Low Voltage Transformer Sizing Calculator

Size transformers using load, margin, and derating. Review current, secondary supply, losses, and spare capacity. Make practical selections before installation, expansion, or equipment changes.

Transformer Input Form

Choose the electrical supply arrangement.
Typical low-voltage distribution side input.
Output voltage feeding downstream loads.
Common values are 50 Hz or 60 Hz.
Total installed load before diversity is applied.
Portion of connected load expected to run together.
Planned expansion allowance for extra equipment.
Used to convert kW demand into kVA size.
Accounts for transformer losses during operation.
125% is common for continuous loading review.
Extra allowance for nonlinear and electronic loads.
Reduce usable capacity for hotter surroundings.
Use lower values when cooling reduces at altitude.
Final engineering reserve before choosing a standard size.
Reset

Sizing Progress Graph

The graph tracks how margins and derating move the transformer from raw load demand to the final selected standard rating.

Example Data Table

Case Phase Primary / Secondary Connected Load Demand Factor Growth Recommended kVA Selected Standard
Office Distribution Panel 3φ 480 / 208 V 62.5 kW 80% 20% 93.44 kVA 100 kVA
Workshop Machine Feeder 3φ 415 / 240 V 118.0 kW 75% 15% 152.68 kVA 167 kVA
Control Circuit Supply 1φ 230 / 120 V 8.4 kW 90% 10% 13.92 kVA 15 kVA

Formula Used

1. Demand Load: Demand Load (kW) = Connected Load × Demand Factor
2. Growth Adjustment: Growth Adjusted Load = Demand Load × (1 + Future Growth)
3. Continuous Load Adjustment: Continuous Adjusted Load = Growth Adjusted Load × Continuous Load Factor
4. Harmonic Allowance: Harmonic Adjusted Load = Continuous Adjusted Load × (1 + Harmonic Margin)
5. Input Power: Input kW = Harmonic Adjusted Load ÷ Efficiency
6. Base Size: Base kVA = Input kW ÷ Power Factor
7. Site Derating: Derated kVA = Base kVA ÷ (Ambient Derating × Altitude Derating)
8. Final Recommendation: Recommended kVA = Derated kVA × (1 + Safety Margin)
9. Standard Selection: Choose the next standard transformer size equal to or above the recommended kVA.
10. Current: Single phase current = kVA × 1000 ÷ V. Three phase current = kVA × 1000 ÷ (√3 × V).

This method combines diversity, growth, continuous loading, nonlinear load allowance, site derating, and engineering reserve into one practical sizing workflow.

How to Use This Calculator

  1. Choose whether the transformer is single phase or three phase.
  2. Enter primary and secondary voltages for the transformer you want to evaluate.
  3. Input the total connected load in kilowatts.
  4. Apply demand factor to reflect expected simultaneous usage.
  5. Add future growth for expansion planning.
  6. Set power factor and efficiency for realistic kVA conversion.
  7. Increase continuous load factor when the load runs for long periods.
  8. Add harmonic margin for drives, computers, UPS systems, or other nonlinear loads.
  9. Enter ambient and altitude derating percentages if site conditions reduce cooling performance.
  10. Use safety margin to keep spare capacity before selecting the nearest standard transformer rating.

FAQs

1. Why is transformer sizing based on kVA instead of only kW?

Transformers carry apparent power, not just real power. kVA includes both useful power and reactive power, so it better reflects actual heating and current demand in windings.

2. What does demand factor change in the result?

Demand factor reduces the connected load to a more realistic operating load. It helps prevent oversizing when every connected device is unlikely to run at full power together.

3. Why should I add future growth?

Future growth reserves capacity for extra circuits, machines, or building changes. It lowers the chance of early replacement and helps maintain acceptable loading over the equipment life.

4. When is a harmonic margin important?

Use harmonic margin when the load includes variable frequency drives, switched-mode power supplies, UPS systems, LED drivers, or dense computer equipment. These can increase heating beyond simple linear assumptions.

5. What is the purpose of ambient and altitude derating?

Higher temperature and altitude reduce cooling effectiveness. Derating lowers usable transformer capacity so the selected unit can still operate within acceptable temperature rise limits on site.

6. Should I size exactly to the calculated kVA?

Usually no. Practical selection uses the next larger standard size above the calculated requirement. That approach preserves headroom and avoids chronic overload during normal operating variation.

7. What does the loading percentage tell me?

Loading percentage compares the recommended requirement with the chosen standard size. Lower values mean more spare capacity, while very high values suggest tighter margins and less flexibility for growth.

8. Can this replace a full electrical design review?

No. It is a planning and checking tool. Final selection should still consider code requirements, insulation class, impedance, fault duty, enclosure type, sound level, and application-specific manufacturer data.