Determine Total Connected Load — Transformer Sizing Guide

Determining the total connected load is the foundational step in transformer sizing. An undersized transformer leads to overheating, accelerated insulation ageing, and premature failure. An oversized transformer wastes capital and increases no-load losses throughout its operational life.

Step-by-Step Methodology

1. List all connected loads — Document every piece of equipment that will draw power through the transformer. Include motors, lighting, HVAC, UPS systems, elevators, and any future planned loads.

2. Apply demand factors — Not all connected loads operate simultaneously at full capacity. Demand factors (also called diversity factors) account for this:

• Residential: 0.4–0.6 (occupants use power at different times)
• Commercial: 0.6–0.8 (office hours drive peak demand)
• Industrial: 0.7–0.9 (process loads run more continuously)
• Data centres: 0.9–1.0 (near-constant load)

3. Convert to kVA — Divide the total kW demand by the expected power factor:

• kVA = kW ÷ Power Factor
• Typical power factors: 0.85 (general commercial), 0.90 (industrial with PF correction), 0.95 (data centres with modern UPS)

4. Add growth margin — Include 15–25% margin for future load growth. Utility specifications often mandate specific margins (e.g., DEWA requires 20% spare capacity).

5. Consider cyclic loading — If the load profile is cyclic (e.g., solar plants with daytime-only generation), the transformer may be rated for cyclic overload per IEC 60076-7, potentially allowing a smaller unit.

Example Calculation

• Connected load: 3,000 kW
• Demand factor: 0.7 (commercial building)
• Maximum demand: 3,000 × 0.7 = 2,100 kW
• Power factor: 0.85
• Required kVA: 2,100 ÷ 0.85 = 2,471 kVA
• Growth margin (20%): 2,471 × 1.2 = 2,965 kVA
• Selected transformer: 2 × 1,600 kVA (with N+1 redundancy) or 1 × 3,150 kVA

At ETS, our application engineers assist with load calculations and recommend optimal sizing for every project.