# 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) %Imp Primary secondary >600Volt >600Volt <600Volt C.B Fuse C.B Fuse C.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) %Imp Primary secondary >600Volt >600Volt <600Volt C.B Fuse C.B Fuse C.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) Protection Primary Protection Secondary Protection Method More than 9A 2A to 9A Less than 2A More than 9A Less than 9A Primary only protection 125% 167% 300% Not required Not required Primary and secondary protection 250% 250% 250% 125% 167%

 Size of Fuse / Inverse Time C.B as per NEC (Amp) 1 25 60 125 250 600 2000 3 30 70 150 300 700 2500 6 35 80 160 350 800 3000 10 40 90 175 400 1000 4000 15 45 100 200 450 1200 5000 20 50 110 225 500 1600 6000

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

Jignesh Parmar has completed M.Tech (Power System Control), B.E(Electrical). He is member of Institution of Engineers (MIE) and CEng,India. Membership No:M-1473586.He has more than 16 years experience in Transmission -Distribution-Electrical Energy theft detection-Electrical Maintenance-Electrical Projects (Planning-Designing-Technical Review-coordination -Execution). He is Presently associate with one of the leading business group as a Deputy Manager at Ahmedabad,India. He has published numbers of Technical Articles in “Electrical Mirror”, “Electrical India”, “Lighting India”,”Smart Energy”, “Industrial Electrix”(Australian Power Publications) Magazines. He is Freelancer Programmer of Advance Excel and design useful Excel base Electrical Programs as per IS, NEC, IEC,IEEE codes. He is Technical Blogger and Familiar with English, Hindi, Gujarati, French languages. He wants to Share his experience & Knowledge and help technical enthusiasts to find suitable solutions and updating themselves on various Engineering Topics.

### 14 Responses to Calculate Size of Circuit Breaker/ Fuse for Transformer (As per NEC)

1. DAKSHESH says:

Can we connect both body & neutral earthing in a common earthing grid ? pls.advice

2. Pankaj says:

What about the rating of Drop out fuse.

3. Dear Jignesh,
I have been referring to this web site for getting solutions / ideas for various electrical engineering related issues and found it very useful.
1. Use of ester based transformer oil in new transformers.
2. Replacement of mineral oil based trf oil in old trf with ester based oil, their reliability and improvement in useful life of old transformers.

Kindly post your views on web site or via mail and also some useful links / papers on this issue if you have with you.
Thanks & regards.

4. Naitik says:

5. Gaurav Kumar says:

Hi Jignesh Ji,
Pls upload the Abstract Of NEC Codes.
I would like to thank you for this such a wonderful Notes and articles that you have posted on your blog.Your explaination Methods of Calculations are quiet simlpe amd better understandable.

Thank you very much.

6. naveen killari says:

hello boss !
thanks for this article ,it really very useful to me. and i want learn about dc machines and its circuits .

7. santosh says:

very nice information

8. Davinder singh says:

In above calculation amp of Ip Sp is wrong but in last calculation it is correct.

9. M.M. Munas says:

Excellent dear

10. Shohel Rana says:

dear sir, Would u kindly provide, how to calculate HT/LT cable size for 100 KVA,200 KVA,250 KVA Trf……….2500 KVA Trf ?

11. kuldip singh says:

thanks a lot

12. Azib says:

In top calculation Ip & Is is wrong but after rating tables Ip and Is are correct…..Calculation on top may confuse anyone if he calculates.

13. Sergiy says:

Primary and secondary currents of transformer calculated incorrectly.
Primary is 52Amp and secondary is 1344 Amp.

14. Vaibhav Rawat says:

I think there is a Calculation mistake in
“Transformer Primary Current (Ip)=1000000/1.732×11000=49Amp”
The Ip will be 52.4 Amp
and
“Transformer Secondary Current (Is)=1000000/1.732×430=71Amp”
The Is will be 1342.7 Amp

This site uses Akismet to reduce spam. Learn how your comment data is processed.