Oversized Neutral for K rated transformer?

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beegee

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My foreman went on vacation for a week and left me in charge. I'm afraid I might be in over my head. Before he left he ordered material for hanging and wiring a transformer that'll feed a 100A panel about 100 feet away. He just left today and I already have a journeyman telling me that I need an oversized neutral because its a K rated transformer. Is that true? Should I go over my boss' head and order more wire for the neutral?
 
A K rated transformer implies that harmonic loads are expected on the secondary.
Harmonic loads with triplen content may result in neutral currents in excess of the phase conductor. So an oversized neutral may be in order.
 
Plans? No plans here. This isn't new construction, its more like a maintenence gig so we rarely get plans, we usually have to figure things out on our own. I usually defer to my foremans 30+ years of experience.
 
Does the primary circuit need a neutral, or is it a Delta-primary transformer? The question implies a Y, but . . .
 
Oversized neutrals are not a code requirement. It is a design issue.
K-rated transformers are used when non-linear loads are expected. A lot of non-linear loads. A lot of PC's, copiers, laser printers, fax machines are common items that produce harmonics.
The mere presence of a K-rated transformer does not justify a design change to use oversize neutrals. He could have just gotten a good deal on it.
You need to take a look at the loads.

BTW, 100 feet away? to panelboard. You need a manin overcurrent device within 10' to 25' of conductor length. 240.21(C)
 
I wasn't planning on running a neutral to feed the X-former. Its getting fed from a 3-phase Bus Plug with no neutral.


Also, the feeders run through a Disconnect before leaving the electric room. The panel feeds a lab space. The load will have no flourescent lighting but will mostly consist of alot of sensitive 120/208v lab equipment, but being that the oversized neutral is not a code issue makes me more hesitant to go over my foremans head to order larger wire for the neutral.
 
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Unless someone can explain to me what a K rated transformer is and why it would make sense to pull an oversized neutral. To me, it just sounds like theory, and seems a little excessive......but I'm no expert in K rated transformers or harmonic loads.
 
beegee said:
Unless someone can explain to me what a K rated transformer is and why it would make sense to pull an oversized neutral. To me, it just sounds like theory, and seems a little excessive......but I'm no expert in K rated transformers or harmonic loads.
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K rated transformers are designed to handle larger than normal neutral currents created by harmonic currents which are additive within the neutral. Typically when using K rated transformers the neutral is sized at 200% of the phase conductors. So for a 100 amp panel the 200% neutral would typically be a #3/0. The neutral bar is sized at 200% also.
 
So what is different about a K rated transformer that makes it able to handle a larger load on the neutral. If that's all it is, couldn't I just add a larger wire and neutral lug to a normal transformer and call it K rated?
 
beegee said:
So what is different about a K rated transformer that makes it able to handle a larger load on the neutral. If that's all it is, couldn't I just add a larger wire and neutral lug to a normal transformer and call it K rated?
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Yes. And if you have it tested and approved for it, you could even sell it that way.
 
To me, it just sounds like theory, and seems a little excessive......but I'm no expert in K rated transformers or harmonic loads.
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It is just theory. There are very very few documented cases of neutral problems caused by nonlinear loads and almost all of the information that you can find on this subject was published by someone with an economic interest in solving this theoretical problem.
Don
 
I understand the K rated transformers to be beefier, in terms of the copper in the coils. The have bigger beefier windings, than your general purpose types.

Some of the 75KVA K-rated I've put in were bigger in size than a general purpose 112.5KVA.
 
Dnkldorf said:
Some of the 75KVA K-rated I've put in were bigger in size than a general purpose 112.5KVA.
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An engineer friend told me the K-rated transformer is typically equivalent to twice the capacity of a standard one.
 
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