Transformer Capacity Selection: Load Calculation & Sizing Guide

Transformer Sizing Fundamentals

Proper transformer sizing ensures optimal efficiency (70–80% loading), prevents premature failure, and accommodates future expansion. Undersized transformers overheat and fail; oversized transformers waste energy through excessive no-load losses.

Step 1: Calculate Total Connected Load

Sum all connected loads from your single-line diagram: lighting, HVAC, motors, UPS, plug loads. Include both continuous and non-continuous loads.

Step 2: Apply Demand Factor

Formula: Demand Load = Connected Load × Demand Factor

Step 3: Apply Diversity Factor

Diversity factor accounts for non-simultaneous operation. Typical values: 1.1–1.3 for industrial, 1.2–1.5 for commercial.

Formula: Diversified Load = Demand Load ÷ Diversity Factor

Step 4: Add Future Expansion

Add 15–25% capacity margin for future load growth. This ensures the transformer remains adequate for 10–15 years.

Step 5: Verify Motor Starting

Large motors draw 6–8× FLC during starting. Ensure transformer can supply starting current without excessive voltage drop (>15%).

Formula: Transformer kVA ≥ (Motor HP × 6) for direct-on-line starting

Step 6: Select Standard Rating

Round up to next standard kVA: 30, 50, 80, 100, 125, 160, 200, 250, 315, 400, 500, 630, 800, 1000, 1250, 1600, 2000, 2500, 3150.

Contact our engineering team for a free load calculation and transformer recommendation.