Isolated Power Panelboards

[mshields]

I'm specifying a 5kVA Isolated Power Panelboard for a hospital application and I have two options for the primary voltage; 120V or 208V (single phase of course). I want to go with the 208V, 1 phase but a colleague is telling me I should go with the 120V primary. He seems to think that a grounded voltage has some advantage over a line to line. I don't know why that would be the case.

Any thoughts?

thanks,

Mike

 

[wirenut1980]

I don't think it would matter either way since you would be running an equipment ground out with 120 V or 208 V. Plus in the isolation panelboard I think you are deriving a new neutral to ground bond anyways.

 

[bob]

What wirenut said. You do not need a neutral input to the transformer. You will establish a new neutral in the panel. Higher voltage smaller wire.

 

[roger]

What wirenut said. You do not need a neutral input to the transformer. You will establish a new neutral in the panel. Higher voltage smaller wire.

There will be no neutral in the secondary of this system, it will be 120 v L-L. In my experience, the most common way (not always though) of feeding these panels / transformers are with L-N feeders, 120 or 277 volts, I don't really know why.

Roger

 

[bob]

There will be no neutral in the secondary of this system, it will be 120 v L-L. Roger

I did not read any thing in the OP that indicated no neutral in the secondary.
I do not see where this fact has anything to do with whether the transformer is fed 208 or 120 volts.

 

[roger]

I did not read any thing in the OP that indicated no neutral in the secondary.

Well, I'd bet that since this is a 5KVA Isolated Power Panel for a hospital, Mike is referring to a 517.160 application and there will be no neutral

I do not see where this fact has anything to do with whether the transformer is fed 208 or 120 volts.

As I said, L-N is the most common way I see Isolated Power Systems fed but, I don't know why this is, there are times they are fed from a L-L source

Roger

 

[mshields]

L-L, 120V output is correct

L-L, 120V output is correct

Thanks for the input

 

[steve66]

I'd feed it with the 208 volts.

If you use 120, you will have a 50 amp or larger breaker. That may be hard to coordinate with the next upstream feeder breaker, and/or the transformer inrush current.

Steve

 

[Dickieboy]

I believe the normal procedure in hospital applications are for line conditioners with integral isolation transformers with bonded neutral to ground secondary,,,,thus getting all noise out and away from the sensitive hospital equipment when needed,as to which voltage,you may need both depending on equipment to be used,208 is not to common for single phase users.

This post brings up a question as well, where are you getting 120V L to L for your primary? Is this a special set up for hospital distribution ?I'm not familiar with this source voltage or where you derive it.

dick

 

[jim dungar]

I believe the normal procedure in hospital applications are for line conditioners with integral isolation transformers with bonded neutral to ground secondary,,,,thus getting all noise out and away from the sensitive hospital equipment when needed,as to which voltage,you may need both depending on equipment to be used,208 is not to common for single phase users.

This post brings up a question as well, where are you getting 120V L to L for your primary? Is this a special set up for hospital distribution ?I'm not familiar with this source voltage or where you derive it.

dick

The OP transformer is for power supplies to operating rooms and therefore is concerned with isolating patients not equipment. There are very special rules in article 517.160.

 

[roger]

This post brings up a question as well, where are you getting 120V L to L for your primary? Is this a special set up for hospital distribution ?I'm not familiar with this source voltage or where you derive it.

dick

The 120 L-L (60-60) is the secondary voltage, the primary can be any voltage under 600 V, see 517.160(A)(2) The only reference to ground is through the Line Isolation Monitor. The combined ground and leakage currents must be less than .0006 amps in it's idle (no load all breakers off) state, the alarm threshold would 5ma.

Roger

 

[roger]

I'd feed it with the 208 volts.

If you use 120, you will have a 50 amp or larger breaker. That may be hard to coordinate with the next upstream feeder breaker, and/or the transformer inrush current.

Steve

Steve, I see many fed at 25 amp 277v, this saves space in the supply panel verses using double pole breakers for the feeders.

Roger

 

[mshields]

To Dick

To Dick

Dick - I am using a 208V single phase primary. Could use a 120V L-N primary? No I was not suggesting a 120V L-L primary.

Mike

 

[steve66]

Steve, I see many fed at 25 amp 277v, this saves space in the supply panel verses using double pole breakers for the feeders.

Roger

Yes, I agree 277 would also be good. Probably even better since it may be eliminating other step-down transformers from the chain of possible failures.

But I think Mike only has 120 or 208 to choose from.

 

[roger]

But I think Mike only has 120 or 208 to choose from.

Agreed

Roger

 

[LarryFine]

My take on this is that the isolated secondary doesn't care whether the primary supply has a grounded conductor, I see no advantage to using a 120v circuit, and I concur with the higher voltage/lower current position, so I agree with using 208v.

 

[roger]

My take on this is that the isolated secondary doesn't care whether the primary supply has a grounded conductor,

Correct, it doesn't care or matter.

Roger

 

[kingpb]

I'm specifying a 5kVA Isolated Power Panelboard for a hospital application and I have two options for the primary voltage; 120V or 208V (single phase of course). I want to go with the 208V, 1 phase but a colleague is telling me I should go with the 120V primary. He seems to think that a grounded voltage has some advantage over a line to line. I don't know why that would be the case.

Any thoughts?

thanks,

Mike

Use 208V single phase, and forget about it.

 

[mshields]

Good advise

Good advise

Yes - came to that same conclusion several days ago.

thanks,

Mike