NEC 450.3
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.
Content
- Introduction
- Overcurrent protection of transformers >600V (NEC450.3A)
- Unsupervised location of transformer (impedance <6%)
- Unsupervised location of transformer (impedance 6% to 10%)
- Supervised location (in primary side only) of transformer
- Supervised location of transformer (impedance up to 6%)
- Supervised location of transformer (impedance 6% to 10%)
- Difference in C.B between supervised & unsupervised Location
- Sumary of overcurrent protection for more than 600V
- Overcurrent Protection of Transformers <600V (NEC 450.3B)
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.
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.
Overcurrent Protection of Transformers >600V (NEC450.3A)
1) 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.
- 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.
2) 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.
- 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.
3) 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.
4) 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.
- 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%)
- 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.
- 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 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.
Summary of overcurrent Protection for more than 600V
| Maximum Rating of Overcurrent Protection for Transformers more than 600 Volts | ||||||
| Location Limitations | Transformer Rated Impedance | Primary Protection (More than 600 Volts) | Secondary Protection | |||
| More than 600V | Less than 600V | |||||
| C. B. | Fuse Rating | C. B. | 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) | |
| Supervised locations 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. | ||||||
Overcurrent Protection of transformers <600V (NEC 450.3B)
1) 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.
2) 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.
Summary of overcurrent Protection for Less than 600V
| Maximum Rating of Overcurrent Protection for Transformers Less than 600 Volts | |||||
| Protection Method | 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. | |||||
Related electrical guides & articles
In your last example, there appears to be a typo mistake:
“Rating of Primary Fuse = 1.25X38A= 63A”
63A should be 47.5A.
I tried a couple of times to save to PDF this article but I was not able to. I became a premium member yesterday. For my understanding change the article to PDF is part of the subscription
Please advise.
is there other articles that you recommend regarding transformer protection?
Thank you
What will be fuse rating 100kva at 33/0.433kv?
thanks
Sir, Let me know the recommended fuse for 100 kvA – 3 pole ; 33kV HV & LV: 440 V .
Thanks.
Sridhar
Great explanation. Do you have a BibTex that I can use for my publications?
Because I am willing to use your work as a reference to my work.
Hi. A ton of good information here.
I am reading the very beginning.
“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.” You think flux builds up over multiple cycles? Anything is saved or stored in a transformer from cycle to cycle?
“Such overcurrent protection will not necessarily protect the primary or secondary conductors or equipment connected on the secondary side of the transformer.
…
To avoid using over sized conductors, overcurrent devices should be selected at about 110 to 125 percent of the transformer full-load current rating.”
You make the good point that the protection is for the transformer and may or may not protect the conductors. But then why the “over sized conductors” comment? The 125% is for the transformer.