Transformers and Power Distribution Systems
Transformer fundamentals
Transformers transfer energy by induction and change voltage in proportion to turns ratio. Primary and secondary currents follow the inverse voltage ratio, and kVA is approximately conserved minus losses. Know isolation transformers, autotransformers, buck-boost units, delta-wye connections, K-rated transformers for harmonics, and the concept of a separately derived system. Transformer protection follows specific tables and next-standard-size rules.
Distribution systems
For a two-winding transformer, compute primary current from kVA and primary voltage, and secondary current from kVA and secondary voltage. Primary overcurrent protection is generally sized at 125% of primary current, with the next standard size permitted; secondary protection and conductor sizing have their own multipliers and rules. Inrush current affects magnetic breaker selection.
Traps
Do not use the secondary current to size primary protection, or ignore the difference between line current and phase current in delta and wye windings. A common error is treating a transformer as a separately derived system when it is not, or bonding the neutral on the load side of a subpanel. Harmonic loads can overload neutrals and require K-rated equipment.
Worked example
A 45 kVA, 480 V to 208Y/120 V transformer: primary current = 45,000 ÷ (1.732 × 480) = 54.1 A; 125% = 67.6 A, so a 70 A primary breaker is permitted. Secondary current = 45,000 ÷ (1.732 × 208) = 125 A. Rule of thumb: kVA divided by √3 times voltage gives line current on either side.