transformer feeding new ups system??

[charlie tuna]

steel office building - law firm buys a new ups system and requests us to install a 100 amp single phase feed to their computer room to serve this unit. the supply voltage to the ups system is 240 volts. we connect the transformer as recommended by the manufacturer for a single phase - 240 volt secondary (primary is single phase 480 volts). the transformer has a welded ground lug common thru the housing. we have installed bond wires on the primary and secondary feeders and secure them thru the use of a dual lug connected to this welded lug bolt. on the common welded bolt on the outside of the frame we connect properly sized ground wire to the steel frame of the building.

problem and question: since we connected the transformer straight 240 single phase - we do not have a center tap connected to the ground lug of the transformer - and this makes this an isolation transformer - ie - we have no voltage from either 240 volt feeder legs to ground. is this correct or do we need to provide this center tap inside the transformer even though we have no use for it? and will the ups sense this???

 

[dereckbc]

Re: transformer feeding new ups system??

Charlie, sounds like you answered your own question. If a neutral is not needed, then it is not needed. What ever the UPS is being used for will be restricted to 240 only.

The configuration is a little odd. Usually a large scale UPS will have 480 delta input and 208Y/120 out, or 480 delta in - 480Y out, or some sort of balanced system.

If I understood you correctly you have single phase 480 input and single phase 240 output. How are you going to balance the 480 supply system?

 

[charlie tuna]

Re: transformer feeding new ups system??

der,
this transformer is 25kw and is fed by a 2 pole 60 amp - 480 volt breaker. i'm not concerned too much about balanced load since the distribution panel is fed from a 4000 amp bus duct. the firm is one of the largest in the state which means they did not confir with anyone prior to ordering this ups - it should have been rated with a 480 volt - three phase primary. the ups is 12kw! it's just strange to have a source that is basically "ungrounded" due to the requirments of it being connected in a straight 240 volt configuration. we could go back and connect a jumper from the center point of the secondary's wiring and ground it to the frame/building steel even though we have no need for it at the ups?
thanks for your response.... charlie

 

[dereckbc]

Re: transformer feeding new ups system??

Charlie, I understand what you are saying now. You will still need to ground the center tap of the secondary, even though no neutral conductor will be ran to loads. Is there any GFD on the secondary?

 

[bwilson]

Re: transformer feeding new ups system??

Charley, 250.20(b)

 

[charlie tuna]

Re: transformer feeding new ups system??

thanks guys i looked it up and will install the jumper from the center point of the two single phase secondary windings before we energize the ups.... thanks again----this is great for our trade! and thanks to mike holt!