What are the accepted rules when applying demand factors when sizing liquid filled transformers? The (hypothetical) building in question has about an 863 kVA connected load (mostly actual connected load (not estimated), although receptacles are estimated per NEC). The load is primarily motors (air handlers & pumps, all on VFDs), broken down as follows:
28 kVA misc. (actual)
57 kVA lighting (actual)
292 kVA receps (per NEC)
486 kVA motors (actual)
I know a 750kVA transformer would be more than adequate. I believe a 500kVA transformer could be used without a problem though. From experience with similar buildings at this site, I know the building will never actually require 700kVA for an extended period, if ever. In reality, the typical load will probably be near half that. The "old hat" engineers I know tell me "just use a 500, it'll be fine, and if it does heat up a little, it'll cool down overnight".
Question #1:
That doesn't seem like a good explanation if the thing did catch on fire. So, my question is, how do I apply demand factors to the connected load to justify sizing the transformer at 500kVA? What are the established guidelines on this?
Question #2:
What is the overload limit for liquid filled transformers? I've seen them loaded to more than 200% for a few hours at a time on a daily basis for an extended period (years), with no apparent ill effects. I'm sure this degrades the insulation and cooling fluid, and I don't think you should PLAN for a 200% overload, but this particular one seemed to be fine. What IS an acceptable maximum overload percentage for liquid filled transformers? 25%? 50%? How does the duration of the overload affect that amount?
28 kVA misc. (actual)
57 kVA lighting (actual)
292 kVA receps (per NEC)
486 kVA motors (actual)
I know a 750kVA transformer would be more than adequate. I believe a 500kVA transformer could be used without a problem though. From experience with similar buildings at this site, I know the building will never actually require 700kVA for an extended period, if ever. In reality, the typical load will probably be near half that. The "old hat" engineers I know tell me "just use a 500, it'll be fine, and if it does heat up a little, it'll cool down overnight".
Question #1:
That doesn't seem like a good explanation if the thing did catch on fire. So, my question is, how do I apply demand factors to the connected load to justify sizing the transformer at 500kVA? What are the established guidelines on this?
Question #2:
What is the overload limit for liquid filled transformers? I've seen them loaded to more than 200% for a few hours at a time on a daily basis for an extended period (years), with no apparent ill effects. I'm sure this degrades the insulation and cooling fluid, and I don't think you should PLAN for a 200% overload, but this particular one seemed to be fine. What IS an acceptable maximum overload percentage for liquid filled transformers? 25%? 50%? How does the duration of the overload affect that amount?
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