Xfmr. Neutrals

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Mike01

Mike01

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:confused: If a K-4 rated xfmr. has a 100% neutral and a K-13 rated xfmr. has a 200% neutral what does a standard dry type xfmr. have a 50% neutral?
 
I wasn't aware that this is how such things are built. But my guess is that the higher the K-rating, the more harmonics are anticipated to be present in the secondary loads, and the larger the neutral wire is needed. This ic because the triplens harmonics (i.e., the 3rd, 9th, 15th, etc.) are in phase with each other, and therefore they add up in the neutral conductor.
 
K-1: This is the rating of any conventional transformer that has been designed to handle only the heating effects of eddy currents and other losses resulting from 60 Hertz, sine-wave current loading on the transformer. Such a unit may or may not be designed to handle the increased heating of harmonics in its load current.

K-4: A transformer with this rating has been designed to supply rated KVA, without overheating, to a load made-up of 100% of the normal 60 Hertz, sine-wave, fundamental current plus: 16% of the fundamental as 3rd harmonic current; 10% of the fundamental as 5th; 7% of the fundamental as 7th; 5.5% of the fundamental as 9th; and smaller percentages through the 25th harmonic. The "4" indicates its ability to accommodate four times the eddy current losses of a K-1 transformer.

K-9: A K-9 transformer can accommodate 163% of the harmonic loading of a K-4 rated transformer.

K-13: A K-13 transformer can accommodate 200% of the harmonic loading of a K-4 rated transformer.

K-20, K-30, K-40: The higher number of each of these K-factor ratings indicates ability to handle successively larger amounts of harmonic load content without overheating.
Click to expand...

So after all that what size neutral do you put on a standard dry type xfmr? just a full size 100% or do you put a half size?
 
For my part, I never use anything less than a 100% neutral. My reasoning is that despite any efforts on my part (or on the part of the installer) to balance the loads, we can't control what will be turned on or off at any given moment. Thus, at some times during any given day, it is possible that most of the load running at the moment might be on one or two phases, with the other phase(s) being lightly loaded. That would put a significant load on the neutral wire. So I prefer to make it a full size neutral, just as an added safety precaution.
 
100%

100%

It sound to me (correct me if I am wrong) that a standard dry type transformer you can install a 100% neutral (or whatever size lug you can fit on the termination stab) the only difference is that the K-4, K-13, etc. (internally) will be able to handle more neutral and or harmonics than the standard dry type transformer prior to overheating (meaning more copper) it could be as easy as a 112.5 just de-rated to a 75kva. Interesting, however I have never looked at the graph for a K vs. Non K rated xfmr. while under load (it would be interesting though)
 
The neutral on the transformers that I'm familiar with are either 100 or 200%.
K1 transformer have 100% neutrals and those that are K4,9, 13, etc have 200% neutrals.
What the installer does with his neutral conductor from that point on must comply with the NEC requirements.
 
My intent is not to answer this question but to address what is takes for the transformer manufacturer it provide a 200% neutral.
Think about it. A 200% neutral doesn't have anything to do with the (3) windings. The finish end of each winding are connected together in a wye, at a center point that is called the X0.
Then what is the distance from the point of that connection to where the actual X0 termination is located? I can't be vary far. It is that short conductor and the size of the termination the should be sized at 200%.
Regarding the NEc, it requires that the transformer be listed by a recognized testing agencty. I UL has listed it then is is accepted by the NEC when applied as directed by the manufacturer.
 
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