Abstract of over current Protection of Transformer (NEC 450.3)
September 18, 2012 14 Comments
Introduction:
- The over current protection required for transformers is consider for Protection of Transformer only.Such over current 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 (approximately0.01 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, over current devices should be selected at about 110 to 125 percent of the transformer full-load current rating. And when using such smaller over current 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.
- Over current 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 over current 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
Over current Protection of Transformers > 600 V (NEC 450.3-A)
1) Unsupervised Location of Transformer (Impedance <6%) 
- Over Current 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.
- Over Current 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.
- Over Current 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 Circuit Breaker =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.
2) Unsupervised Location of Transformer (Impedance 6% to 10 %) 
- Over Current 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.
- Over Current 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.
- Over Current 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 =4X87A=348A, So Next Standard Size of Circuit Breaker =400A.
- 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.
3) Supervised Location (in Primary side only) of Transformer: 
- Over Current 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.
4) Supervised Location of Transformer (Impedance Up to 6%): 
- Over Current 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.
- Over Current 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.
- Over Current 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.
5) Supervised Location of Transformer (Impedance 6% to 10%): 
- Over Current 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.
- Over Current 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.
- Over Current 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.
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 Over current 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 Over current 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 Over current Protection increased from 125% to 250% for unsupervised to Supervised Location.
Summary of over current Protection for more than 600V:
| Maximum Rating of Over current Protection for Transformers more than 600 Volts | ||||||
| LocationLimitations | TransformerRated Impedance | Primary Protection (More than 600 Volts) | Secondary Protection | |||
| More than 600 Volts | Less than 600 Volts | |||||
| Circuit Breaker | Fuse Rating | Circuit Breaker | Fuse Rating | C.B or Fuse | ||
| Any location | Less 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) | |
| Supervisedlocations
only |
Any | 300%(NH) | 250%(NH) | Not required | Not required | Not required |
| Less than 6% | 600% | 300% | 300% | 250% | 250% | |
| 6% To 10% | 400% | 300% | 250% | 225% | 250% | |
| NH: Next Higher Standard Size. | ||||||
Over current Protection of Transformers< 600 V (NEC 450.3-B)
1) Only Primary side Protection of Transformer: 
- Over Current 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.
- Over Current 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.
- Over Current 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.
2) Primary and Secondary side Protection of Transformer: 
- Over Current 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.
- Over Current 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.
- Over Current 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.
- Over Current 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.
- Over Current 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= 48A, So Next Higher Standard Size of Fuse =50A.
Summary of over current Protection for Less than 600V:
| Maximum Rating of Over current Protection for Transformers Less than 600 Volts | |||||
| ProtectionMethod | Primary Protection | Secondary Protection | |||
| More than 9A | 2A to 9A | Less than 2A | More than 9A | Less than 9A | |
| Primary only protection | 125%(NH) | 167% | 300% | Not required | Not required |
| Primary and secondary protection | 250% | 250% | 250% | 125%(NH) | 167% |
| NH: Next Higher Standard Size. | |||||
JIGNESH PARMAR
Dear Sir,
You are doing a great service to Electrical Engineers. Can I have a note explaining how symmetrical component circuits are derived ,e.g. series combination of +ve , -ve and zero ph seq impedances for single ph to ground faults in a 3-ph ckt.
Best regards & thanks
New Delhi
Dear Sir,
Pl. note that selection of current rating of CB at Primary and secondary side of transformer should be in accordance with fault level at the point and Full load current of Transformer only. To cope up with inrush current of transformer pri.side CB over load portection relay can be set accordingly.
regards
Arun Jain
Yes Arun I completely agree with You.
S.C Current play vital role for selecting Size of CB and Relay grading & Relay setting for Protection.
This Article elaborate NEC code 450.3 for Transformer. I have tried to write in simple language what this code say only.
Grading for Over current Relay is always depend upon lower side systems min Fault current for Radial feeder and for Transformer
The primary winding over current protection is typically set with:
(1) The O/L Relay (I>>) operates as back-up protection for short circuits at the secondary side of the transformer and is adjusted to 80% to 90% of the minimum possible short circuit at the low voltage terminal.
(2)The O/L Relay (I>>>) operates in instantaneous time and is adjusted to 110% to 120% of the maximum possible short circuit at the secondary side of the transformer.
Thank you.Can you send me how to set the over current,earth fault,relay setting .
Wait for few days my next post will be setting of IDMT O/L & E/F Relay and Relay Grading..
Thank yousir. Cant wait to have it.
Thanks for lovely information. I can appreciate everything that you have said here.
Thanks,nice work and good information.
VERY GOOD INFORMATION TO ME. THANKYOU.
very nice discussion that have increased my knowledge.
Thanx
Mazhar Ali
you need to include the “exceptions” as noted in the NEC book – they are very important.
Thank you for these information.But I want to understand for example :”Maximum Rating of Over current Protection for Transformers more than 600 Volts”.Does “maximum ” mean that primary protection by CB shoud be <=600 * full load currant or exactly 600%.
FOR VERY INVERSE RELAY CARACTERISTIC THAT HAS THE FOLLOWING EQUATION I WANT TO KNOW HOW THE SET THE APPROPRIATE TMS
t=TMS.13,5/Ir-1 where Ir=Is/Id
Please correct what is wrong.
For a transformer for example:
1-setting the Is value :according to the Abstract of over current Protection of Transformer (NEC 450.3) that you made ,for transformer more than 600v in supervised location with rated impedance less than 6% ,Is=600% of full load current
2-So for that Is ,taking in account the coordination ,it must be 0.3 sec between the transformer OC relay and the upstream relay so if if the upstream relay has 0.5 s at Is=600% FLC then the OC transformer OC must be 0.2 s and so the
TMS=t .(Ir-1)/13.5=0.2(6.Is-1)/13,5
Are these steps correct?
I have 25KVA 3 ph 415/415 V transformer. Its secondary has 250A fuse with individual secondary feeders have 125A fuse. I have doubt as primary Fuse is given as 63A only. Is it right? if yes thn Why and if No than what is right fuse rating? I think any fault in secondary feeder would trip transformer primary fuse first. Plz reply.