Transformer Sizing Calculator Formula
Understand the math behind the transformer sizing calculator. Each variable explained with a worked example.
Formulas Used
Maximum Demand
demand_load = demand_kwRequired kVA (with growth)
required_kva = demand_kw / power_factor * (1 + growth_margin / 100)Variables
| Variable | Description | Default |
|---|---|---|
total_load_kw | Total Connected Load(kW) | 200 |
demand_factor | Demand Factor(%) | 80 |
power_factor | Average Power Factor | 0.85 |
growth_margin | Growth Margin(%) | 20 |
demand_kw | Derived value= total_load_kw * demand_factor / 100 | calculated |
How It Works
Transformer Sizing
The transformer must handle the maximum demand load with adequate margin.
Method
Maximum demand = Total connected load x Demand factor
Required kVA = Max demand (kW) / Power factor x (1 + Growth margin)
The demand factor accounts for the fact that not all loads operate simultaneously. The growth margin reserves capacity for future expansion (typically 15-25%).
Worked Example
200 kW total connected load, 80% demand factor, 0.85 PF, 20% growth.
- 01Maximum demand = 200 x 0.80 = 160 kW
- 02Required kVA = 160 / 0.85 x 1.20 = 225.9 kVA
- 03Select the next standard size: 250 kVA transformer.
Frequently Asked Questions
What are standard transformer sizes?
Common three-phase ratings: 100, 160, 200, 250, 315, 400, 500, 630, 800, 1000, 1250, 1600, 2000, 2500 kVA. Always select the next size above the calculated requirement.
What is a typical demand factor?
Commercial buildings: 60-80%. Industrial plants: 50-70%. Residential complexes: 40-60%. Hospitals: 70-85%. Higher demand factors apply when loads are more likely to operate simultaneously.
Should I account for starting current of motors?
For the transformer sizing, demand factor typically covers motor starting since not all motors start simultaneously. However, for large individual motors (above 15% of transformer rating), verify the voltage dip during starting is acceptable.
Ready to run the numbers?
Open Transformer Sizing Calculator