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Calculating Fault Current

Discussion in 'Electrical Codes' started by jar546, Nov 9, 2018 at 10:41 PM.

  1. jar546

    jar546 *****istrator

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    I noticed that on good prints from a more respectable MEP firm, they normally provide the calculations for fault current on new construction but not as often on renovation jobs.

    NEC 110.9 and 110.10 are pretty clear but 110.24 is relatively new the past 3 code cycles and often overlooked for old work, aka, renovations and changes to existing electrical systems.

    How many of you require updated fault current calculations on the prints and labeling on the equipment under 110.24(A)&(B)?

    Just for fun, here is a label for a 3 phase transformer. If the primary will be 208 3ph and the secondary will be 400/231 3ph for some equipment manufactured in Europe, what is the fault current for this newly installed transformer? All of the info you need is on the data plate seen in the photo below:

    Please post your answers to the questions above and your fault current calculation. Let's see who takes a stab at this.

    IMG_1615.JPG
     
  2. chris kennedy

    chris kennedy Sawhorse

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    Infinite primary bus gives me 1,444A
    Lower than I would have guessed.
     
  3. jar546

    jar546 *****istrator

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    Hmmm, get 1299. Might as well round it off to 1,300A at the secondary based on infinite bus.
     
  4. jar546

    jar546 *****istrator

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    Did you use the secondary phase to phase of 400?
     
  5. chris kennedy

    chris kennedy Sawhorse

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    Yes, using Bussmans on-line calculator.

    Doing it myself I get this;

    45,000÷692=65A
    65÷.045= 1,444.444
     
  6. jar546

    jar546 *****istrator

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    45,000/ (400 x 1.732 x 5%)
    45,000 / 34.64
    Isca=1299
     
  7. ICE

    ICE Sawhorse

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    There’s a phone number on the label.
     
  8. jar546

    jar546 *****istrator

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    There is also a simple calculation to use:

    3ph
    Isca=VA / (E x 1.732 x Imp%)

    All of that is on the label for us to use.
     
  9. ICE

    ICE Sawhorse

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    Apparently there is more than one simple calculation to use and one wildly complicated pdf formula. You and Kennedy are 145 amps apart. So there’s a phone number on the label.
     
  10. chris kennedy

    chris kennedy Sawhorse

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    Your using the 5% impedance shown on the plate. ANSI allows the manufacturer +/- 10% tolerance for Z, therefore I used .045 as worst case scenario.
     
  11. jar546

    jar546 *****istrator

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    That's because he is using worst case.. We are close enough but still different numbers. I used two different methods to come up with the same number and then used an online calculator from a manufacturer and still came up with the same number. 145A for fault current is a moot point.

    #fearofmath?
     
  12. ICE

    ICE Sawhorse

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    I never ignore 110.9 and seldom does a contractor pay attention to 110.24.


    110.9 Interrupting Rating. Equipment intended to interrupt current at fault levels shall have an interrupting rating at nominal circuit voltage sufficient for the current that is available at the line terminals of the equipment.

    Equipment intended to interrupt current at other than fault levels shall have an interrupting rating at nominal circuit voltage sufficient for the current that must be interrupted.

    110.24 Available Fault Current.

    (A) Field Marking. Service equipment in other than dwelling units shall be legibly marked in the field with the maximum available fault current. The field marking(s) shall include the date the fault-current calculation was performed and be of sufficient durability to withstand the environment involved.


    Because I have inspected mostly residential for the past ten years I don't encounter many transformers. The fault levels I deal with are directly from Edison. The construction below has no Edison power because the TPP they installed is rated 10,000 amp and the available interrupting current is 42,000 amp. It's been four months getting this far with another six to go.

    Everybody from the contractors to Edison tell me that I am the only inspector that enforces 110.9 on a temporary power pole. Series rating will be an exercise for this bunch. Years ago we had a chart with conductor sizes and lengths.....now there's a simple calculation....or two and a pdf.

    [​IMG]

    Obviously a car wash under construction. The case prior to that was a restaurant. I talked that contractor into installing 42K equipment for the TPP and then he played dumb when it came to 110.24

    [​IMG]

    While I see the utility of the formulas, we have electrical plan check engineers for that stuff. Most everybody else should use the phone number.
     
    #13 ICE, Nov 11, 2018 at 9:11 AM
    Last edited: Nov 11, 2018 at 9:30 AM
    jar546 likes this.
  13. ICE

    ICE Sawhorse

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    According to the PDF ANSI allows 7.5% and UL is at 10%.
     
    chris kennedy likes this.
  14. chris kennedy

    chris kennedy Sawhorse

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    Thank you, I stand corrected.
     
  15. ICE

    ICE Sawhorse

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    I would call that dumb luck on my part.
     

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