the switch to the HO buss" The "switch" on the secondary side? Where does this second neutral come from?

And this: "The inspector looked at it today and asked that the neutral and equipment ground be removed between the Main panel and the transformer, and install a second cold water bond for the transformer HO terminal and bond between the HO and the case with the equipment ground." That makes it sound like a Delta-Wye secondary.


It also sounds like the ground for the tranny case, and system is dependant on the EGC. A #6?


Mark Heller
"Well - I oughta....." -Jackie Gleason


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