Transformers and Power Distribution Equipment
What the exam targets
Article 450 transformer protection, 250.30 separately derived systems, 240.21(C) secondary conductor rules, plus transformer math and working clearances.
Concepts to own
- Full-load current: single phase I = kVA × 1000 ÷ E; three phase I = kVA × 1000 ÷ (1.732 × E).
- 450.3(B): with primary-only protection, the primary device may be up to 125% of rated primary current (167% where primary current is under 9 A). With both primary and secondary protection, 125% primary and 125% secondary.
- Secondary conductors must be protected by a secondary overcurrent device, a 240.21(C) tap rule (10 ft or 25 ft), or approved transformer thermal protection.
- Separately derived systems (250.30): system bonding jumper at the transformer, grounding electrode conductor to the building electrode system, equipment grounding conductor run with the secondary conductors.
- Working space (110.26) and ventilation (450.9) apply to transformer installations.
Traps
Sizing the primary device at 100% instead of 125%; forgetting the 167% allowance for small primary currents; treating a transformer secondary as a service and bonding neutral to ground at the first panel in addition to the transformer; ignoring that the secondary of a 480-to-208Y/120 transformer is a separately derived system.
Worked example
A 45 kVA, 480-to-208Y/120 V three-phase transformer. Primary FLA = 45,000 ÷ (1.732 × 480) = 54.1 A; 54.1 × 1.25 = 67.6 A, so a 70 A primary device. Secondary FLA = 45,000 ÷ (1.732 × 208) = 124.9 A; 124.9 × 1.25 = 156 A, so the next standard size is 175 A.