Overcurrent Protection of Transformer (NEC 450.3)

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

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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About Author //

author-pic

Jignesh Parmar

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.



14 Comments


  1. Ajay John
    Jan 26, 2015

    Dear Parmar,what about the over protection of transformer with respect of IEC following countries?.Whether there is any connection and formula for design of the transformer protection circuits?

  2. […] NEC Article 450 gives specific primary and secondary overcurrent device ratings that may not be exceeded. These vary depending upon the accessibility of the transformer to unqualified persons and the impedance of the transformer. The smallest protective device that allows the rated full-load current of the transformer gives the best practical overcurrent protection. […]


  3. Ricardo Brown
    Jan 20, 2015

    Mr Engineer
    I am looking for a simulator program that can detect faults scenario that can exit on a single phase or three phase transformers

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