A transformer is rated in kVA, not in kW.

Because a transformer supplies both real power and reactive power.

📌 Power Types in Electrical System

Relationship:$$kVA = \frac{kW}{Power\ Factor}$$kVA=Power FactorkW​

🔌 Simple Example

Suppose the Demand Load = 80 kW

Power factor of building = 0.8

Transformer must supply apparent power.$$kVA = \frac{80}{0.8}$$kVA=0.880​ $$kVA = 100\ kVA$$kVA=100 kVA

👉 So we select 100 kVA transformer.

💡 Why Transformer Cannot Be Rated in kW

Transformer does not know the load power factor.

It only sees:

• Voltage
• Current

And$$kVA = Voltage \times Current$$kVA=Voltage×Current

So the transformer capacity is based on current carrying capability, not real power.

⚡ Practical Electrical Design Steps

Engineers usually do:

1️⃣ Calculate Connected Load (kW)
2️⃣ Apply Demand Factor → get Demand Load (kW)
3️⃣ Convert kW → kVA using power factor
4️⃣ Select nearest higher transformer rating

Example:

Connected Load = 120 kW
Demand Factor = 0.7

Demand Load
= 120 × 0.7 = 84 kW

Power Factor = 0.8

Transformer Size$$kVA = 84 / 0.8 = 105\ kVA$$kVA=84/0.8=105 kVA

👉 Select 125 kVA transformer.

✅ In one sentence:
Demand load is converted to kVA because transformers supply apparent power (kVA), not real power (kW).

Why Transformers Are Usually Loaded Only 70–80% in Design

Engineers usually do not run a transformer at 100% continuously. Instead, they design it to operate around 70–80% loading. There are several practical reasons.

1️⃣ Future Load Expansion 📈

Buildings or industries often add more equipment later.

Example:

• Current demand = 80 kVA
• If you install a 100 kVA transformer, it will run at 80% load.

Later if more machines or AC units are added, the transformer still has extra capacity.

2️⃣ Heat and Insulation Life 🌡️

Transformers produce heat due to losses.

If a transformer runs near 100% continuously:

• Temperature increases
• Insulation ages faster
• Transformer life reduces

Running at 70–80% keeps temperature lower, increasing equipment life.

3️⃣ Safety Margin ⚠️

Actual load sometimes changes suddenly.

Examples:

• Motors starting
• Multiple loads turning ON at once
• Seasonal loads (summer AC load)

The extra 20–30% margin prevents overloading.

4️⃣ Voltage Stability 🔌

When transformers are heavily loaded:

• Voltage drop increases
• Power quality reduces

Operating below full capacity helps maintain stable voltage.

🔧 Simple Practical Example

Demand Load = 80 kVA

Instead of selecting 80 kVA transformer, engineers choose:

➡ 100 kVA transformer

Load percentage:$$\frac{80}{100} \times 100 = 80\%$$10080​×100=80%

So the transformer runs at 80% load, which is safe.

✅ Short Answer:
Transformers are designed for 70–80% loading to allow future expansion, reduce heating, improve reliability, and avoid overloading.

💡 Since you’re learning electrical design and SLD, another very important concept is:

Connected Load → Demand Load → Maximum Demand → Transformer Size