Site Transformer Calculator – Connexion Electrical
Connexion Electrical

Site Transformer Calculator

Professional tool for calculating transformer parameters, load currents, turns ratio & cable sizing for construction sites.

Transformer Parameters

Enter the transformer specifications below

kVA
Enter a valid kVA rating
Select or enter voltage
Select or enter voltage
PF
Calculation Results

Transformer Efficiency Calculator

Calculate output power and efficiency percentage

VA
W
W
PF

Efficiency at Various Loads

Typical efficiency curve for site transformers

Load (%)Approximate EfficiencyNotes
25%94–96%Light load, higher iron losses dominate
50%96–97%Good efficiency range
75%97–98%Near-optimal operation
100%95–97%Full load, copper losses increase
125%+<95%⚠️ Overload — avoid sustained operation

Cable Sizing Guide

Recommended cable size based on current & run length

A
m
V
%

Standard Cable Sizes Reference

110V site cable current carrying capacity (BS 7671)

Cable Size (mm²)Max Current (A)Typical Use
1.5 mm²15ALight tools, lighting
2.5 mm²20ASmall power tools
4 mm²27AMedium tools, 16A circuits
6 mm²34A32A outlets
10 mm²46ASub-distribution boards
16 mm²61ASite transformer feeds
25 mm²80ALarge transformers

How to Use This Calculator

Quick reference guide

Step 1 – Choose Phase: Select Single Phase (1Ø) for most construction site tools. Use Three Phase (3Ø) for large industrial machinery.
Step 2 – Enter kVA Rating: Find this on the transformer nameplate. Common UK site values: 1.2kVA, 1.5kVA, 3.3kVA, 5kVA, 10kVA, 20kVA.
Step 3 – Set Voltages: Primary is usually 240V (UK mains). Secondary is typically 110V for UK construction sites (BS EN 60742 compliant).
Step 4 – Power Factor: Use 0.85 for mixed loads, 0.8 for motors, 1.0 for purely resistive heater loads.
Results explained:
• Primary Current: Current drawn from the mains supply
• Secondary Current: Current available at the output terminals
• Turns Ratio: Ratio of primary to secondary windings
• Real Power: Actual usable power (kW) accounting for power factor
⚠️ Safety Notice: All 110V site transformers should comply with BS EN 61558 & IEC 61558. The centre-tap earth system (55V to earth) greatly reduces electrocution risk. Always inspect transformer before use. If in doubt, contact a qualified electrician.
I = kVA × 1000 / V
I₃ = kVA × 1000 / (V × √3)
Turns = Vp / Vs
kW = kVA × PF

Which Transformer Do I Need?

Quick selection guide from Connexion Electrical

ApplicationRecommended SizeProduct
1–2 power tools1.2–1.5 kVAPortable 110V
Small site, 4 tools3.3 kVA3.3kVA Intermittent
Medium site5 kVA5kVA Continuous
Large site10 kVA10kVA Single Phase
Major construction20 kVA20kVA Three Phase
Site heaters3–5 kVA5kVA Heater Transformer
Calculator provided by Connexion Electrical Ltd  |  9 Carlisle Rd, London NW9 0HD  |  020 8200 5451  |  Results are indicative — always verify with a qualified electrician.

What is a Transformer kVA Calculator?

A Transformer kVA Calculator is an essential tool used to calculate the apparent power (kVA) of a transformer based on voltage and current values. Whether you’re an electrician, engineer, or student, this tool helps you quickly determine transformer capacity without complex manual calculations.

Our Transformer Calculator simplifies the process of calculating transformer kVA for both single-phase and three-phase systems.

How to Use the Transformer Calculator

Using this transformer kVA calculator is simple:

  1. Select the system type (Single Phase or Three Phase)
  2. Enter the voltage (V)
  3. Enter the current (A)
  4. Click on calculate

The tool will instantly display the transformer rating in kVA (kilovolt-amperes).

Transformer kVA Formula

Single Phase:

kVA = (Voltage × Current) / 1000

Three Phase:

kVA = (√3 × Voltage × Current) / 1000

This calculator automatically applies the correct formula, making calculating transformer kVA fast and error-free.

Why Use This Transformer Calculator?

✔ Accurate results in seconds
✔ No manual calculations required
✔ Supports single-phase & three-phase systems
✔ Ideal for engineers, electricians & students
✔ Helps in selecting the right transformer size

Applications of Transformer kVA Calculation

The transformer calculator is widely used in:

  • Electrical system design
  • Load calculation
  • Transformer sizing
  • Industrial and residential power planning
  • Energy management systems

Example of Calculating Transformer kVA

If you have:

  • Voltage = 400V
  • Current = 50A

For a three-phase system:

kVA = (1.732 × 400 × 50) / 1000 = 34.64 kVA

Using our transformer kVA calculator, you can get this result instantly without manual effort.

FAQs – Transformer Calculator

What does kVA mean in transformers?

kVA (kilovolt-amperes) represents the apparent power capacity of a transformer.

  1. Why is kVA used instead of kW?

Because transformers deal with apparent power, which includes both real power (kW) and reactive power.

  1. Is this transformer calculator accurate?

Yes, this tool uses standard electrical formulas to ensure precise results.

  1. Can I calculate both single and three-phase kVA?

Yes, this transformer kVA calculator supports both system types.