The transformer is the heartbeat of every commercial and industrial electrical system — stepping 4,160V or 13,800V utility voltage down to the 480V, 208V, or 120V that drives building loads. Undersizing a transformer guarantees premature failure from overheating; oversizing wastes capital and increases no-load losses that run 24/7/365. NEC Article 450 governs transformer installation and overcurrent protection, but proper sizing requires understanding the interplay between connected load, demand factors, power factor, harmonic content, and future growth.

Transformer sizing begins with the total demanded load in kVA — not the total connected load. Apply NEC Article 220 demand factors to reduce the connected load to the expected maximum demand. Convert from kW to kVA using the system power factor: kVA = kW / PF. A building with 400 kW of demanded load at PF 0.85 requires 400 / 0.85 = 470 kVA. Standard transformer sizes (75, 112.5, 150, 225, 300, 500, 750, 1000, 1500, 2000, 2500 kVA) follow a preferred number series. Select the next standard size above the calculated demand, typically loading the transformer to 75-80% of its kVA rating to allow for future growth and harmonic derating.

Overcurrent protection for transformers is governed by NEC 450.3(B) for transformers rated ≤1000V. Primary-only protection allows sizing at 125% of rated primary current (next standard size up if 125% doesn't match a standard fuse/breaker). When both primary and secondary protection are provided, the primary may be sized up to 250% (fuses) or 300% (breakers) of rated current, and the secondary must be protected at 125% of rated secondary current. The choice between primary-only and primary+secondary protection affects coordination with downstream devices and available fault current.

K-factor rated transformers are designed to handle non-linear loads that produce harmonic currents. Standard transformers (K-1) are designed for sinusoidal loads. K-4 transformers handle moderate harmonic content (typical office building with computers). K-13 transformers serve heavy electronic loads (data centers, broadcast facilities). K-20 transformers handle the most severe harmonic environments (large VFD installations, SCR-controlled loads). The K-factor derating method uses the formula: K = Σ(Ih² × h²), where Ih is the harmonic current as a fraction of fundamental and h is the harmonic number.

Dry-type vs liquid-filled transformers present distinct trade-offs. Dry-type transformers are standard for indoor commercial installations — no flammable liquid, less fire risk, NEC 450.21 governs installation clearances. Liquid-filled (mineral oil or less-flammable FR3/Envirotemp) transformers offer higher efficiency, better overload capacity, quieter operation, and longer life. NEC 450.23-450.28 governs oil-filled transformer installations including vault requirements, drainage, and ventilation. For transformers over 35 kV or above 112.5 kVA in certain occupancies, a transformer vault per NEC 450.21-450.27 may be required.

Transformer paralleling requires matching voltage ratio, impedance, polarity, and phase rotation. Two transformers with different impedance percentages will not share load proportionally — current divides inversely proportional to impedance, overloading the lower-impedance unit. NEC 450.7 allows paralleling with overcurrent protection based on the combined kVA rating. Impedance should match within 5% (relative). Phase rotation must be identical (verify with a phase rotation meter before closing tie). Voltage taps must be set to the same position. Paralleling doubles available fault current — verify downstream equipment SCCR ratings.