Size of Circuit Breaker & Fuse are as per NEC 450.3
This MS Excel spreadsheet calculates the following:
- Size of Circuit Breaker on Primary side of Transformer as per NEC 450.3
- Size of Fuse on Primary side of Transformer as per NEC 450.3
- Size of Circuit Breaker on Secondary side of Transformer as per NEC 450.3
- Size of Fuse on Secondary side of Transformer as per NEC 450.3
- Size of Transformer
- Transformer Full Load Losses.
Overcurrent Protection of Transformer (NEC 450.3)
This technical article was written by author Jignesh Parmar explains step by step NEC 450.3 (Transformer protection).
- 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)
| Software: | Calculation of Size of Transformer, Fuse and Circuit Breaker |
| Version: | 10.6.13 |
| Developer: | Jignesh Parmar |
| Size: | 90.3 KB |
| Price: | Free |
| Download: | Right here | Video Courses | Membership | Download Updates |
Sheet not useful.
The values for the circuit breakers are not correct as per NEC and CSA. As example, 45 kVA – 480/208 3 phase transformer shoiuld have 70 AMP primary and 175 AMP secondary protection. Please advise.
Great tool
How to size the circuit breaker based on the inrush current? Can a formula be included in the excel file to calculate the inrush current, and pick a required CB based on the Long/Medium delay, due to inrush current?
I want to know about transformer rating related to HRC fuse to be fixed.
Hello Jignesh Parmar,
Will it be possible to access the hidden sheets from the excel please. Also, how do I get the password to remove the protection on the sheet.
Thanks for your response in advance.
Regards,
I have a 150kVA transformer 480VAC 3 phase. Could you recommend what size of cables and fuse to install. Thank you.
diversity factor
Many Thanks .
We remain grateful .
Hello sir,
hope to be fine all.
I am using your tutorial files every they,
thank you
I am not able to download the excel sheet
What is mean by DF load in this page? How can we calculate the DF load?
I don’t want to repeat allready bespoken aspects. Just a reminder about ISO – standardized naming and abreviations for physical values:
Kva : it must be written as kVA
Amp : it must be written as A or Ampère
Volt: in formulas and sheets simply V , or kV …
Kw : it must be written as kW
Power Factor and working points of losses should be keeped at values conform to IEC (as it takes matter in the most countries and nearly everywhere 50Hz is..)
that’s right
how to calculate feeder short circuit current(KA)?kindly if u know mean inform me how to calculate this one
@Anmol978
The ‘0.97’ is the stacking factor. The core is made up of Si steel/CRGO laminations due to which, some correction factor for the area is used.
Dear Mr.Parmar,
The spread sheet is ok. but the breaker selection is out of range as per the standards used. for example for 1000KVA breaker only 1600A breaker is sufficient. Please correct me if i am wrong. In the spreadsheet for 1000KVA 3000A is breaker suggest. Please check it
For 1000 KVA, 3-phase, 50Hz, 400V system:
IL= 1440A
So, the next standard breaker size would be: 1600A
Since it is more than 800 or 1000A, it should be of ACB type.
Yes, for 1000 kVA Tx, 1600 A CB is enough. For 2500 kVA , 2500 A CB is enough.
https://electrical-engineering-portal.com/overcurrent-protection-transformer-nec-450-3
NEC Article 450.3 upsizes the secondary breaker 250% per the table. This is to protect the transformer and prevent nuisance tripping. XFMR Magnetizing (EMF) currents causes breakers to trip if not sized per NEC 450.3
Read the Intro in above hyperlink…
This article too also goes into why the NEC 430.5 has the table for the various conditions.
https://www.ecmweb.com/national-electrical-code/article/20900665/nec-guidelines-for-transformer-and-transformer-feeder-protection
I haven’t run exhausted checks with respect to the results to verify its accuracy but I have checked a few and they agree with the table.
Please check you data inputs? Are you selecting a supervised location? If so that is why you are getting 3000….
Is there any difference between this standard and those within the UK?
this program make over design for breaker and this breaker out of standard range
thank you for trying
best regard
Eng. the engineer
I was just checking some State electricity Board inspection sheets on transformers. (Not related with spreadsheet) that while calculating the flux density of a transformer (MAX value is 1.600 T) a basic formula is used B=E(phase)/(A*4.44*f*N*.97), What is that .97 factor multiplied???
P.s the above work is fantastic …