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equipment ground for transformer

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shocker3218

Senior Member
I am installing A FIRE PUMP WITH A TRANSFORMER TO STEP UPFROM 208 TO 480 VOLT. sERVICE ENTERS BUILDING VIA A 200 AMP METER SOCKET, THEN HITS A 600 AMP MAIN DISCONNECT FUSED AT 450 AMPS (ALL PER LOCAL CODES AND AHJ) .

MY QUESTION IS, SINCE NEUTRAL IS BONDED AT MAIN DISCONNECT, AND THE LOW VOLTAGE SIDE OF THE STEP UP TRANSFORMER (75 KVA 208Y-480DELTA) AM i REQUIRED TO RUN AN EQUIPMENT GROUND CONDUCTOR? i AM USING 2 FT. OF FLEX FOR FINAL CONNECTION TO TRANSFORMER. ALSO, WHAT ABOUT GROUNDING ELECTRODE CONNECTION FROM TRANSFORMER? WHAT DO I BASE THIS WIRE SIZE ON? 200 AMP SERVICE, OR 450 AMP FUSES?
 
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jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
PE (Retired) - Power Systems
Re: equipment ground for transformer

Quick comment.

You inferred that the X0 terminal of your transformer was connected to the system neutral and bonded to ground at the transformer. When wye windings are connected as the primary in a step up transformer configuration the X0 terminal should always be left unconnected (floating no connection to ground or neutral).
 
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shocker3218

Senior Member
Re: equipment ground for transformer

That's what I was missing. It was connected as a step up previously and X0 was bonded to frame. (by a previous contractor)

So do I need to bond a phase on the Secondary (480 volt) side, or just leave it float and connect equipment and grounding electrode to case only?
 
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bennie

Esteemed Member
Re: equipment ground for transformer

What is the size of this fire pump? 75 KVA is a good sized transformer. Calculate the cost of running this transformer 24/7, even unloaded.

If the transformer only goes on line when a fire is annunciated, and no overcurrent devices are in the circuit clear back to the utility bank, this should work.

My advice is to buy the correct transformer.
 
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shocker3218

Senior Member
Re: equipment ground for transformer

fire pump is 30 HP

There are 450 amp fuses located on the primary (low voltage) side. Nothing on the secondary side as per AHJ and 695-5.

Cost is not a real factor to owner. Electricity is relatively cheap in Nebraska.
 
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bennie

Esteemed Member
Re: equipment ground for transformer

You may have a good point in running the 480 volts ungrounded. Might add to the continuity of service.
 
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stew

Senior Member
Location
federal way,washington
Re: equipment ground for transformer

Something doesnt sound right here. Why would you "never" ground the x0? If you were to feed a subpanel I beleive that this is what you would do.As long as the neutral was not grounded in the panel I believe this is perfectly ok. If you were feeding a service panel then you would float it at the transformer and bond the neutral at the panel.Anyway unless there is some requirement for 277 volts lighting or something you wouldnt need the neutral anyway right?

[ April 20, 2004, 11:00 PM: Message edited by: stew ]
 
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bennie

Esteemed Member
Re: equipment ground for transformer

Stew: Look back at the secondary that is feeding this wye primary. If you connect both star points (X-O), there is a complete circuit from the phase to neutral conductors.

Any imbalance, current, on the wye primary will flow in the source transformer. Imbalance in a floating wye will cause an elevated voltage in the high phase,to make up for the zero sequence, normally furnished by the neutral.

Another issue is by grounding the X-O primary and bonding to the can, you will have another earthing past the main.
 
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bennie

Esteemed Member
Re: equipment ground for transformer

I am just confusing this topic. I'll bow out and let someone who can do a better job than I am doing.
 
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curt swartz

Electrical Contractor - San Jose, CA
Location
San Jose, CA
Occupation
Electrical Contractor
Re: equipment ground for transformer

Stew

Since this is a delta secondary the only way to ground it would be on one of the corners (A, B or C phase). The NEC does permit ungrounded systems for certain applications and many times they are desired. With a grounded system a ground fault on any of the phases will take out the circuit. With an ungrounded system 2 phases would need to develop ground faults.
 
iwire

iwire

Moderator
Staff member
Location
Massachusetts
Re: equipment ground for transformer

Basically you must bond the secondary of the transformer to provide a fault clearing path unless you are trying for an ungrounded system.

You will have to bond one of the secondary conductors just like you would bond XO if there was one.

[ April 21, 2004, 01:35 PM: Message edited by: iwire ]
 
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stew

Senior Member
Location
federal way,washington
Re: equipment ground for transformer

and thats precisley how I understood a proper method of grounding to be done i wre. I agree and the only difference would be where it is done depending upon the application ie: subpanel or service right?

[ April 21, 2004, 01:38 PM: Message edited by: stew ]
 
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shocker3218

Senior Member
Re: equipment ground for transformer

transformer is up and running. No connection to X0 and I left the secondary (480 volt side) ungrounded. I felt that since this was a fire pump application I needed to insure that it would run as long as possible even with a fault condition.

Also, my boss found some wire that is 2 hour fire rated when installed in EMT conduit. A real cost savings on this application because trenching outside (then trhough a 4 1/2 ft. thick wall) was very impractable, and Mineral insulated wire is very expensive.

I don't have the manufacturs' info in front of me, but am more than willing to share if there is an interest.

Thanks for the help!! This was great!!
 
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hurk27

Senior Member
Location
Portage, Indiana NEC: 2008
Re: equipment ground for transformer

Stew
I think what your not understanding is this transformer is a step down transformer being used as a step up transformer and where the "Y" would normally be the secondary is now the primary and what would have been the primary delta is now the secondary. when you wire the transformer like this you don't bond the X0 on the primary side just like what Bennie has said will happen. and the NEC allows secondary delta's to remain ungrounded when continuity of service is required.
 
brian john

brian john

Senior Member
Location
Leesburg, VA
Re: equipment ground for transformer

In my opinion Bennie is right, the best way to complete this installation would be to obtain a DELTA (at 208 VAC) to WYE at (277/480 VAC). The big advantage of this installation is a transformer that is designed for the installation. The liability involved with something that may go wrong after installation (TOTALLY OUTSIDE YOUR CONTROL) could be turned agaist you in court.

The transformer you have at present is a 480 delta to 208/120 wye. In your installation the load connections will be on the H1, H2, H3 delta windings, You can ground anyone of these connections or choose to ground none of these connections (see 250.20 (B). The line connections should be 208 VAC X1, X2, X3 no grounded conductor/neutral on the XO termination. As stated by others an ungrounded system on a fire pump can be a plus.

You are required to run an equipment grounding conductor no matter what type of system you install grounded or ungrounded.
 
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explorer

Member
Re: equipment ground for transformer

Jim,
I'm not saying your wrong. But if you could provide some references to the NEC or other books about not grounding the XO terminal if a wye winding is used as the primary in this step-up case it would be very helpful.

Thanks
John Cosmo
State Electrical Inspector
 
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