Waste of money and resources.
There is generally no proven need for K-rated transformers at the design stage. If they were regularly required they would be available ' over the counter' and not need to be special ordered.
Well, technically, all transformers are “K rated”. The K ratings go from 1 to 50 and are related to their ability to handle the added heating effects of non-linear loads, especially on the neutrals. A “standard” COTS distribution transformer is a K-1 rating, which is good for resistive and basic induction motor loads (those without drives of any sort) where less than 5% of the loads are non-linear.
But we no longer live in that world. Incandescent lighting is out, LED, HID and modern fluorescent lighting now ALL use switch mode power supplies of one sort or another, all of them “non-linear”. Add up that with the prevalence of VFDs and the need for having a K rating higher than 1 is more important than ever.
That said, does every transformer need to be K-13 or higher? No. It’s really something to look at when selecting. To your (Don) later point, if a transformer is only 50% loaded, a K-1 might be fine. But if the load is closer to the full rating, and is up to 50% non-linear loads, then you will want a K-4 at least.
That said, the de-facto standard is to jump immediately to a K-13, which is good for up to 100% non-linear loads, and yes, that is usually overkill. In my experience it tends to be done out of “apathy” to having to do the work to make a good decision. (I said apathy because “laziness” seemed too derogatory…). But years ago when I worked at Boeing Computer Services in the mid-80s, we were burning up transformers outside of older engineering buildings right and left. We all knew what “harmonics” were, but we didn’t know a lot about it. Turned out, after investigating, that the drafting buildings, which used to have rooms full of row upon row of drafting tables and guys with pencils in them, were now being filled with IBM CATIA CAD workstation computers (between a PC and a mini-computer at the time), all of them with big new switch mode power supplies. Turned lout that the loads in those buildings had gone from less than 1% non-linear, to over 75% in less than 5 years! So not only were the transformer neutrals melting, but so were the one-size-down neutral wires in the walls! We were seeing scorched marks on some walls where the conduits inside got hot enough to affect the paint! All of those fireball transformers got replaced with K-13 and the problems went away. It was quite the eye opener. But again, we KNEW what the non-linear loading was. Doing so (at that level) prophylactically is probably a waste of money.
