Calculation of Size of Transformer, Fuse and Circuit Breaker

Size of Circuit Breaker & Fuse are as per NEC 450.3

This MS Excel spreadsheet calculates the following:

1. Size of Circuit Breaker on Primary side of Transformer as per NEC 450.3
2. Size of Fuse on Primary side of Transformer as per NEC 450.3
3. Size of Circuit Breaker on Secondary side of Transformer as per NEC 450.3
4. Size of Fuse on Secondary side of Transformer as per NEC 450.3
5. Size of Transformer
6. 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)
  1. Unsupervised location of transformer (impedance <6%)
  2. Unsupervised location of transformer (impedance 6% to 10%)
  3. Supervised location (in primary side only) of transformer
  4. Supervised location of transformer (impedance up to 6%)
  5. Supervised location of transformer (impedance 6% to 10%)
  6. Difference in C.B between supervised & unsupervised Location
  7. Sumary of overcurrent protection for more than 600V
• Overcurrent Protection of Transformers <600V (NEC 450.3B)
  1. Only primary side protection of transformer
  2. Primary and secondary side protection of transformer
  3. Summary of overcurrent protection for less than 600V