A clear, step-by-step explanation for choosing a transformer
size.
1. What’s the Big Picture?
You have a 100 kW heat pump that runs on electricity.
Transformers are rated in kVA (kilo–volt-amps), but heat pump specs use kW
(kilo–watts). We need to translate kW to kVA and add margin, so we end up
selecting a 75 kVA transformer.
2. Determining the kVA Load
• Electricity input is expressed in FLA on the
rating plate: FLA 58 Amps.
• 58 Amps x 480 V x 1.732 = 48.2 kVA (power
required for input)
3. Why Not Pick Exactly 48.2 kVA?
Although 48.2 kVA covers the apparent load, we need extra
capacity for:
1. Startup surges
2. Heat losses in the transformer
3. Future expansion
4. Standard practice to de-rate to 80% continuous load
Rule of thumb: size at 125% of continuous load:
48.2 kVA × 1.25 = 60.3 kVA → Next standard size: 75 kVA.
4. Putting It All Together
1. Heat demand: 100 kW unit has an input of : 48.2 kVA
electric input.
2. De-rate for continuous service: 48.2 kVA ÷ 0.8 ≈ 60.3 kVA.
3. Choose the next standard size: 75 kVA transformer.
5. Summary
• Load Requirement ≈ 48.2 kVA
• With margin, spec ≈ 60.3 kVA continuous
• Standard size = 75 kVA transformer
A 75 kVA transformer reliably supports your 100 kW heat pump (COP 4) on 480 V
three-phase, handles surges, and leaves room for future needs.