75kva 240-480

[dwalker]

First time here with an important question I hope can be answered. Ive done a search and read all the posts but couldn't find the answer i'm looking for.
I have a 200A - 240v Delta 3 phase service that has just been installed. I'm having a 75kva transformer hooked up right after the 200A fused disconnect. The disconnect is hooked up directly back to back of the meter can. It is a 480v - 240v transformer which will be reversed fed.This service is dedicated soley for 1 machine running on 480v. I have 3 questions.
1) When reversing the transformer does the 240v line in go to the (x) and the 480v line out go to the (H) which in turn goes to the machine. And / or do you change the 3 wires on the coils currently going to (H) that are for different voltages.
2)NEC says ocd rated at not more than 125% which is 225A. So does that mean it can be less as in my 200A fuses?
3) How is this suppose to be grounded? Currently the ground from the disconnect goes to ground on copper strip to transformer case. With ground from machine going to this same location. And solid copper from this same location to metal beam of building.
The inspector came and looked at the meter can and fused disconnect and ok'd this but he wants the transformer and machine hooked up before he gives his final approval.
I have been getting multiple different answers with the questions above and the electrician is saying none of this is right and that the inspector will not pass it. I hope I can get the RIGHT answers here so that I may get this machine up and running as soon as possible.
If more info is needed let me know.
All help is greatly appreciated,
Doug

 

[augie47]

transformer

transformer

240 to x
the "coil wires" are for voltage adjustment (taps)..match them to your voltage.
200 amp primary fuse should be o.k.
unless I overlooked something. grounding sounds o.k. if its a delta-delta transformer

 

[jim dungar]

On transformers terminals, H means high voltage (not primary) and X means low.

Your transformer is designed for the magnetic flux to be supplied by the 480V windings. Because you are connecting it in reverse the 240V windings will not be as effective in creating the flux so you can expect that it will have a higher inrush current. It is not unusual to see reverse fed transfromers need primary OCP of over 200%, which requires them to have a single secondary OCP also.

 

[dwalker]

Thanks for your responses.
The inspector will be out tomorrow so we will see what happens.
I've checked a lot of different electrical forums and talked with various electricians and I'm amazed at all the different opinions on transformer installs. No wonder there are so many installs that are not right.
Regards,
Doug

 

[dwalker]

Update

Update

Here is an update. The electrical Inspector came today and passed everything except the ground from meter can to main disconnect as it was bonded to neutral. He told me to just remove it and all is well. He couldn't believe they did that though. He had to get out the bible to see if I also needed protection on the secondary side, but i didn't.
Thanks again guys,
Doug

 

[winnie]

So what exactly did you do on the secondary side with respect to grounding?

A 480V delta is not required to be grounded...but if you have an ungrounded system then you need ground fault detection. Plus you need to make sure that the equipment being fed is suitable for an ungrounded system.

-Jon

 

[augie47]

transformer

transformer

I'm curious about the lack of protection also,.....is the secondary delta or wye ??

(side note: jon, I was going to mention the gf detection earlier and for the life of me I couldn't rememeber or find it in the Code. Direct me, please ?)

 

[winnie]

On ground fault detection.

I was sure that it was in the book, and as old as the hills. When you have an ungrounded 480V system you must have ground detection. But then I went looking in the 2002 book, and couldn't find it.

The closest that I came was 250.21(3)...which applies only to control circuits.

I then went to the 2005 book, and it is there clear as day. 250.21 in its last paragraph explicitly states that most ungrounded systems must have ground detectors.

-Jon

 

[dwalker]

Ground

Ground

Hi Guys,
Secondary is delta. And it is grounded to the steel building along with the xfmr.Does this sound correct to you? It's kinda funny but the first inspector passed the metercan and fused disconnect but the inspector today didn't pass the ground to neutral from meter can to disconnect. I don't know what to say or who to believe anymore. I just hope this machine starts, runs, and doesn't burn up because of the electrical.
Regards,
Doug

 

[winnie]

Hi Guys,
Secondary is delta. And it is grounded to the steel building along with the xfmr.Does this sound correct to you?

How exactly is the secondary grounded? There has to be some electrical connection from your grounding conductor to the secondary; what is it?

-Jon

 

[dwalker]

Hi Jon,
There is a #6 ground wire that goes from the xfmr to the panel of the motor controls. There is also a ground wire that goes from the xfmr to a metal beam in building and also to the main fused disconnect. Not sure if this is what you wanted or if its even right.
Regards,
Doug

 

[Pierre C Belarge]

On ground fault detection.

I was sure that it was in the book, and as old as the hills. When you have an ungrounded 480V system you must have ground detection. But then I went looking in the 2002 book, and couldn't find it.

The closest that I came was 250.21(3)...which applies only to control circuits.

I then went to the 2005 book, and it is there clear as day. 250.21 in its last paragraph explicitly states that most ungrounded systems must have ground detectors.

-Jon

230.95 is the reference I believe you are looking for.

 

[don_resqcapt19]

Doug,
What is the #6 grounding conductor connected to at the transformer.
Don

 

[coulter]

... Plus you need to make sure that the equipment being fed is suitable for an ungrounded system. ...

jon -
What could make it unsuitable (other than a required neutral connection)?

carl

 

[dwalker]

Hi Don,
The #6 wire is anchored to a lug that is on top of a brass strip anchored to the metal cabinet. The other side of the brass strip is anchored to the metal frame of the coils.
Regards,
Doug

 

[don_resqcapt19]

Doug,
Then you don't have a "grounded" secondary. To be a grounded system, there must be a grounding connection to one of the circuit conductors.
Don

 

[winnie]

jon -
What could make it unsuitable (other than a required neutral connection)?
carl

On some of the VFDs that I've worked with, there were specific instructions with requirements and modifications when used on ungrounded systems. Mostly this had to do with MOV devices connected from phase to ground.

I am not aware of other specific circumstances.

Question: can 'slash' rated circuit breakers be applied to ungrounded systems?

-Jon

 

[winnie]

Doug,

A grounded electrical system has one of the transformer secondary terminals electrically connected to ground. This performs a similar function to the neutral-ground bond in a service entrance; it provides a defined fault current path, as well as solidly referencing the other transformer terminal voltages to ground.

If you do not have this solid, intentional connection between the transformer and ground, then you have an ungrounded system. In the event of a ground fault (say a wire touches a metal conduit), then you will _not_ have a short circuit, and you will not get high current flow. In fact, the system can continue to operate in this state, and this is why ungrounded systems are sometimes intentionally used. However there are several downsides to this. One is that faults can go unnoticed for a long period of time, until a second fault causes an even worse short circuit; another is that some fault conditions can 'pump' the voltage between the wires and ground to well above the supply voltage. In this 'restriking' ground faults, lots of equipment can be damaged do to severe overvoltage and insulation failure.

With a delta secondary such as you are considering using, your grounding choices are either to use a 'corner grounded' system, or to add some sort of grounding transformer, or to go ungrounded, or (possibly) to use a center tap on one of the 480V coils.

-Jon

 

[jim dungar]

Question: can 'slash' rated circuit breakers be applied to ungrounded systems?

No.

Slash rated breakers can only be used on solidly grounded systems where the line-ground voltage does not exceed the lower of the two voltages.