Calculation of Size of Transformer, Fuse and Circuit Breaker

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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

Overcurrent Protection of Transformer (NEC 450.3)

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

 Software: Calculation of Size of Transformer, Fuse and Circuit Breaker Version: 10.6.13 Developer: Jignesh Parmar Size: 90.3 KB Price: Free Download: Right here | Get Download Updates | Get Technical articles

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1. Tony
Oct 15, 2015

2. 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..)

3. sakthibalan
Oct 14, 2014

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

4. Sahil
Mar 21, 2014

@Anmol978
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.

5. 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

• Shaukat Ali Khan
Sep 05, 2014

For 1000 KVA, 3-phase, 50Hz, 400V system:
IL= 1440A
So, the next standard breaker size would be: 1600A
Since it is more than 800 or 1000A, it should be of ACB type.

6. Eric.Bernays
Sep 07, 2013

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

7. 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

8. 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 …