Tuesday, January 29, 2019

Calculate Size of Transformer, Fuse, Circuit Breaker Excel Spreadsheet


Calculate Size of Circuit Breaker/ Fuse for Transformer (As per NEC)

  • Calculate Size of Circuit Breaker or Fuse on Primary and Secondary side of Transformer having following Detail
  • Transformer Details(P)= 1000KVA
  • Primary Voltage (Vp)= 11000 Volt
  • Secondary Voltage (Vs)= 430 Volt
  • Transformer Impedance= 5%
  • Transformer Connection = Delta / Star
  • Transformer is in unsupervised condition.

Calculations:

  • Transformer Primary Current (Ip)= P/1.732xVp
  • Transformer Primary Current (Ip)=1000000/1.732×11000=49Amp
  • Transformer Secondary Current (Is)= P/1.732xVs
  • Transformer Secondary Current (Is)=1000000/1.732×430=71Amp
  • AS per NEC 450.3, Max.Rating of C.B or Fuse is following % of its Current according to it’s Primary Voltage,% Impedance and Supervised/Unsupervised Condition.
Max Rating of Over current Protection for Unsupervised Transformer More than 600 Volts (As per NEC)
%ImpPrimarysecondary
>600Volt>600Volt<600Volt
C.BFuseC.BFuseC.B/Fuse
 Up to 6%600%300%300%250%125%
More than 6%400%300%250%225%125%

Max Rating of Over current Protection for Supervised Transformer More than 600 Volts (As per NEC)
%ImpPrimarysecondary
>600Volt>600Volt<600Volt
C.BFuseC.BFuseC.B/Fuse
 Up to 6%600%300%300%250%250%
More than 6%400%300%250%225%250%

Max Rating of Over current Protection for Transformers Primary Voltage Less than 600 Volts (As per NEC)
ProtectionPrimary ProtectionSecondary Protection
MethodMore than 9A2A to 9ALess than 2AMore than 9ALess than 9A
Primary only protection125%167%300%Not requiredNot required
Primary and secondary protection250%250%250%125%167%

Size of Fuse / Inverse Time C.B as per NEC (Amp)
125601252506002000
330701503007002500
635801603508003000
10409017540010004000
154510020045012005000
205011022550016006000
For Primary Side:
  • Transformer Primary Current (Ip) =52.49Amp and impedance is 5%
  • As per above table in not supervised condition Size of Circuit Breaker= 600% of Primary Current
  • Size of Circuit Breaker = 52.49 x 600% =315Amp
  • If Transformer is in supervised condition then Select Circuit Breaker near that size but if Transformer is in unsupervised condition then Select Circuit Breaker next higher size.
  • Rating of Circuit Breaker =350Amp (Next Higher Size of 300Amp)
  • Size of Fuse = 52.49 x300% =157Amp
  • Rating of Fuse =160Amp (Next Higher Size of 150Amp)
For Secondary Side:
  • Transformer Secondary Current (Is) =1342.70Amp and impedance is 5%
  • As per above table in not supervised condition Size of Circuit Breaker= 125% of Secondary Current
  • Size of Circuit Breaker = 1342.70 x 125% =1678Amp
  • If Transformer is in supervised condition then Select Circuit Breaker near that size but if Transformer is in unsupervised condition then Select Circuit Breaker next higher size.
  • Rating of Circuit Breaker =2000Amp (Next Higher Size of 1600Amp)
  • Size of Fuse = 1342.70 x125% =1678Amp
  • Rating of Fuse =2000Amp (Next Higher Size of 1600Amp)

 Results:


  • Size of Circuit Breaker on Primary Side=350Amp
  • Size of Fuse on Primary Side=160Amp
  • Size of Circuit Breaker on Secondary Side=2000Amp
  • Size of Fuse on Secondary Side=2000Amp

File Size: 91 Kb


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.



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