Transformer Sizing in EPC

Electrical EPC Design Course — Beginner to Professional Level

Transformer sizing is a very important EPC electrical engineering skill because it connects several concepts you’ve already learned:

Load List → kW → Power Factor → kVA → Design Margin → Transformer Selection

In this lesson, we’ll learn the basic transformer-sizing workflow step by step. You don’t need to memorize everything at once. 👍

1. What Does a Transformer Do?

A transformer transfers electrical power between circuits and changes the voltage.

For example:

        11 kV
          │
          ▼
   ┌──────────────┐
   │ Transformer  │
   │ 11 / 0.415 kV│
   └──────────────┘
          │
          ▼
        415 V

In an industrial plant, a transformer may supply:

• MCCs
• PCCs
• Pumps
• Fans
• Lighting
• Small power
• HVAC systems

The transformer therefore becomes an important part of the plant’s electrical distribution system.

2. Why Is a Transformer Rated in kVA?

This is extremely important.

You may remember the relationship:

kVA = kW / Power Factor

But why don’t we simply rate transformers in kW?

Because transformer loading is fundamentally related to voltage and current, and its thermal loading is associated with apparent power.

Therefore, transformers are commonly rated in:

• 500 kVA
• 1000 kVA
• 1600 kVA
• 2000 kVA
• 2500 kVA
• 3150 kVA
• 5 MVA
• 10 MVA

and other standard ratings.

3. Start With the Electrical Load List

Let’s assume our LV transformer supplies the following loads:

Total Connected Load

Adding all the loads:

100 + 100 + 400 + 50 + 50 + 40 + 20

Total connected load = 760 kW

But wait! 🚨

Don’t immediately select a 760 kVA transformer.

There are several more steps to complete.

4. Step 1 — Determine the Operating or Demand Load

Remember what we learned in Lesson 4 — Electrical Load List.

Not every installed load necessarily operates at the same time.

Suppose the operating philosophy is:

• Pump 1 → Duty
• Pump 2 → Standby
• Fan 1 → Duty
• Fan 2 → Standby
• Compressor → Running
• Lighting → Running
• Small Power → Running

Therefore, the normal running load is:

100 + 400 + 50 + 40 + 20

Normal operating load = 610 kW

So now we have:

Connected Load

760 kW

Normal Operating Load

610 kW

This is why the electrical load list is so important.

5. Step 2 — Convert kW to kVA

Now suppose the design power factor is:

0.85

The basic relationship is:

kVA = kW / PF

Therefore:

kVA = 610 / 0.85

Approximately:

718 kVA

So our transformer needs to support approximately:

718 kVA

before considering additional design requirements.

6. Step 3 — Add Design Margin

Suppose the project requires:

20% spare capacity

Then:

718 × 1.20 = 862 kVA

Therefore, our calculated requirement becomes approximately:

862 kVA

7. Step 4 — Select a Standard Transformer Rating

We generally don’t say:

“Let’s manufacture an 862 kVA transformer.”

Instead, we select an appropriate standard transformer rating that meets the calculated requirement.

For example:

1000 kVA

could be a possible selection.

So the basic thought process becomes:

Operating Load
      ↓
    610 kW
      ↓
Power Factor
      ↓
    718 kVA
      ↓
20% Design Margin
      ↓
    862 kVA
      ↓
Standard Rating
      ↓
1000 kVA Transformer

🎯 This is the basic transformer-sizing workflow.

8. But We’re Not Finished Yet!

A professional EPC engineer doesn’t stop after:

862 kVA → 1000 kVA

We need to ask several more questions before finalizing the transformer.

9. Transformer Primary Voltage

First, we need to know:

What voltage is coming into the transformer?

For example:

11 kV

This would be the transformer high-voltage side in our example.

10. Transformer Secondary Voltage

Next, we need to know:

What voltage do we need on the secondary side?

For example:

415 V

Therefore, a simplified transformer specification could be:

1000 kVA, 11/0.415 kV Transformer

However, an actual EPC transformer specification contains many more parameters.

11. Transformer Impedance

You may see a transformer specification such as:

% Impedance = 6%

or another project-specific value.

Transformer impedance is important because it affects:

• Short-circuit current
• Voltage drop
• Fault level
• Parallel operation
• Overall system performance

We will study this in detail when we reach short-circuit calculations.

For now, remember:

Transformer impedance is an important design parameter—not just a nameplate number.

12. Transformer Cooling

You may encounter transformer cooling terms such as:

• ONAN
• ONAF

For example:

ONAN = Oil Natural Air Natural

ONAF = Oil Natural Air Forced

These terms describe the transformer cooling method.

Don’t worry about memorizing them yet.

We’ll cover transformer construction and technical specifications separately.

13. Transformer Redundancy

Now let’s make our example more realistic.

Suppose the plant requires:

2000 kVA

Do we necessarily install:

1 × 2000 kVA transformer?

Not always.

The project philosophy may require redundancy.

For example, a possible arrangement could be:

             11 kV BUS
              │    │
              │    │
              ▼    ▼
            TX-1  TX-2
           1000   1000
           kVA    kVA
              │    │
              └─┬──┘
                │
                LV

Another arrangement may be selected depending on the required reliability and project philosophy.

14. What Does Transformer Redundancy Mean?

Simply:

If one piece of equipment fails, another can continue supplying the required load.

For example:

TX-1 → Normal Supply
TX-2 → Normal / Backup Arrangement

However, exactly how much load the remaining transformer must carry depends on the project philosophy.

You may hear terms such as:

• N
• N+1
• 2N

These represent different redundancy philosophies.

We’ll study them in more detail later.

15. Transformer Sizing Is Not Just About kW

This is one of the most important lessons in transformer sizing.

Suppose someone tells you:

“Plant load = 2000 kW.”

You cannot immediately say:

“Use a 2500 kVA transformer.”

You need to understand several factors.

Load

What loads are connected?

Demand

What loads operate simultaneously?

Power Factor

What is the actual kVA requirement?

Future Load

Will the plant expand?

Spare Capacity

What margin does the client require?

Motor Starting

Can the transformer handle important starting conditions?

Harmonics

Are there VFDs or other nonlinear loads?

Redundancy

What happens if one transformer fails?

Environment

What are the ambient temperature, altitude, installation and cooling conditions?

Standard Rating

What suitable standard transformer rating is available?

This is how an EPC engineer thinks.

16. Motor Starting Can Affect Transformer Selection

This connects directly to Lesson 9 — Motor Starting Methods.

Suppose:

Transformer = 1000 kVA

and a large motor starts.

The motor’s high starting current can cause a voltage dip.

Therefore, the transformer may have enough steady-state kVA capacity, but the starting condition could still be unacceptable.

So remember:

Transformer sizing must consider both running conditions and important transient or starting conditions.

This is a very important EPC concept.

17. Transformer Full-Load Current

Another useful calculation for EPC engineers is transformer full-load current.

For a three-phase transformer:

I = S / (√3 × V)

Where:

• I = Current in amperes
• S = Apparent power in VA
• V = Line voltage in volts

Suppose:

Transformer = 1000 kVA

Secondary voltage:

415 V

Then:

I = 1,000,000 / (√3 × 415)

Approximately:

1391 A

So the transformer secondary full-load current is approximately:

1390 A

This value becomes important when selecting:

• LV switchboard incomer
• Busbar
• Circuit breaker
• Cable or busduct

18. See the Connection?

Look at what just happened:

Transformer
1000 kVA
    ↓
Secondary Current
≈ 1391 A
    ↓
LV Switchboard
    ↓
Incomer Breaker
    ↓
Busbar
    ↓
Outgoing Feeders

One transformer-sizing decision affects several other parts of the electrical system.

That’s why EPC electrical design is highly interconnected.

19. A Realistic EPC Transformer Sizing Flow

Eventually, you’ll be able to follow this complete engineering chain:

             LOAD LIST
                 ↓
         Connected Loads
                 ↓
         Operating Loads
                 ↓
        Demand / Diversity
                 ↓
                kW
                 ↓
          Power Factor
                 ↓
               kVA
                 ↓
          Future Margin
                 ↓
          Motor Starting
                 ↓
            Reliability
                 ↓
          Standard Rating
                 ↓
           TRANSFORMER
                 ↓
       Short-Circuit Study
                 ↓
        Switchgear Rating

🔥 This is the engineering chain I want you to understand.

🧠 Lesson 10 — Remember These 6 Things

Don’t try to memorize everything.

Focus on these six points:

1. Transformer Changes Voltage

A transformer transfers electrical power between circuits while changing the voltage level.

2. Transformer Rating Is Normally in kVA or MVA

Transformer capacity is generally expressed in apparent power.

3. Remember the kW-to-kVA Relationship

kVA = kW / PF

4. Don’t Use Connected Load Blindly

Consider actual operating and demand conditions.

5. Add the Required Project Margin

Consider future load and the spare capacity required by the project.

6. Check More Than Steady-State Load

Consider:

Starting + Fault Level + Redundancy + Future Expansion + Project Specifications

🎯 Practical EPC Exercise

Let’s test your understanding.

A transformer supplies the following loads:

Operating Philosophy

• Pump A = Duty
• Pump B = Standby
• Pump C = Running
• Compressor = Running
• Fan A = Duty
• Fan B = Standby
• Lighting = Running

Design Information

Power Factor = 0.8

Future/Spare Margin = 20%

Calculate:

1. What is the connected load?

2. What is the normal running load?

3. What is the running kVA?

4. What is the kVA after adding 20% margin?

5. Based on these simplified assumptions, would a 1000 kVA standard transformer be reasonable?

Don’t worry if you make a mistake.

The goal is not just to get the answer. The goal is to understand the engineering process.

🚀 What Comes Next?

So far, we’ve learned how to calculate a basic transformer capacity from the electrical load.

Now we’ll go one step deeper.

⚡ Lesson 11 — Transformer Selection & Technical Specification

In the next lesson, we’ll learn what the important parameters on a real EPC transformer datasheet mean, including:

• MVA
• HV/LV voltage
• Transformer impedance
• Vector group
• Tap changer
• Neutral
• Insulation level
• Cooling
• Losses
• Temperature rise
• Transformer specifications

You’ll start learning how to read and understand an actual transformer datasheet like an EPC electrical engineer. ⚡