Views: 0 Author: Zhejiang Shengxian Electric Technology Co., Ltd Publish Time: 2026-07-07 Origin: Zhejiang Shengxian Electric Technology Co., Ltd
"My factory has a total load of about 300kW – what size transformer do I need?"
This is one of the most common questions customers ask. Choose too small, and the transformer runs overloaded – excessive heating, shortened lifespan. Choose too big, and you are paying higher demand charges for capacity you don't use.
This article gives you a simple method to estimate transformer capacity – so you know what size to choose and why.
This is the most common confusion customers have.
Unit | Meaning | Explanation |
|---|---|---|
kW (kilowatt) | Active power | The actual power consumed by equipment |
kVA (kilovolt-ampere) | Apparent power | Transformer capacity rating – includes both active and reactive power |
Power factor | kW ÷ kVA | Typically around 0.8 (lower with many motors) |
The relationship:
kVA × Power Factor = kWExample: A 1000kVA transformer at 0.8 power factor can supply approximately 800kW of active load – not 1000kW.
This is why you cannot size a transformer just by adding up kW loads. You need to convert to kVA.
Step 1: Calculate total load (kW)
Add up the rated power of all equipment
Note: not all equipment runs simultaneously (apply a diversity factor of 0.7–0.9)
Step 2: Determine power factor
General factory: around 0.8
Motor-heavy factory: 0.7–0.8
With correction capacitor bank: 0.9 or above
Step 3: Calculate transformer capacity (kVA)
Transformer capacity (kVA) = Total load (kW) ÷ Power Factor ÷ Load FactorLoad factor: recommended 70–80% (the economic operating range for transformers)
Actual Load (kW) | Power Factor | Recommended Transformer (kVA) | Note |
|---|---|---|---|
50 | 0.8 | 100 | With margin |
100 | 0.8 | 160–200 | Common configuration |
200 | 0.8 | 315–400 | |
300 | 0.8 | 500 | |
500 | 0.8 | 800 | |
800 | 0.8 | 1250 | |
1000 | 0.8 | 1600 |
These are reference values. Actual selection should consider load type and diversity factor.
Application | Capacity Range | Common Ratings |
|---|---|---|
Small factory, shop | 50–200kVA | 100, 160, 200kVA |
Medium factory, shopping mall | 200–800kVA | 315, 400, 500, 630, 800kVA |
Large factory, commercial building | 800–2500kVA | 1000, 1250, 1600, 2000, 2500kVA |
Situation | Suggestion |
|---|---|
Current load is near capacity, with future expansion planned | Choose one size up (e.g., 500kVA → 630kVA) |
Current load is moderate, future is uncertain | Size for current needs – expand later if needed |
Severe phase unbalance | Size based on highest phase load × 3 |
Multiple transformers | Choose same ratings if possible – easier for spares and parallel operation |
Oil-immersed: 100, 125, 160, 200, 250, 315, 400, 500, 630, 800, 1000, 1250, 1600, 2000, 2500kVA
Dry-type: 100, 125, 160, 200, 250, 315, 400, 500, 630, 800, 1000, 1250, 1600, 2000, 2500kVA
Item | Check |
|---|---|
Current total load (kW) | □ |
Power factor (with or without correction?) | □ |
Any expansion planned in the next 2–3 years? | □ |
Redundancy required (N+1)? | □ |
Voltage level (10kV / 35kV / other) | □ |
Installation environment (indoor/outdoor, IP rating) | □ |
The principle for transformer sizing is simple: size for actual needs, leave moderate margin – don't oversize, don't undersize.
Too big: Higher demand charges, slower return on investment
Too small: Overload risk, frequent tripping
Right size: Stable operation, cost-effective in the long run
If you have a specific project and need sizing advice, send us your total load and load type – we can help you calculate the recommended capacity.
