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Thread: Utility Transformer Exceeds FLA Clarification

  1. #11
    Join Date
    Jul 2006
    Location
    San Francisco, CA, USA
    Posts
    6,434
    This whole subject reminds me of an encounter I once had with PG&E on a pump station service. We were starting up a project with large pumps and even with soft starters, the voltage drop was so bad that everything shut down, including the HID lighting. After hooking up a PQ meter, I confirmed that the voltage was collapsing. Walked around to the outside to look at the pad mount utility xfmr, and it was WAY too small to be correct for a pump station with 400HP of total potential load. Called PG&E, they said it was a 750kVA transformer, just as specified. The pad mount was maybe 50" cube, nowhere NEAR what I would expect of a 750kVA. Nonetheless, they insisted it was right. Went back in and turned everything down as low as possible on the soft starters, this time the transformer fuse blew. PG&E technician shows up 2 hours later, opens the box and it says, right on the inner panel, 75kVA! I point that out to him, but he keeps insisting that their records show that it was 750kVA. He replaces the MV fuses, we try again, fuse clears immediately right in front of him. Finally he calls their Engineering dept., they start over with the ridiculousness of how "You can't go by what you see, we have different rules for transformers." I told them I understood that, but this was NOT that kind of situation. 4 hours after we began this issue, 4 PG&E engineers showed up to look at it and eventually came to the conclusion that yes, someone had "dropped a zero" on the order.
    __________________________________________________ ____________________________
    Many people are shocked when they discover I am not a good electrician...

  2. #12
    Join Date
    Feb 2003
    Location
    Massachusetts
    Posts
    60,020
    Quote Originally Posted by peter d View Post
    And even the overloading scenario is not very likely because the poco knows the true load will be far less that what the NEC calls for.
    For sure often the case but add up just his motors.

  3. #13
    Join Date
    Mar 2003
    Location
    New England
    Posts
    13,009
    Quote Originally Posted by iwire View Post
    For sure often the case but add up just his motors.
    I did, it's very interesting to see how the poco goes about sizing.

  4. #14
    Join Date
    Nov 2007
    Posts
    744
    Quote Originally Posted by RRoc View Post
    My Pump Station Load is listed below and the Main Utility Transformer Supplying this load is 150KVA. From adding up all the current draw. The Pump Station Load is greater than the Secondary Transformer FLA. The Utility Engineer informed although the FLA has been exceeded the Operational Limits of the transformer has not. Can someone explain why this is ok? Also what NEC standard section to reference as well?

    Pump Station Load
    50HP, Current Draw = 65
    50HP, Current Draw = 65
    50HP, Current Draw = 65
    15HP, Current Draw = 21
    15HP, Current Draw = 21
    25HP, Current Draw = 34
    2HP, Current Draw = 3.4
    Total = 274.4

    Utility Transformer
    Pri Volt: 16.34 KV, Secondary Volt: 480VAC
    Rating: 150KVA, Secondary FLA: 180 FLA
    I noticed your nameplate amps refer to the FLA of a 3-phase transformer, 150kVA, 480V secondary!
    Your load amps when computed as single-phase loads will give you 131.712 kVA (very near the 150kVA rating); but should be 228.13kVA if computed as 3-phase!

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