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Utility Conductor Sizing

  1. Jun 18, 2015 #1
    Hello, this is my first post and I've been out of school for a few years and now working a job in my field at a distribution utility, but I'm just really rusty or green on a lot of things now that I'm being challenged. Seems like the simplest questions elude me!

    Anywho, I've been tasked with finding the primary conductor size for a new commercial facility. We'll have a 3-phase 1000kVA, 12470/480 V pad mount transformer. The conductor will be bored to the transformer. Never done the calculation before.

    My initial thoughts are: 1000 kVA = 46.30 Amps
    (√3)(12.47 kV)

    Do I take 125% of the amps to account for voltage drop as I have read somewhere? Which would lead me to approx 58A? However, both of those numbers seem low for such a large transformer.

    Moving on with my theory, would I then account 58A as the maximum conductor ampacity? Would I divide that by 3 to account for the ampacity of the 3 individual primary conductors (or by 4 to include the neutral wire)?

    Or is the 58A the required ampacity for each conductor?

    As I said earlier, the 58A looks like very small conductor ampacity for such a large transformer. I'm thinking I should get an answer around a 4/0 conductor or something. Please guide my thought process on this.

    Thank you!
  2. jcsd
  3. Jun 18, 2015 #2


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    What country is this in? What regulatory standards cover those conductor sizings? :smile:
  4. Jun 18, 2015 #3
    Yeah I know my numbers are ridiculous! But I'm just hoping to not make the same mistakes.
  5. Jun 18, 2015 #4


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    Most utilities wont use anything smaller than #2 wire on their primary side. You should probably contact your local utility, their linemen or reference the local utilities standards for their xfmr installations. #4 Cu is sometimes use for campus distribution, but the local power companies in the Northeast USA generally use wire connection tools that do not work for smaller than #2 wire, hence their minimum size.
    Getting back to your calculation. Utility transformers are not installed under NEC rules. I am not sure what your utility feels comfortable for protection, but most utilities have no issues at running their transformers at 125% (so you should upsize your conductor to that value at the very least). Voltage drop is NOT a concern as the primary (and secondary) have voltage correction taps. I would even suspect your oversizing the primary wire by 200% would be considered fine (never know when you might expand or extend service).
    Last edited: Jun 18, 2015
  6. Jun 18, 2015 #5
    Thanks @CalcNerd but I may not have been clear, I work for the utility. I'm new here.
  7. Jun 18, 2015 #6


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    I edited my post.
    I also suggest you speak to your linemen. They will probably tell you that they have a minimum sized, medium voltage cable that they can install and nothing smaller. Just because some yahoo on this site (who may even be quoting NEC) says something, your utility has standards and procedures, some of which are not covered or controlled by the NEC.
    Utility companies often run their transformers at 140% (or more). That practice isn't kosher for the NEC. However power companies use UTILITY GRADE transformers which are a bit better than the dry type used by the commercial world.
  8. Jun 18, 2015 #7


    Staff: Mentor

    What about short circuit conditions? Primary, secondary, close in, far out, unbalanced, ...
  9. Jun 24, 2015 #8
    I worked once for an utility company in electric design department.

    If the transformer it was one for public service I had to use an one year statistic compared with the City Hall developing office intention for the service area and calculating the load according to a good statistics -and NEC also-taking the maximum and a 5-10% safety factor .

    Since I worked in design of developed area always, I have to consult the City architect.

    However, I could not gather never enough information. In this case I did my self statistics of mine studying other existing areas.

    If the transformer was destined for a single consumer I get an official inquire for power supply from the beneficiary .
  10. Jun 24, 2015 #9

    jim hardy

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    What's the let-through current of upstream fuse?
  11. Jun 24, 2015 #10
    We have-usually-three criteria in order to choose the right cross-section area of conductors :
    1)steady state load- continuous or average variable load.
    2) voltage drop
    3) short-circuit withstand current.
    In medium voltage systems voltage drop it is not a required criterion- usually- but short-circuit
    current always it will be.
    It is good to check the clearance time for the fuse or breaker at the close end of supply, but I think a second disconnected mean clearance time-reserve time-has to be taken into consideration.
    Let's say the close disconnecting time it is one cycle-1/60 or 1/50 - 0.02 seconds, but a good one
    has to be 0.2 sec-at least.
    A short-circuit power at 13 kV system could be 500 MVA.
    I"k=500/sqrt(3)/13=22 kA.
    In this case a minimum 3/0 awg [for a copper conductor cable XLPE insulated 90oC initial temperature 0.2 sec. clearance time ] will be required.
    If the cable runs underground in duct, 3 single-core cables in single duct [20oC 90 RHO Earth ] can withstand 250-260 A 100%.[from my experience the actual load of a public service transformer is not more than 70%] and delivered power could be 5520 kVA.:biggrin:
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