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Calculation of Size of Transformer, Fuse and Circuit Breaker
Calculation of Size of Transformer, Fuse and Circuit Breaker

Size of Circuit Breaker & Fuse are as per NEC 450.3

This MS Excel spreadsheet calculates the following:

  1. Size of Circuit Breaker on Primary side of Transformer as per NEC 450.3
  2. Size of Fuse on Primary side of Transformer as per NEC 450.3
  3. Size of Circuit Breaker on Secondary side of Transformer as per NEC 450.3
  4. Size of Fuse on Secondary side of Transformer as per NEC 450.3
  5. Size of Transformer
  6. Transformer Full Load Losses.

Overcurrent Protection of Transformer (NEC 450.3)

This technical article was written by author Jignesh Parmar explains step by step NEC 450.3 (Transformer protection).

Software:Calculation of Size of Transformer, Fuse and Circuit Breaker
Developer:Jignesh Parmar
Size:90.3 KB
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Calculation of Size of Transformer, Fuse and Circuit Breaker
Calculation of Size of Transformer, Fuse and Circuit Breaker

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  1. Shimelis Teklemariam
    Jun 11, 2022

    I want to know about transformer rating related to HRC fuse to be fixed.

  2. Muritala
    Aug 21, 2020

    Hello Jignesh Parmar,
    Will it be possible to access the hidden sheets from the excel please. Also, how do I get the password to remove the protection on the sheet.
    Thanks for your response in advance.

  3. LIEM T LE
    Apr 28, 2020

    I have a 150kVA transformer 480VAC 3 phase. Could you recommend what size of cables and fuse to install. Thank you.

  4. Pascualino Guercio
    Sep 08, 2019

    diversity factor

  5. Julius Echemita
    Sep 07, 2019

    Many Thanks .
    We remain grateful .

  6. aziz
    Dec 15, 2018

    Hello sir,

    hope to be fine all.

    I am using your tutorial files every they,

    thank you

  7. Umakant D Joshi
    Jun 11, 2018

    I am not able to download the excel sheet

  8. Tony
    Oct 15, 2015

    What is mean by DF load in this page? How can we calculate the DF load?

  9. Cosy
    Jun 02, 2015

    I don’t want to repeat allready bespoken aspects. Just a reminder about ISO – standardized naming and abreviations for physical values:

    Kva : it must be written as kVA
    Amp : it must be written as A or Ampère
    Volt: in formulas and sheets simply V , or kV …
    Kw : it must be written as kW

    Power Factor and working points of losses should be keeped at values conform to IEC (as it takes matter in the most countries and nearly everywhere 50Hz is..)

    • Pascualino Guercio
      Sep 08, 2019

      that’s right

  10. sakthibalan
    Oct 14, 2014

    how to calculate feeder short circuit current(KA)?kindly if u know mean inform me how to calculate this one

  11. Sahil
    Mar 21, 2014

    The ‘0.97’ is the stacking factor. The core is made up of Si steel/CRGO laminations due to which, some correction factor for the area is used.

  12. MANU
    Dec 02, 2013

    Dear Mr.Parmar,
    The spread sheet is ok. but the breaker selection is out of range as per the standards used. for example for 1000KVA breaker only 1600A breaker is sufficient. Please correct me if i am wrong. In the spreadsheet for 1000KVA 3000A is breaker suggest. Please check it

  13. Eric.Bernays
    Sep 07, 2013

    Is there any difference between this standard and those within the UK?

  14. moon172011
    Jun 16, 2013

    this program make over design for breaker and this breaker out of standard range
    thank you for trying
    best regard
    Eng. the engineer

  15. Anmol978
    Jun 16, 2013

    I was just checking some State electricity Board inspection sheets on transformers. (Not related with spreadsheet) that while calculating the flux density of a transformer (MAX value is 1.600 T) a basic formula is used B=E(phase)/(A*4.44*f*N*.97), What is that .97 factor multiplied???
    P.s the above work is fantastic …

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