Overcurrent Protection of Transformer (NEC 450.3)

Overcurrent Protection of Transformer (NEC 450.3) - Photo by Efrem Oshinsky @ Flickr

Content


Introduction

The overcurrent protection required for transformers is consider for Protection of Transformer only. Such overcurrent protection will not necessarily protect the primary or secondary conductors or equipment connected on the secondary side of the transformer.

When voltage is switched on to energize a transformer, the transformer core normally saturates.

This results in a large inrush current which is greatest during the first half cycle (approximately 0.01 second) and becomes progressively less severe over the next several cycles (approximately 1 second) until the transformer reaches its normal magnetizing current. To accommodate this inrush current, fuses are often selected which have time-current withstand values of at least 12 times transformer primary rated current for 0.1 second and 25 times for 0.01 second. Some small dry-type transformers may have substantially greater inrush currents.

To avoid using over sized conductors, overcurrent devices should be selected at about 110 to 125 percent of the transformer full-load current rating. And when using such smaller overcurrent protection, devices should be of the time-delay type (on the primary side) to compensate for inrush currents which reach 8 to 10 times the full-load primary current of the transformer for about 0.1 s when energized initially.

Protection of secondary conductors has to be provided completely separately from any primary-side protection.

A supervised location is a location where conditions of maintenance and supervision ensure that only qualified persons will monitor and service the transformer installation. Overcurrent protection for a transformer on the primary side is typically a circuit breaker. In some instances where there is not a high voltage panel, there is a fused disconnect instead.

It is important to note that the overcurrent device on the primary side must be sized based on the transformer KVA rating and not sized based on the secondary load to the transformer.

Go to Content ↑


Overcurrent Protection of Transformers >600V (NEC450.3A)

1) Unsupervised Location of Transformer (Impedance <6%)

Unsupervised Location of Transformer (Impedance <6%)

Unsupervised Location of Transformer (Impedance <6%)


  • OverCurrent Protection at Primary Side (Primary Voltage >600V):
  • Rating of Pri. Fuse at Point A= 300% of Pri. Full Load Current or Next higher Standard size. or
  • Rating of Pri. Circuit Breaker at Point A= 600% of Pri. Full Load Current or Next higher Standard size.
  • OverCurrent Protection at Secondary Side (Secondary Voltage <=600V):
  • Rating of Sec. Fuse / Circuit Breaker at Point B= 125% of Sec. Full Load Current or Next higher Standard size.
  • OverCurrent Protection at Secondary Side (Secondary Voltage >600V):
  • Rating of Sec. Fuse at Point B= 250% of Sec. Full Load Current or Next higher Standard size. or
  • Rating of Sec. Circuit Breaker at Point B= 300% of Sec. Full Load Current.
Example: 750KVA, 11KV/415V 3Phase Transformer having Impedance of Transformer 5%

  • Full Load Current At Primary side = 750000/(1.732X11000) = 39A
  • Rating of Primary Fuse = 3X39A = 118A, So Standard Size of Fuse = 125A.
  • OR Rating of Primary Circuit Breaker = 6X39A = 236A, So standard size of CB = 250A.
  • Full Load Current at Secondary side = 750000/(1.732X415)  = 1043A.
  • Rating of Secondary of Fuse / Circuit Breaker = 1.25X1043A = 1304A, so standard size of Fuse = 1600A.

Go to Content ↑

2) Unsupervised Location of Transformer (Impedance 6% to 10%)

Unsupervised Location of Transformer (Impedance 6% to 10%)

Unsupervised Location of Transformer (Impedance 6% to 10%)


  • OverCurrent Protection at Primary Side (Primary Voltage >600V):
  • Rating of Pri. Fuse at Point A= 300% of Primary Full Load Current or Next higher Standard size.
  • Rating of Pri. Circuit Breaker at Point A= 400% of Primary Full Load Current or Next higher Standard size.
  • OverCurrent Protection at Secondary Side (Secondary Voltage <=600V):
  • Rating of Sec. Fuse / Circuit Breaker at Point B= 125% of Sec. Full Load Current or Next higher Standard size.
  • OverCurrent Protection at Secondary Side (Secondary Voltage >600V):
  • Rating of Sec. Fuse at Point B= 225% of Sec. Full Load Current or Next higher Standard size.
  • Rating of Sec. Circuit Breaker at Point B= 250% of Sec. Full Load Current or Next higher Standard size.
Example: 10MVA, 66KV/11KV 3Phase Transformer, Impedance of Transformer is 8%

  • Full Load Current At Primary side = 10000000/(1.732X66000) = 87A
  • Rating of Pri.  Fuse = 3X87A = 262A, so next standard size of Fuse = 300A.
  • OR Rating of Pri. Circuit Breaker = 6X87A = 525A, so next standard size of CB = 600A.
  • Full Load Current at Secondary side = 10000000/(1.732X11000) = 525A.
  • Rating of Sec. Fuse = 2.25X525A = 1181A, so next standard size of fuse = 1200A.
  • OR Rating of Sec. Circuit Breaker = 2.5X525A = 1312A, so next standard size of circuit breaker = 1600A.

Go to Content ↑

3) Supervised Location (in Primary side only) of Transformer

Supervised Location (in Primary side only) of Transformer

Supervised Location (in Primary side only) of Transformer


  • OverCurrent Protection at Primary Side (Primary Voltage >600V):
  • Rating of Pri. Fuse at Point A= 250% of Primary Full Load Current or Next higher Standard size.
  • Rating of Pri. Circuit Breaker at Point A= 300% of Primary Full Load Current or Next higher Standard size.

Go to Content ↑


4) Supervised Location of Transformer (Impedance Up to 6%)

Supervised Location of Transformer (Impedance Up to 6%)

Supervised Location of Transformer (Impedance Up to 6%)


  • OverCurrent Protection at Primary Side (Primary Voltage >600V):
  • Rating of Pri. Fuse at Point A= 300% of Pri. full load current or next lower standard size.
  • Rating of Pri. Circuit Breaker at Point A= 600% of Pri. full load current or next lower standard size.
  • OverCurrent Protection at Secondary Side (Secondary Voltage <=600V):
  • Rating of Sec. Fuse / Circuit Breaker at Point B= 250% of Sec. Full Load Current or Next higher Standard size.
  • OverCurrent Protection at Secondary Side (Secondary Voltage >600V):
  • Rating of Sec. Fuse at Point B= 250% of Sec. Full Load Current or Next Lower Standard size.
  • Rating of Sec. Circuit Breaker at Point B= 300% of Sec. Full Load Current or Next Lower Standard size.
Example: 750KVA, 11KV/415V 3Phase Transformer having Impedance of Transformer 5%

  • Full Load Current At Primary side = 750000/(1.732X11000) = 39A
  • Rating of Primary Fuse = 3X39A = 118A, so next lower standard size of fuse = 110A.
  • OR Rating of Primary Circuit Breaker = 6X39A = 236A, so next lower standard size of Circuit Breaker = 225A.
  • Full Load Current at Secondary side = 750000/(1.732X415) =1043A.
  • Rating of Secondary of Fuse / Circuit Breaker = 2.5X1043A=2609A, so standard size of Fuse = 2500A.

Go to Content ↑

5) Supervised Location of Transformer (Impedance 6% to 10%)

Supervised Location of Transformer (Impedance 6% to 10%)

Supervised Location of Transformer (Impedance 6% to 10%)


  • OverCurrent Protection at Primary Side (Primary Voltage >600V):
  • Rating of Pri. Fuse at Point A= 300% of Pri. full load current or next lower standard size.
  • Rating of Pri. Circuit Breaker at Point A= 400% of Pri. full load current or next lower standard size.
  • Overcurrent protection at secondary side (Secondary voltage <=600V):
  • Rating of Sec. Fuse / Circuit Breaker at Point B= 250% of Sec. full load current or next higher standard size.
  • Overcurrent protection at secondary side (Secondary voltage >600V):
  • Rating of Sec. Fuse at Point B= 225% of Sec. full load current or next lower standard size.
  • Rating of Sec. Circuit Breaker at Point B= 250% of Sec. full load current or next lower standard size.
Example: 750KVA, 11KV/415V 3Phase Transformer having Impedance of Transformer 8%

  • Full Load Current At Primary side = 750000/(1.732X11000) = 39A
  • Rating of Primary Fuse = 3X39A = 118A, so next lower standard size of Fuse = 110A.
  • OR Rating of Primary Circuit Breaker = 4X39A = 157A, so next lower standard size of Circuit Breaker = 150A.
  • Full Load Current at Secondary side = 750000/(1.732X415) = 1043A.
  • Rating of Secondary of Fuse / Circuit Breaker = 2.5X1043A=2609A, so standard size of Fuse = 2500A.

Go to Content ↑

Difference in C.B between Supervised & Unsupervised Location

Here we see two notable conditions while we select Fuse / Circuit Breaker in Supervised Location and Unsupervised Location.

First notable condition is Primary Overcurrent Protection. In unsupervised location fuse in primary side is 300% of primary current or Next Higher Standard size and in supervised location is 300% of primary current or Next Lower Standard size. Here primary overcurrent protection is same in both conditions (300%), but selecting size of Fuse/Circuit Breaker is different.

Lets us Check with the Example for 750KVA, 11KV/415V 3Phase Transformer.

  • Full Load Current At Primary side = 750000/(1.732X11000) = 39A
  • In Unsupervised Location: Rating of Primary Fuse = 3X39A = 118A, so next higher standard size = 125A
  • In Supervised Location: Rating of Primary Fuse = 3X39A = 118A, so next lower standard size = 110A
  • Second notable condition is Secondary Overcurrent Protection increased from 125% to 250% for unsupervised to Supervised Location.

Go to Content ↑


Summary of overcurrent Protection for more than 600V

Maximum Rating of Overcurrent Protection for Transformers more than 600 Volts
Location LimitationsTransformer Rated ImpedancePrimary Protection
(More than 600 Volts)
Secondary Protection
More than 600VLess than 600V
C. B.Fuse RatingC. B.Fuse RatingC.B or Fuse
Any locationLess than 6%600%(NH)300%(NH)300 %( NH)250%(NH)125%(NH)
6% To 10%400%(NH)300%(NH)250%(NH)225%(NH)125%(NH)
Supervised locations onlyAny300%(NH)250%(NH)Not requiredNot requiredNot required
Less than 6%600%300%300%250%250%
6% To 10%400%300%250%225%250%
NH: Next Higher Standard Size.

Go to Content ↑


Overcurrent Protection of transformers <600V (NEC 450.3B)

1) Only Primary side Protection of Transformer

Only Primary side Protection of Transformer

Only Primary side Protection of Transformer


  • OverCurrent Protection at Primary Side (Less than 2A):
  • Rating of Pri. Fuse / C.B at Point A = 300% of Pri. full load current or next lower standard size.
  • Example: 1KVA, 480/230 3Phase transformer, full load current at Pri. side = 1000/(1.732X480) = 1A
  • Rating of Primary Fuse = 3X1A = 3A, so next lower standard size of Fuse = 3A.
  • OverCurrent Protection at Primary Side (2A to 9A):
  • Rating of Sec. Fuse / C.B at Point A = 167% of Pri. full load current or next lower standard size.
  • Example: 3KVA, 480/230 3Phase transformer, full load current at Pri. side = 3000/(1.732X480) = 4A
  • Rating of Primary Fuse = 1.67X4A = 6A, so next lower standard size of Fuse = 6A.
  • OverCurrent Protection at Primary Side (More than 9A):
  • Rating of Pri. Fuse / C.B at Point A = 125% of Pri. full load current or next higher standard size.
  • Example: 15KVA, 480/230 3Phase transformer, full load current at Pri. side = 15000/(1.732X480) = 18A
  • Rating of Primary Fuse = 1.25X18A= 23A, so next higher standard size of Fuse = 25A.

Go to Content ↑


2) Primary and Secondary side Protection of Transformer

Primary and Secondary side Protection of Transformer

Primary and Secondary side Protection of Transformer


  • OverCurrent Protection at Primary Side (Less than 2A):
  • Rating of Pri. Fuse / C.B at Point A = 250% of Pri. full load current or next lower standard size.
  • OverCurrent Protection at Primary Side (2A to 9A):
  • Rating of Sec. Fuse / C.B at Point A= 250% of Pri. full load current or next lower standard size.
  • OverCurrent Protection at Primary Side (More than 9A):
  • Rating of Pri. Fuse / C.B at Point A= 250% of Pri. Full Load Current or Lower Higher Standard size.
  • Example: 25KVA, 480/230 3Phase Transformer, Full Load Current at Pri. Side=125000/(1.732X480)=30A
  • Rating of Primary Fuse = 2.50X30A= 75A, So Next Lower Standard Size of Fuse =70A.
  • OverCurrent Protection at Secondary Side (Less than 9A):
  • Rating of Pri. Fuse / C.B at Point B= 167% of Sec. Full Load Current or Lower Standard size.
  • Example: 3KVA, 480/230 3Phase Transformer, Full Load Current at Sec. Side=3000/(1.732X230)=8A
  • Rating of Primary Fuse = 1.67X8A= 13A, So Next Lower Standard Size of Fuse =9A.
  • OverCurrent Protection at Secondary Side (More than 9A):
  • Rating of Pri. Fuse / C.B at Point A= 125% of Pri. Full Load Current or Higher Standard size.
  • Example: 15KVA, 480/230 3Phase Transformer, Full Load Current at Sec. Side=15000/(1.732X230)=38A
  • Rating of Primary Fuse = 1.25X38A= 63A, So Next Higher Standard Size of Fuse =70A.

Go to Content ↑


Summary of overcurrent Protection for Less than 600V

Maximum Rating of Overcurrent Protection for Transformers Less than 600 Volts
Protection MethodPrimary ProtectionSecondary Protection
More than 9A2A to 9ALess than 2AMore than 9ALess than 9A
Primary only protection125%(NH)167%300%Not requiredNot required
Primary and secondary protection250%250%250%125%(NH)167%
NH: Next Higher Standard Size.

Go to Content ↑


author-pic

jiguparmar - Jignesh Parmar has completed his B.E(Electrical) from Gujarat University. He is member of Institution of Engineers (MIE),India. Membership No:M-1473586.He has more than 12 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 Assistant Manager at Ahmedabad,India. He has published numbers of Technical Articles in "Electrical Mirror", "Electrical India", "Lighting India", "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.



9 Comments


  1. Lona
    Jul 22, 2014

    Hi

    Tks so much for such educating website. I’ve got only 1 question – what do you mean by supervised and unsupervised locations.


  2. David S
    Feb 26, 2014

    In your example of “OverCurrent Protection at Primary Side (More than 9A)” why did you use 125000 instead of 25000 (25KVA) in your calculation of the full load current at the primary side?


  3. Mohit Bhatia
    Jun 19, 2013

    Nice Article.
    But I understand this article is related to component sizing selection (Fuse and CB) not Overcurrent protection.
    e.g. This article explained the selection of FUSE rating, CB rating only. it doesn;t explain the protection setting (Pick Up point for Inst and IDMT).
    If there any article for Overcurrent Protection for transformer Setting (Instantaneous and IDMT), please share.


  4. chourishiraj
    Jun 17, 2013

    How can we calculate the power losses due to harmonics in the power system or power equipment like Transformer, cables, motors, etc. please reply

  5. [...] to Index ↑ 2.2 Earth fault protectionThe general practice is to employ a set of two or three overcurrent relays and a separate overcurrent relay for single line to ground fault. Separate earth fault relay [...]

  6. [...] cause serious damage to the device.Remember that a differential relay is basically an instantaneous overcurrent relay that operates on the difference of current flowing into and out of the protected zone.For [...]

  7. [...] control lighting circuits covers:Switch requirementsSwitch locationExterior lightsThe section on overcurrent protection covers accessibility of branch-circuit overcurrent devices (fuses and circuit breakers) and [...]


  8. CHAKWESHA CHAKWESHA
    Oct 09, 2012

    This is a very good website.I really enjoy logging to it.


    • Edvard
      Oct 09, 2012

      Thank you Chakwesha! Glad you find EEP usefull for your education/work. Technical writers like Jignesh and others are really contrubuting a lot.

Leave a Comment

Tell us what you're thinking... we care about your opinion!
and oh, not to forget - if you want a picture to show with your comment, go get a free Gravatar!


nine + 4 =

FOLLOW EEP!

Subscribe to Weekly Download Updates: (free electrical software, spreadsheets and EE guides)

EEP's Android Application
Electrical Engineering Daily Dose
Daily dose of knowledge and news from
Electrical Engineering World

Advertisement

Get
PDF