kVA Sizing
Step 1: Calculate total connected load in VA. Step 2: Apply demand factors per NEC 220 or engineering analysis. Step 3: Select the next standard kVA size that meets or exceeds the demand load.
Standard three-phase dry-type transformer sizes: 15, 30, 45, 75, 112.5, 150, 225, 300, 500, 750, 1000 kVA. For single-phase: 15, 25, 37.5, 50, 75, 100, 167, 250 kVA.
Example: 80 kVA calculated demand load → select 112.5 kVA transformer. Provides 40% growth capacity. Never size transformers below the calculated demand — typical practice is 20-40% spare capacity.
Primary and Secondary Current
Three-Phase: I = kVA × 1000 / (V × √3). Single-Phase: I = kVA × 1000 / V.
Example: 225 kVA, 480V primary, 208/120V secondary. Primary FLC = 225,000 / (480 × 1.732) = 270.6A. Secondary FLC = 225,000 / (208 × 1.732) = 624.7A.
These currents are used for sizing conductors (at 125%), overcurrent protection, and upstream circuit breaker selection.
Impedance and Its Effects
Transformer impedance (%Z) determines available fault current on the secondary. Lower impedance = higher available fault current = higher AIC requirements for downstream equipment.
Typical %Z values: <500 kVA = 2-5.75%, 500-2500 kVA = 5.75%, >2500 kVA = 6.5%. The industry standard for commercial/industrial is 5.75%.
Fault current: Isc = (kVA × 1000) / (V_secondary × √3 × %Z/100). Example: 500 kVA, 208V, 5.75%Z → Isc = 500,000 / (208 × 1.732 × 0.0575) = 24,148A. All downstream equipment requires ≥ 25 kAIC rating.
Low-impedance transformers (2-3%Z) provide better voltage regulation under load but produce dangerously high fault currents. High-impedance transformers (5.75%+) limit fault current but have worse voltage regulation.
NEC 450.3 Overcurrent Protection
Transformers with primary over 9A: primary protection max 125% of primary FLC. If 125% doesn't correspond to a standard size, the next higher standard size is permitted.
Transformers with primary 9A or less: primary protection max 167% of primary FLC.
Transformers with primary 2A or less: primary protection max 300% of primary FLC.
Secondary protection is NOT required if primary protection is provided per Table 450.3(B), and the transformer impedance is ≤ 6%. Otherwise, secondary protection at max 125% of secondary FLC is required.
NEC 450.9 requires ventilation for all transformer installations. Dry-type transformers need adequate air circulation — typical requirement: 100 CFM per kW of transformer losses. Minimum clearance from walls: 12 inches for transformers up to 112.5 kVA, 3 feet for larger units.
Common Mistakes to Avoid
Undersizing for harmonics — Non-linear loads (VFDs, LED drivers, computers) generate harmonic currents that increase transformer heating without proportionally increasing kVA meter readings. K-rated transformers (K-4, K-13, K-20) are designed for harmonic loading.
Forgetting derating for altitude or temperature — Standard transformer ratings assume 40°C ambient at ≤ 3,300 feet elevation. Above 3,300 feet, derate capacity by 0.3% per 330 feet. Above 40°C ambient, derate per manufacturer specifications.
Using NEC Table 310.16 for transformer conductors — Transformer secondary conductors must be sized per NEC 240.21(C), which has specific tap rules with distance limitations. These rules differ from standard feeder sizing.
Not marking available fault current — NEC 110.24 requires field-marking of available fault current at service equipment downstream of transformers.