## Size of Circuit Breaker & Fuse are as per NEC 450.3

**This MS Excel spreadsheet calculates the following:**

- Size of Circuit Breaker on Primary side of Transformer as per NEC 450.3
- Size of Fuse on Primary side of Transformer as per NEC 450.3
- Size of Circuit Breaker on Secondary side of Transformer as per NEC 450.3
- Size of Fuse on Secondary side of Transformer as per NEC 450.3
- Size of Transformer
- Transformer Full Load Losses.

### 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).

- Introduction
- Overcurrent protection of transformers >600V (NEC450.3A)
- Unsupervised location of transformer (impedance <6%)
- Unsupervised location of transformer (impedance 6% to 10%)
- Supervised location (in primary side only) of transformer
- Supervised location of transformer (impedance up to 6%)
- Supervised location of transformer (impedance 6% to 10%)
- Difference in C.B between supervised & unsupervised Location
- Sumary of overcurrent protection for more than 600V

- Overcurrent Protection of Transformers <600V (NEC 450.3B)

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 |

Umakant D Joshi

I am not able to download the excel sheet

Tony

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

Cosy

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..)

sakthibalan

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

Sahil

@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.

MANU

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

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.

Eric.Bernays

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

moon172011

this program make over design for breaker and this breaker out of standard range

thank you for trying

best regard

Eng. the engineer

Anmol978

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 …