Calculate Size of Circuit Breaker/ Fuse for Transformer (As per NEC)
April 1, 2019 18 Comments
- 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
Can we connect both body & neutral earthing in a common earthing grid ? pls.advice
What about the rating of Drop out fuse.
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.
I would like to know about your views / experience regarding:
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.
Thanks, it is very helpful……
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.
hello boss !
thanks for this article ,it really very useful to me. and i want learn about dc machines and its circuits .
very nice information
In above calculation amp of Ip Sp is wrong but in last calculation it is correct.
Excellent dear
dear sir, Would u kindly provide, how to calculate HT/LT cable size for 100 KVA,200 KVA,250 KVA Trf……….2500 KVA Trf ?
thanks a lot
Please correct typing error…
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.
Primary and secondary currents of transformer calculated incorrectly.
Primary is 52Amp and secondary is 1344 Amp.
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
the transformer power signed by the letter S [kVA]
P is for active Power [kW]
what’s the difference between supervised and unsupervised?
How many Amp breaker a 250 kva transformers can use?
Thank you so much Sir.