Page 1 of 2 12 LastLast
Results 1 to 10 of 14

Thread: Utility Transformer Exceeds FLA Clarification

  1. #1
    Join Date
    Mar 2017
    Location
    Florida
    Posts
    4

    Utility Transformer Exceeds FLA Clarification

    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

  2. #2
    Join Date
    Feb 2003
    Location
    Massachusetts
    Posts
    60,020
    It is very straight forward, the NEC load calculations do not apply to the power company.

    The power company knows what they are doing and will sometimes run their transformers beyond FLA.

  3. #3
    Join Date
    Jan 2016
    Location
    PA
    Posts
    3,494
    Utility sets its own ratings basically
    xfmrs like motors have a service factor, ie, 1.15 can run at 115% fla indefinitely

    on a more technical note
    do all your motors run at the same time?
    or for example the 50's: 2 will alternate (both will run in hi flow conditions) the third is a back-up
    Last edited by Ingenieur; 03-16-17 at 02:27 PM.



  4. #4
    Join Date
    Feb 2003
    Location
    Wisconsin
    Posts
    9,771
    NEC article 450 is about Amps not kVA. There is no NEC requirement or information, that assists you in determining either the primary or secondary FLA of a transformer.

    Things like forced air cooling and acceptable loading profiles (e.g. motor starting) allow the manufacturer to be flexible with terms like Nameplate kVA, Continuous kVA, Emergency kVA. But 450.3 seems to be primarily concerned with transformer conductors protection, so just stay with FLA and transformers might be easier.
    Just because you can, doesn't mean you should.

  5. #5
    Join Date
    Mar 2017
    Location
    Florida
    Posts
    4
    I have asked this question about the motors all running at the same time and the answer is yes. I am assuming not all pumps are running at 100%.
    I don't believe there are any issues because the pump station is up and running. I just don't understand how is this possible.

    Maybe a Utility Engineer/Lineman can help out.

  6. #6
    Join Date
    Jul 2006
    Location
    San Francisco, CA, USA
    Posts
    6,670
    And to reiterate, the NEC does not apply to Utilities. Don't do things by their example.
    __________________________________________________ ____________________________
    Many people are shocked when they discover I am not a good electrician...

  7. #7
    Join Date
    Feb 2003
    Location
    Massachusetts
    Posts
    60,020
    Quote Originally Posted by Jraef View Post
    And to reiterate, the NEC does not apply to Utilities. Don't do things by their example.
    If you do the inspector will have a good laugh as they fail you.

  8. #8
    Join Date
    Feb 2003
    Location
    Massachusetts
    Posts
    60,020
    Quote Originally Posted by RRoc View Post
    I just don't understand how is this possible.
    What specifically are you not understanding?

    A transformer is capable of supplying more current than the FLA, it is just a mater of how much over the FLA, how long you do it and what you do to keep it cool.

    With an oil-filled power company distribution transformer there is a lot of thermal mass so you can run it above the FLA for many hours without actually overheating the unit.

  9. #9
    Join Date
    Mar 2003
    Location
    New England
    Posts
    13,025
    I use this example a lot (kind of like someone who wired a lot of condos in the 1980's) but there is a street near me where there's 11 houses on a single 50 kva transformer. The houses are fairly small, the street has a gas main, and central A/C is uncommon around here, but that's a lot even for this area. Still, haven't seen any sign of a problem with it.

  10. #10
    Join Date
    Mar 2003
    Location
    New England
    Posts
    13,025
    Quote Originally Posted by iwire View Post
    What specifically are you not understanding?

    A transformer is capable of supplying more current than the FLA, it is just a mater of how much over the FLA, how long you do it and what you do to keep it cool.

    With an oil-filled power company distribution transformer there is a lot of thermal mass so you can run it above the FLA for many hours without actually overheating the unit.
    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.

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •