
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 |

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?