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Thread: Utility specs large transformers for motors

  1. #1
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    Utility specs large transformers for motors

    Have a job where we brought in a 200 amp 480 service for two motors, a 20 hp and a 30 hp. I recently got the design from the utility and they are using 3x100 KVA transformers. Is this typical? I Was expecting them to install 3 15's or 3 25's. It seems like they sized the bank to the LRC of the larger motor. Just seems a little over the top to me. Opinions?
    Ethan Brush - East West Electric. NY, WA. MA

    "You can't generalize"

  2. #2
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    sqrt 3 x 200 A x 480 ~ 170 kva
    I would expect 3 x 50 kva
    15 or 25 too small imo, they must have sized on service size, not load
    probably used what they had
    perhaps for 480 the smallest they stock is 100's
    perhaps prim v determined what they stock
    who knows
    The difference between genius and stupidity is that genius has its limits.

  3. #3
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    I guess they want a stiff supply to keep flicker and VD within target during motor starting. This might mess up my fault current calcs. IT was a catch 22 when I set this up: the utility asked me a few questions and then said just said build it and get it inspected, and after I did that they sent it to engineering I assumed a full 200 amps for transformer sizing as absolute worst case. Its going to be close. Hopefully the transformer impedance is less than what the book says, probably is.
    Ethan Brush - East West Electric. NY, WA. MA

    "You can't generalize"

  4. #4
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    At 200 amp the service drop or lateral will probably negate the issue but you might want to check your available fault current.
    At my age, I'm accustomed to restaurants asking me to pay in advance, but now my bank has started sending me their calendar one month at a time.

  5. #5
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    Quote Originally Posted by augie47 View Post
    At 200 amp the service drop or lateral will probably negate the issue but you might want to check your available fault current.
    yeah that is what I am hoping. I remember that I was right there at max with 3 75's and not counting the service drop and SEC impedance.
    Ethan Brush - East West Electric. NY, WA. MA

    "You can't generalize"

  6. #6
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    there is no telling why pocos do what they do. I suspect in your case maybe that is just the smallest 480V xfmrs they stock. When you buy 1000s of them at a time it may well be more cost effective to just get a unit that can handle most of your installations rather than stocking a bunch of different sizes.
    Bob

  7. #7
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    Quote Originally Posted by electrofelon View Post
    Have a job where we brought in a 200 amp 480 service for two motors, a 20 hp and a 30 hp. I recently got the design from the utility and they are using 3x100 KVA transformers. Is this typical? I Was expecting them to install 3 15's or 3 25's. It seems like they sized the bank to the LRC of the larger motor. Just seems a little over the top to me. Opinions?
    3-100kVA units? total 300 kVA bank?

    Around here we typically only see 3-50 kVA maximum for service to 100 HP irrigation wells, and those can be loaded 24/7 at times in mid summer. Anything larger then that usually is more then they want to hang on poles and they will provide padmount transformer.
    I live for today, I'm just a day behind.

  8. #8
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    Wouldn't you want a higher transformer impedance than what is listed for a lower fault current?
    Electricians do it until it Hertz!

  9. #9
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    Quote Originally Posted by petersonra View Post
    there is no telling why pocos do what they do. I suspect in your case maybe that is just the smallest 480V xfmrs they stock. When you buy 1000s of them at a time it may well be more cost effective to just get a unit that can handle most of your installations rather than stocking a bunch of different sizes.
    Unless they just never run into 480 volt services that are low capacity I find it hard to believe they would only stock 100kVA units as their smallest unit. 100 kVA is enough for ~100 hp of load at .85+ power factor.

    We see 10 and 15 kVA (not just 240 volt but 480 volt also) pots all over the place on limited load services around here.
    I live for today, I'm just a day behind.

  10. #10
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    Quote Originally Posted by kwired View Post
    Unless they just never run into 480 volt services that are low capacity I find it hard to believe they would only stock 100kVA units as their smallest unit. 100 kVA is enough for ~100 hp of load at .85+ power factor.

    We see 10 and 15 kVA (not just 240 volt but 480 volt also) pots all over the place on limited load services around here.
    I don't know how it is now but for decades around here the cost of a 240 V delta service was dirt cheap but a 480 V delta service was insane. The difference for the same kVA worth of service was so much that it was common to see 240-480 V transformer banks fed from the service. Weirdly, ComEd sold and serviced a lot of those 240 V transformers. I have absolutely no idea why that was cost effective. My understanding is the electricity costs were even lower for 240 services as opposed to 480.

    These days if you want three phase it is either 208/120 or 480/277, unless you can convince ComEd you have to have a 480 V delta, and I have been told it is not easy to do. I have been told they still allow some 240 V delta services with one side center grounded, but they limit them to pretty small services.

    My brother told me today that the new smart meters will actually pay you to use electricity during certain times of the day. Literally. It gets billed to him at a negative rate. What the heck is smart about that.
    Bob

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