Transformers and Power Distribution
Transformers and Power Distribution
This domain tests Article 450 transformer ratings, primary and secondary currents, overcurrent protection, grounding, and distribution equipment. Expect kVA, turns ratio, and three-phase current calculations.
Core rules
- Transformer capacity is rated in kVA. For a three-phase transformer, I = kVA × 1,000 / (√3 × V_L). For single-phase, I = kVA × 1,000 / V.
- Turns ratio equals voltage ratio: V_p / V_s = N_p / N_s. Current ratio is inverse: I_p / I_s = V_s / V_p.
- Article 450.3 provides overcurrent protection percentages and next-standard-size rules for transformers. Primary protection does not replace secondary conductor protection.
- Separately derived systems require a grounded conductor, grounding electrode conductor, and bonding jumper; the neutral is bonded at the transformer.
- Distribution equipment includes panelboards, switchboards, and busways; working clearances and labeling still apply.
Worked example
A 75 kVA, three-phase transformer has a 480 V primary and a 208Y/120 V secondary. Primary current = 75,000 / (1.732 × 480) = 90.2 A. Secondary current = 75,000 / (1.732 × 208) = 208.2 A. The turns ratio is 480/208 = 2.31:1. A 125% primary OCPD target would be about 112.8 A; verify the exact 450.3(B) allowance before selecting the next standard size.
Traps
- Using line-to-neutral voltage for a three-phase current calculation.
- Forgetting √3 for three-phase kVA.
- Sizing transformer OCPD at 100% of current without the 450.3 allowance.
- Treating a transformer neutral like a subpanel neutral.