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Thread: Utility Diversification of Load

  1. #1
    Join Date
    May 2008
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    Utility Diversification of Load

    My local utility very actively diversifies connected load calculations. I understand that not everything will be turned on at the same time and that NEC requires to include safety margins.

    However, in my most recent project, my calculated connected load was right at 1800 amps with 208/120v secondary. This load was calculated carefully to make sure no duplication in heating/air conditioning loads were included. The utility is offering to install 3 50kVA overhead transformers instead of an appropriately sized pad-mount. They diversified the load down to 139kVA from almost 650kVA.

    How can I dispute their installation of 3 50kVA transformers for this?

    Secondly, my NEC requirements have 5 parallel sets of 600kcmil conductors for the service. The utility will be providing 2 parallel sets of 500kcmil from their OH bank. How do I connect the two? I decided on a tap box. Any other ideas?

    The service at this point will be OH transformer with riser down to CT/VT & meter instead of OH line with riser down to pad-mount transformer to CT/VT.

  2. #2
    Join Date
    Feb 2003
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    Massachusetts
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    48,433
    Quote Originally Posted by Montanaboy
    How can I dispute their installation of 3 50kVA transformers for this?
    Why would you want to?

    If it turns out to be undersized they will have to replace it.

    But it will not be undersized they have many years of data that they use to size a service and the NEC is not part of it.

    Where is the point of connection?

    We would have to run it up the pole and they would take it from there.

  3. #3
    Join Date
    May 2008
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    Ya, but...

    So as long as I comply with NEC on my side of the tap box, I'm covered. It just seems that a ~80% decrease in load based on their diversification seems extreme, regardless of the occupancy.
    Edward Fransen, PE, PMP

  4. #4
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    Feb 2003
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    Indianapolis
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    Thumbs up

    Quote Originally Posted by Montanaboy
    . . . It just seems that a ~80% decrease in load based on their diversification seems extreme . . .
    Sorry but it is rare that we actually miss. If we size the transformer wrong or put in too small of a service drop or lateral, we get to replace it with a larger one. We will be there for years to come and our guarantee will be honored. You are required to build the service and size it so it will carry the load in accordance with Article 220. Will you downsize the service and be able to say that you will always be there to replace it if it is too small? :smile:
    Charlie Eldridge, Indianapolis, Utility Power Guy
    Responses based on the 2011 NEC, unless stated otherwise.

  5. #5
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    Jan 2008
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    Montanaboy, did you diversify your loads in your calculations? Is your load totally unique as compared to other loads the utility might serve? The utility has a broad database of historic load shapes and characteristics from which they draw.

    Almost all of the customers over-estimate their loads. In our area, governmental installations are notorious for this. If you think you have a legitimate concern, talk with the utility engineer and he can give you his reasons and you can show him your detailed calculations that might refute that reasoning.

    We have offered to charge the customer an excess kVA charge if they really want to insist on a larger service but have yet to be taken up on it.

    We have had some larger installations buy spare transformers, pay for loop feeds, etc. that were beyond the scope of a regular service. Your POCO should be willing to install anything that you are willing to pay for that is beyond the norm. They just don't want to waste equipment and cost the rest of the customers money.

  6. #6
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    Mar 2003
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    New England
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    Quote Originally Posted by mivey
    We have had some larger installations buy spare transformers, pay for loop feeds, etc. that were beyond the scope of a regular service. Your POCO should be willing to install anything that you are willing to pay for that is beyond the norm. They just don't want to waste equipment and cost the rest of the customers money.
    I was reading through one of our local poco's green book and they will primary meter any transformer that is larger than the poco's calculation installed at the customer's request.

  7. #7
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    Jan 2008
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    Quote Originally Posted by peter d
    I was reading through one of our local poco's green book and they will primary meter any transformer that is larger than the poco's calculation installed at the customer's request.
    That may cover the losses but that still does not cover the waste. Plus you now have added cost of primary metering which may be 10 times the cost of secondary metering.

    If you have an extra $40,000 cost (say $7000 for primary metering and $33,000 for the pad and the over-sized UG service drop) just because the customer wants it, you might have a carrying cost of $600 per month (assuming a flat 18% carrying cost per year). I would say this could be added to the customer's monthly bill as an excess facilities charge.

  8. #8
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    Apr 2006
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    Upstate S.C.
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    I see this all the time. Fortunately I started out working for a utility and am aware of their excellent historical billing records. They rarely miss. And as said by others, it they do, they make if good and replace with a larger service.

    Yet, I still grouse about the excessively conservative demand factors in the NEC. There have been several projects I really had to sharpen my pencil to have a reasonable sized service, but I sure didn't cut things so short I would be libel for an undersized service. That would cost serious $$$!



    RC
    It's my name going on that drawing, not yours. If what you want ain't right, it ain't going on the drawings!

  9. #9
    Join Date
    May 2008
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    13

    Connecting NEC load to Utility Diversified Load Source

    New problem on theme: How do we join the conductors sized for the connected NEC load to the conductors provided by the utility that are sized for their diversified load?

    NEC diversified load: 1800 amp load on 2000 amp service provided by 5 parallel 600 kcmil conductors (2100 amps) at 120/208 (649kVA) on the load side.

    Utility service for diversified load: 2 parallel 500 kcmil conductors (760 amps) via OH 3-50kVA transformers (150kVA) for diversified load of 139kVA.

    Point of Delivery: load side of CT

    CT enclosure: maximum allowable 4 parallel 750kcmil conductors per phase (1900 amps), but with bus bar rated for 2000 amps

    I am assuming that the limiting factor here is the CT enclosure with its 4-750kcmil limit. Does that sound right?
    Edward Fransen, PE, PMP

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