Transformers: ratios, kVA, and connections

A transformer trades voltage for current at (nearly) constant power. The turns ratio sets everything: Vp/Vs = Np/Ns, and current runs the opposite way — step the voltage down by 4 and the secondary current rises by 4. A 480→120 V transformer has a 4:1 ratio, so 10 A on the primary becomes 40 A on the secondary.

Transformers are rated in kVA, not kW, because the nameplate must cover apparent power regardless of the load's power factor. Secondary current is the number that matters for sizing conductors and protection: single-phase, I = VA ÷ V; three-phase, I = VA ÷ (√3 × V). A 45 kVA transformer with a 208 V three-phase secondary delivers 45 000 ÷ (1.732 × 208) ≈ 125 A.

Connections matter. Delta-wye is the commercial default: delta primary rejects triplen harmonics, wye secondary provides the neutral that 120 V loads need. The wye secondary is a separately derived system — it gets its own grounding electrode connection and its own neutral-to-ground bond, exactly one, at the transformer or first disconnect.

Exam focus: ratio problems in both directions, secondary current with the √3 for three-phase, and knowing that a separately derived system is bonded once at its own source — the Unit 2.5 rule reappearing at commercial scale.

Quiz — 3 questions