kVA Transformer Sizing Calculator

Transformer Sizing Inputs

Use the grid below. Large screens show three columns.

Phase Type

Input Method

Line Voltage (V)

Line Current (A)

Load Power (kW)

Power Factor

Efficiency (%)

Demand Factor (%)

Continuous Load Factor (%)

Design Margin (%)

Future Growth (%)

Standard Sizes (kVA)

Example Data Table

Example	Phase	Input	Voltage	Load	PF	Planning kVA	Recommended Size
Office Panel	Single	240 V, 80 A	240 V	19.20 kVA	0.95	27.32 kVA	30 kVA
Motor Panel	Three	415 V, 120 A	415 V	86.25 kVA	0.90	120.96 kVA	150 kVA
Workshop Feed	Three	95 kW	400 V	105.56 kVA	0.90	139.55 kVA	150 kVA

Formula Used

Single Phase Load kVA = (Voltage × Current) ÷ 1000

Three Phase Load kVA = (√3 × Voltage × Current) ÷ 1000

Load kVA from Power = kW ÷ Power Factor

Demand Adjusted kVA = Load kVA × Demand Factor

Planning kVA = Demand kVA × Continuous Factor × (1 + Margin) × (1 + Future Growth)

Recommended Size = Next standard rating above Planning kVA

This calculator first determines the actual load kVA. It then applies demand reduction, continuous duty allowance, design margin, and future growth. The final answer is matched to the next available standard transformer size.

Efficiency is used here to estimate source-side kVA and approximate losses. It does not replace project-specific thermal checks, harmonics review, or code compliance review.

How to Use This Calculator

Select single phase or three phase service.
Choose whether you know current or real power.
Enter system voltage and the remaining load values.
Set power factor, efficiency, and demand factor.
Add continuous duty, margin, and future growth percentages.
Review the planning kVA and recommended standard size.
Use the CSV or PDF buttons for reporting.
Compare the graph and example table for quick validation.

FAQs

1) What does kVA mean in transformer sizing?

kVA is apparent power. Transformers are rated in kVA because they supply voltage and current together, regardless of the load power factor. This makes kVA the standard sizing basis.

2) Why is power factor included?

Power factor connects real power and apparent power. If you enter kW, the calculator needs power factor to convert that value into kVA, which is the proper transformer rating unit.

3) Why add a design margin?

A design margin helps prevent undersizing. It gives room for startup peaks, small future changes, and practical operating conditions that may not appear in a simple connected load estimate.

4) What is the continuous load factor?

Continuous load factor increases the planned transformer size for long-duration loads. Many designers use 125% when a load is expected to run for extended periods.

5) Should I use single phase or three phase mode?

Use the mode that matches your actual electrical system. Single phase uses one phase relationship. Three phase uses the square root of three in the kVA and current formulas.

6) Does this calculator pick the exact transformer model?

No. It recommends a rating size. Final model selection still depends on insulation class, cooling method, impedance, harmonics, altitude, temperature, and local code requirements.

7) Why may the recommended size look larger than load kVA?

The planning value includes demand, continuous duty, design margin, and future growth. The calculator then rounds upward to the next standard transformer size for safer selection.

8) Can I change the standard size list?

Yes. Edit the standard size field with your preferred ratings. The calculator reads the list, sorts it, and selects the first available size that meets or exceeds planning kVA.