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Understanding kVA: How to Choose the Right Site Transformer
On any construction or industrial site, reliable power is essential. A site transformer ensures tools and equipment receive a safe, consistent supply. Whether you need a 3kVA site transformer for portable tools or a 20kVA unit for larger equipment, understanding how kVA works will help you avoid costly mistakes and keep your team safe.
What is kVA and Why It Matters for Transformers?
kVA (kilovolt-amperes) is a unit that measures a transformer’s apparent power capacity in simple terms, how much load it can handle.
- kW (kilowatts) measures real power (what equipment actually consumes).
- kVA includes both real power and reactive power (losses in the system).
Most site transformers are rated in kVA because they need to account for both real and reactive loads when powering tools and machinery.
Put simply: The higher the kVA rating, the more power the transformer can safely deliver.
How to Calculate Your Site’s Power Requirements
Before you choose a site transformer, you need to calculate the total load of the equipment it will power.
Step-by-Step Guide
- List your equipment
Check the wattage (W) or kW rating on each tool or machine you plan to use. - Convert kW to kVA
Use the standard formula for site equipment:
kVA = kW ÷ Power Factor
(A power factor of 0.8 is typically used for construction tools.) - Add simultaneous loads
Only include tools that will be running at the same time. - Add a safety margin
Increase the total by at least 20% to allow for startup currents and future expansion.
Simple Example
- Two drills: 1kW each
- One lighting tower: 2kW
Total load: 4kW
Convert to kVA: 4 ÷ 0.8 = 5kVA
Choosing the Right kVA Rating for Site Transformers
The correct kVA rating depends on the tools, equipment, and number of users on site. Use the table below as a quick reference when selecting 110V site transformers.
| kVA Rating | Typical Load Capacity | Best For | Common Applications |
|---|---|---|---|
| 3kVA site transformer | Up to light-duty tools | Small tools and single users | Drills, grinders, saws, minor repairs |
| 5kVA site transformer | Multiple light tools | Small to medium site setups | Construction sites, workshops, outdoor work |
| 10kVA site transformer | Medium equipment + lighting | Multi-user environments | Larger construction projects, temporary site setups |
| 20kVA unit | Heavy-duty and industrial loads | High-demand sites | Factories, infrastructure projects, compressors, plant equipment |
Common Applications of Different kVA Site Transformers
- 3kVA Site Transformers → Portable tools, small-scale construction tasks.
- 5kVA Site Transformers → Powering site lighting, drills, and saws simultaneously.
- 10kVA Site Transformers → Multiple workers using tools and lighting together.
- 20kVA+ Site Transformers → Industrial machinery, welders, compressors, and plant equipment.
Safety and Compliance Considerations
When selecting a construction site transformer, compliance is just as important as capacity. Look out for:
- BS EN Standards → Ensure the transformer meets BS EN 61558 for site safety.
- IP Ratings → Outdoor use requires IP44 or higher for protection against dust and water.
- Step-Down to 110V → UK construction sites legally require 110V site transformers to reduce electric shock risk.
- Thermal Protection → Built-in overload and reset functions protect against overheating.
Common Mistakes to Avoid
- Under-sizing the transformer → Causes overload, overheating, and tool damage.
- Ignoring site regulations → Using 240V instead of 110V on site can lead to legal issues.
- Overloading multiple tools → Always check total power draw before plugging in.
- Skipping regular inspection → Transformers must be tested for safety and compliance.
FAQs
What is a site transformer?
A site transformer safely steps down mains power to 110V for use on construction and industrial sites.
Why are kVA ratings important?
kVA ratings show how much load a transformer can handle without overloading or overheating.
How do I choose a site transformer?
To choose a site transformer, calculate total tool load in kW, divide by 0.8, and add a safety margin.
When should I use a 3kVA site transformer?
A 3kVA site transformer is ideal for small power tools like drills and grinders.
What is a 5kVA site transformer used for?
A 5kVA site transformer powers multiple light-duty tools at the same time.