K Rated Isolation Transformer vs. Isolated Grounding - Electrician Talk - Professional Electrical Contractors Forum
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Old 02-25-2020, 02:41 PM   #1
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Default K Rated Isolation Transformer vs. Isolated Grounding

I am working on a project in which I would like to limit harmonics caused by nonlinear loads such as switches, UPS's, computers etc..

To do this, I plan on specifying K-rated isolation transformers to feed these loads. My question, do I need to call for oversized neutrals on the panelboards feeding these loads?

Also, what is the relationship between K-rated transformers and isolated ground systems? Can I have isolated ground receptacles going back to panelboard that is fed from K-rated transformer?
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Old 02-25-2020, 03:27 PM   #2
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K rated transformers really don't do anything to remove harmonics, they are just biggger winding transformers that don't heat up so much. Depending on how big the place is and how much non-linear stuff you have, phase shifting zig-zag transformers may be a better option.

No, bigger sized neutrals are not needed to the panelboards, nor are they needed for branch circuits.

Yes, you can have IG receptacles installed as normally as they are without K-rated transformers.
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Old 02-25-2020, 03:27 PM   #3
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"Also, what is the relationship between K-rated transformers and isolated ground systems? Can I have isolated ground receptacles going back to panelboard that is fed from K-rated transformer?"


I've done that so many times I've lost count.

"To do this, I plan on specifying K-rated isolation transformers to feed these loads. My question, do I need to call for oversized neutrals on the panelboards feeding these loads?"


The neutral's harmonics stop at the source of the current -- ie your K-rated transformer. That XFMR is expected to be supplied delta -- with no neutral at all.

The Xo of the isolation transformer is to be bonded into the GEC System as a whole -- in most situations. ( Everyone I've installed, BTW.) I always ran a GEC conductor back to the Service. But the NEC will allow you to get away with bonding to everything in sight, locally.

( Building steel, UFER, water, gas, XFRM chassis, etc. )
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Old 02-25-2020, 03:31 PM   #4
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One last point to consider. If you are really worried about harmonics, install single phase 240/120 transformers, and you won't have to worry about harmonics.
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Old 02-25-2020, 04:08 PM   #5
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Quote:
Originally Posted by telsa View Post

The neutral's harmonics stop at the source of the current --
Huh?

They circulate inside the transformer windings. They don't stop at all. They can be mitigated by phase shifting transformers or filters, but they don't stop on a neutral.
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Old 02-25-2020, 06:04 PM   #6
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Wrong.

The magnetic circuit kills them... muffles them into oblivion.

If it didn't then the isolation XFMR would have no function.

It functions as a barrier in both directions -- up the line and towards the load.

That's why ferrite cores are used in every switching power supply.

The TYPICAL load run to an isolation transformer is that dedicated to ELECTRONICS -- and their switching power supplies which can crank out harmonics like nothing else.

Indeed, it's was a common spec back in the day for each isolated ground circuit to get a dedicated neutral along with its isolated ground.

With today's electronics, all of this is regarded as a total waste of money and effort.

I haven't seen such a specification since the aughts.

Of course, all switching power supplies -- like those used in LED drivers and much else -- still generate harmonics. It's just that they don't foul up the mind of CPU... the ones in the cash registers and elsewhere.
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