Transformer Installation 5

[Ripcord13]

I have a 175 KVA transformer to step up the voltage from 208 to 460. The nameplate reads Primary 208 Delta H1 H2 H3
Secondary 460/266 X0 X1 X2 X3 We have a 208/120 3 Phase Y system. I do not need a neutral on the secondary side. Will this transformer suffice?

 

[whycliffrussell]

yes you will need a neutral for 120/208 on secondary.yes transformer is fine (it has a neutral, x0 on the wye side)

 

[Ripcord13]

Perhaps I didn't make myself clear, I am stepping UP the voltage from 208 to 460 3 Phase, the name plate on the transfomer shows a 208 delta connection to H1 H2 H3 and a secondary connection 460/266 volt X0 X1 X2 X3 I only want 460 volt 3 phase to run a motor I need no neutral on the secondary side. our system is 208 Y not 208 Delta will this transformer work?

 

[rcw retired EE]

The transformer will work. You are feeding it 3-phase 208 V power. The 208 volt widings can't tell and don't care if they are fed from a delta or wye system.

 

[wareagle]

The trransformer is ok. You don't need a neutral but you do need to install a systen grounding conductor(EGC).

 

[rbulsara]

Ripcord13:

I read this as you mean that you do not need neutral on the 460/266V side and it is OK. But you must ground the neutral (X0) at the transformer. You need not take it out to the load. The transformer is fine as is.

You still need the ground conductor as wareagle said.

 

[Ripcord13]

Thanks Guys, The transformer will be located at the main 2000 amp 208/120 volt 3 phase service. The motor (Air Compressor) will be in another room 200 feet away. Ground XO to the system ground. The load will be straight 460 volt 3 phase with a equipment bonding jumper necessary because the conduit run is PVC. Thanks Again

 

[tommom]

How are you using a 460/266 volt transformer step up from 208 to 480 volts - does it have extensive taps?

You don't need the neutral.

 

[resqcapt19]

rbulsara,

I read this as you mean that you do not need neutral on the 460/266V side and it is OK. But you must ground the neutral (X0) at the transformer.

Why does he need to ground XO? It is not required by the NEC.
Don

 

[rbulsara]

resqcapt:

Why would you not ground it? What do you gain? NEC is not a design manual. NEC does not require most of things we do.

 

[davidbeach]

If you don't ground X0 you would have to install ground detectors on the system. Ungrounded systems are good ways for the unsuspecting to get into lots of trouble. Best to ground it in this case, if you don't want a solidly grounded system, a high resistance grounded system would be far better than an ungrounded system.

 

[resqcapt19]

rbulsara,
I read your post as saying it must be grounded. I agree that it is a good design practice to ground the sytem in most cases, but it is a design issue and not a code requirement.
Don

 

[rbulsara]

resqcapt19:

Great, we agree. Why would we advise someone other than a what is a good practice/design?

 

[resqcapt19]

rbulsara,

Why would we advise someone other than a what is a good practice/design?

I just work under the assumption that anytime someone says that we "must" do something, it is because there is a code that requires it.
Don

 

[rbulsara]

resqcapt19:

With all due respect, firstly you are hung up in semantics.

Secondly, NEC does require the system to be grounded to keep the L-G voltage not to exceed 150V for systems rated less than 1000V. See NEC 250.20 (B). Where it is permitted to be ungrounded, there are many other requirements that need to be met, see NEC 250.21.

 

[resqcapt19]

rbulsara,

Secondly, NEC does require the system to be grounded to keep the L-G voltage not to exceed 150V for systems rated less than 1000V.

How do we get a voltage to ground of less than 150 volts for a 480 volt system? The question is about the grounding of the secondary of a step-up transformer.
Don

 

[apowerengr]

How (by code) can you not ground the wye-connected neutral point on the secondary of the proposed transformer? NEC 250.21? You need a path for zero-sequence fault currents to flow on both sides of this transformer. Bond the transformer case to the 208 volt supply system grounding conductor, separately derive the 460/266 volt system with a grounded neutral (since the transformer is wound wye) and carry an equipment grounding conductor to the motor.

It seems that Ripcord had it correct in the posting dated August 16th.?

 

[rbulsara]

well, i may have copied one of the lines that has 150V in it but code still requires grounded system, if not it has to have ground detectors. I will look again...

 

[rbulsara]

OK, I re-read 2002 version that I have handy (will check 2005 in the office later). It is correct that if the neutral is not used as a circuit conductor, it does not have to be grounded. I believe 2005 edition added the requirement of a ground detector. I am not sure how to interprete 250.20(D), but it seems it only covers system covered by 250.20A or B.

Still I would stand by my recommendations. I would not advise a ungrounded system, unless there is a dire need for it for some reason.

 

[waross]

I quote the Canadian code, because of it's close (but not perfect) harmonization with the NEC.
I hope that others may find similar rules in the NEC,
and,
I am several thousand miles away from my copy of the NEC just now.
I believe that if a system may be grounded so that the maximum voltage to ground will be not more than 150 volts to ground, it must be grounded.
If the maximum voltages will be more than 150 volts to ground it may be grounded.
If the system has a neutral, it must be grounded. (I am not sure if your transformer will be considered a system and require a ground under this clause.)
If the system is ungrounded it must be equiped with suitable ground detection devices.
Grounding is often cheaper than ground detection devices.
respectfully