Voltage Drop Calculation in EPC Electrical Projects

Introduction

In Lesson 13, we learned how to select an electrical cable.

In Lesson 15, we learned how to trace the electrical system using a Single Line Diagram (SLD).

Now we take the next practical step:

How do you actually calculate voltage drop in a cable?

Voltage drop is an important electrical design consideration because a cable may have sufficient current-carrying capacity and still be unsuitable because of excessive voltage drop.

In real EPC projects, voltage-drop calculations help engineers ensure that equipment receives an acceptable voltage under the required operating conditions.

1. What Is Voltage Drop?

Imagine a 415 V supply feeding a motor:

415 V
  │
  ▼
MCC
  │
  │
  │────── 100 m CABLE ──────│
  │                          │
  ▼                          ▼
SOURCE                      MOTOR
415 V                        ??? V

The voltage at the motor terminals will normally be slightly lower than the voltage at the MCC.

For example:

MCC       = 415 V
Motor     = 407 V

Therefore:

Voltage Drop = 415 - 407
             = 8 V

So the voltage drop is:

8 V

In simple terms, voltage drop is the reduction in voltage between the source and the equipment receiving power.

2. Why Does Voltage Drop Happen?

A cable has electrical resistance.

In AC systems, the cable also has reactance.

When current flows through the cable:

Current
   ↓
Cable impedance
   ↓
Voltage drop

A simple way to remember it is:

The longer the cable and the higher the current, the greater the voltage drop tends to be.

Cable size also has a major influence because larger conductors generally have lower resistance.

3. Why Is Voltage Drop Important in EPC?

Excessive voltage drop can result in:

• Low voltage at equipment terminals
• Motor starting problems
• Poor motor performance
• Maloperation of sensitive equipment
• Increased losses
• Reduced system performance

Therefore, voltage drop is checked as part of electrical design.

For a motor, this becomes particularly important because the motor needs an adequate voltage at its terminals to operate and start properly.

4. What Does Voltage Drop Depend On?

Voltage drop mainly depends on several factors.

1. Current

Higher current generally produces greater voltage drop.

2. Cable Length

A longer cable generally produces greater voltage drop.

3. Cable Size

A larger conductor generally has lower resistance and therefore lower voltage drop.

4. Power Factor

Power factor affects the resistance and reactance components of voltage drop in an AC circuit.

5. Cable Reactance

Cable reactance also contributes to voltage drop in AC systems.

5. Basic Voltage-Drop Relationship

At a simple level:

Voltage Drop ≈ Current × Cable Impedance

This gives you the basic idea.

If:

Current ↑

then generally:

Voltage Drop ↑

If:

Cable Length ↑

then:

Voltage Drop ↑

If:

Cable Size ↑

then:

Resistance ↓

and therefore:

Voltage Drop ↓

This simple relationship is extremely useful when reviewing cable selections.

6. Three-Phase Voltage Drop Formula

For a three-phase AC circuit, a commonly used voltage-drop formula is:

ΔV = √3 × I × (R cosφ + X sinφ) × L

Where:

• ΔV = Voltage drop
• I = Load current
• R = Cable resistance per unit length
• X = Cable reactance per unit length
• cosφ = Power factor
• L = Cable length

⚠️ Important Unit Check

If the cable manufacturer provides resistance and reactance in:

Ω/km

then the cable length must be entered in:

km

For example:

100 m = 0.1 km

Unit consistency is essential when performing the calculation.

7. Understanding Resistance (R)

Resistance represents opposition to current flow.

For a conductor:

Smaller cable
     ↓
Higher resistance
     ↓
Higher voltage drop

A larger conductor generally has lower resistance.

Therefore:

Cable Size ↑
     ↓
Resistance ↓
     ↓
Voltage Drop ↓

This is one of the reasons an engineer may increase cable size when voltage drop is excessive.

8. Understanding Reactance (X)

Reactance is associated with the magnetic effects of AC current.

Its value depends on factors such as the cable construction and arrangement.

Reactance becomes important when calculating voltage drop accurately in AC systems.

Therefore:

Voltage drop is not always simply current multiplied by resistance.

For practical AC cable calculations, both R and X may need to be considered.

9. Understanding Power Factor

Power factor was introduced in earlier lessons.

Suppose:

PF = 0.85

In the voltage-drop equation:

cosφ = PF

Therefore:

cosφ = 0.85

We can calculate:

sinφ = √(1 - PF²)

For PF = 0.85:

sinφ = √(1 - 0.85²)

Approximately:

sinφ = 0.527

Therefore:

cosφ = 0.85
sinφ ≈ 0.527

These values can then be used in the voltage-drop calculation.

10. Practical Voltage-Drop Example

Let’s take a 75 kW motor.

Motor Data

Assume the cable data is:

R = 0.125 Ω/km
X = 0.080 Ω/km

These values are example values for learning.

First convert the cable length:

100 m = 0.1 km

Now use:

ΔV = √3 × I × (R cosφ + X sinφ) × L

Therefore:

ΔV = √3 × 137 ×
     [(0.125 × 0.85) +
      (0.080 × 0.527)] × 0.1

Let’s calculate it step by step.

11. Calculate the Resistance Component

First:

R cosφ

Therefore:

= 0.125 × 0.85

= 0.10625

12. Calculate the Reactance Component

Now:

X sinφ

Therefore:

= 0.080 × 0.527

Approximately:

= 0.0422

13. Add the Components

Now:

R cosφ + X sinφ

Therefore:

= 0.10625 + 0.0422

≈ 0.14845

14. Calculate the Voltage Drop

Now substitute the values:

ΔV = √3 × 137 × 0.14845 × 0.1

Approximately:

ΔV ≈ 3.53 V

Therefore, the cable voltage drop is approximately:

3.5 V

15. Calculate Voltage-Drop Percentage

The percentage voltage drop is:

%ΔV = (ΔV / V) × 100

Therefore:

%ΔV = (3.53 / 415) × 100

Approximately:

0.85%

So the result is:

Voltage drop ≈ 3.5 V
Voltage drop ≈ 0.85%

Whether this is acceptable depends on the allowable voltage-drop limit specified by the project and applicable design requirements.

16. Calculate Motor Terminal Voltage

Suppose the source voltage is:

415 V

and the calculated voltage drop is:

3.5 V

Then:

Motor Voltage = 415 - 3.5

Therefore:

Motor terminal voltage ≈ 411.5 V

The simplified system becomes:

MCC
415 V
  │
  │
  │ Cable
  ▼
Motor
≈411.5 V

This shows what voltage the motor approximately receives after the cable voltage drop.

17. What Happens If the Cable Is Longer?

Suppose the cable length increases from:

100 m → 200 m

If all other conditions remain approximately the same:

Length ×2
     ↓
Voltage Drop ×2

Our previous voltage drop was approximately:

3.5 V

For 200 m, it would be approximately:

7.1 V

This demonstrates why cable route length is an important input in cable design.

18. What Happens If Current Increases?

Suppose the cable current increases.

For example:

Current = 137 A

compared with:

Current = 250 A

The higher current generally produces greater voltage drop.

Therefore:

High-current feeders require careful voltage-drop consideration.

This becomes particularly important for large motors and high-power industrial equipment.

19. What Happens If Cable Size Increases?

Suppose we have:

70 mm² cable

and increase the conductor size to:

120 mm²

The larger conductor generally has lower resistance.

Therefore:

Cable Size ↑
     ↓
Resistance ↓
     ↓
Voltage Drop ↓

This is one of the most common ways engineers reduce excessive voltage drop.

However, increasing cable size is not the only design consideration. The final selection must also satisfy the other applicable design checks.

20. Voltage Drop vs Current-Carrying Capacity

This distinction is extremely important in EPC electrical design.

Imagine:

70 mm² Cable

Current Capacity
      ↓
     PASS ✓

Voltage Drop
      ↓
     FAIL ✗

Can the cable be accepted?

No.

Even though the cable can carry the required current, the voltage drop does not meet the design requirement.

The engineer may then evaluate a larger cable:

70 mm²
   ↓
95 mm²
   ↓
120 mm²

until the relevant design criteria are satisfied.

Therefore:

Current-carrying capacity and voltage drop are separate checks.

21. Normal Voltage Drop vs Starting Voltage Drop

For motors, voltage drop may need to be considered under different operating conditions.

Normal Running

Motor current might be:

137 A

Starting

Motor current might temporarily be:

6 × 137 = 822 A

Therefore:

Running:
137 A
  ↓
Normal voltage drop

Starting:
822 A
  ↓
Much higher voltage drop

This is why large motors may require a motor-starting study.

22. Starting Voltage Drop

Suppose the normal running voltage drop is:

3.5 V

If starting current is approximately six times the running current, a simple proportional view suggests that the voltage drop during starting could be much higher.

However:

⚠️ Do not simply multiply the normal voltage drop by the starting-current ratio for a final engineering calculation.

Motor-starting analysis can involve:

• Motor starting impedance
• Network impedance
• Transformer impedance
• Generator impedance
• Cable impedance
• Other connected loads
• Motor acceleration

Therefore, a detailed motor-starting study is more comprehensive than a simple proportional calculation.

23. Voltage Drop Across the Entire System

Don’t always consider only one cable.

An industrial electrical system may look like:

Transformer
    │
    │ Cable 1
    ▼
PCC
    │
    │ Cable 2
    ▼
MCC
    │
    │ Cable 3
    ▼
Motor

There can be voltage drops across several sections.

Therefore, the engineer may need to consider the overall voltage drop from the source to the final load.

Conceptually:

Transformer
   ↓
Cable
   ↓
PCC
   ↓
Cable
   ↓
MCC
   ↓
Cable
   ↓
Motor

The relevant voltage drops across the system contribute to the voltage available at the final load.

24. Voltage Drop and Transformer

A transformer can also contribute to voltage variation because of its impedance.

Therefore, from a system perspective, we should not think only about:

Transformer → Cable → Motor

We can think of the system as:

Source
  ↓
Transformer impedance
  ↓
Switchboard
  ↓
Cable impedance
  ↓
Motor

During heavy loading or motor starting, these effects become more important.

25. Voltage Drop and Generator

Generator-fed systems can be particularly sensitive during motor starting.

Consider:

GENERATOR
    │
    ▼
TRANSFORMER
    │
    ▼
MCC
    │
    ▼
LARGE MOTOR

When the motor starts:

Motor starting current
       ↓
Generator voltage response
       ↓
Transformer voltage variation
       ↓
Cable voltage drop
       ↓
Motor terminal voltage

Therefore, motor-starting studies can be particularly important for generator-fed systems.

26. Voltage Drop in Lighting and Small Power

Voltage drop is not only a motor issue.

It can also be relevant to:

• Lighting
• Small power
• Heating loads
• UPS feeders
• Control systems
• Instrumentation
• Long-distance distribution

The allowable voltage drop depends on the project’s design criteria and applicable standards.

27. Information Required for Voltage-Drop Calculation

Before performing a voltage-drop calculation, collect the required information.

Load Information

• Load power
• Load current
• Power factor

System Information

• System voltage
• Single-phase or three-phase system

Cable Information

• Cable size
• Cable resistance
• Cable reactance
• Cable type

Installation Information

• Cable length
• Number of parallel runs
• Installation arrangement

Once these inputs are available, the voltage-drop calculation can be performed.

28. Practical EPC Voltage-Drop Workflow

A useful EPC workflow is:

LOAD
  ↓
LOAD CURRENT
  ↓
SYSTEM VOLTAGE
  ↓
CABLE SIZE
  ↓
CABLE R & X
  ↓
CABLE LENGTH
  ↓
POWER FACTOR
  ↓
VOLTAGE DROP
  ↓
VOLTAGE DROP %
  ↓
COMPARE WITH PROJECT LIMIT
  ↓
PASS / FAIL
  ↓
IF FAIL → REVIEW / INCREASE CABLE SIZE

This is the practical design process you should remember.

29. Cable Selection Is an Iterative Process

Cable selection in EPC projects is not always a one-step decision.

For example, you may initially select:

70 mm²

Then check:

Current Check → PASS
Voltage Drop → FAIL

So you evaluate:

95 mm²

Then:

Current Check → PASS
Voltage Drop → PASS

But the design still needs to verify other relevant requirements:

Short Circuit → PASS?
Installation → PASS?
Protection → PASS?
Termination → PASS?

Only after the applicable checks are satisfied should the cable selection be finalized.

30. Example of an Engineering Decision

Suppose we have:

Can the cable be approved?

❌ No.

The voltage-drop requirement has failed.

The engineer may evaluate alternative sizes:

The objective is not simply to select the biggest cable.

The objective is to select a technically suitable and practical cable that satisfies the applicable design requirements.

🧠 The EPC Engineer’s Mental Model

Whenever you hear:

Voltage Drop

think:

CURRENT
   +
CABLE IMPEDANCE
   +
CABLE LENGTH
   ↓
VOLTAGE DROP

And remember:

Higher Current
      ↓
Higher Drop

Longer Cable
      ↓
Higher Drop

Larger Cable
      ↓
Lower Resistance
      ↓
Lower Drop

This simple mental model will help you understand many cable-sizing decisions in EPC projects.

Key Takeaways

After completing this lesson, you should understand that:

1. Voltage drop is the reduction in voltage between the source and load.
2. Cable resistance and reactance contribute to AC voltage drop.
3. Longer cables generally produce greater voltage drop.
4. Higher current generally produces greater voltage drop.
5. Larger conductors generally reduce voltage drop.
6. A cable can pass the current-capacity check and still fail the voltage-drop check.
7. Motor starting can produce much greater voltage drop than normal running.
8. Transformer and generator characteristics can also influence system voltage during heavy loading and motor starting.
9. Cable selection is an iterative engineering process, not simply a matter of choosing the largest cable.

🎯 EPC Engineer Exercise

A 100 kW motor has:

• Voltage = 415 V
• PF = 0.85
• Efficiency = 95%
• Cable length = 150 m

Try answering these questions:

1.

What is the approximate motor full-load current?

2.

If cable resistance is 0.10 Ω/km and reactance is 0.08 Ω/km, what information is required before calculating voltage drop?

3.

Why do we convert 150 m into kilometres when R and X are given in Ω/km?

4.

If the cable length doubles, what generally happens to voltage drop?

5.

If cable size is increased, what generally happens to resistance?

6.

Can a cable pass current capacity but fail voltage drop?

7.

Why is starting voltage drop important for a large motor?

8.

Which generally gives lower voltage drop, 70 mm² or 120 mm², assuming the same cable type and installation conditions?

🚀 Electrical EPC Learning Path

So far, the course has progressed through:

LESSON 13
Cable Sizing
      ↓
LESSON 14
Motor Electrical Design
      ↓
LESSON 15
Practical SLD Reading
      ↓
LESSON 16
VOLTAGE DROP

The next major calculation is:

⚡ Lesson 17 — Short-Circuit Calculation

In the next lesson, we’ll answer one of the most important questions in electrical design:

“If a fault occurs at this point in the electrical system, how much current will flow?”

We’ll connect transformer impedance → fault current → kA → breaker short-circuit rating → switchgear selection → protection.

For example, we’ll see why a 1000 kVA transformer with 6% impedance at 415 V can produce a prospective fault current of roughly 23 kA at its LV terminals.

That will connect the calculations you’ve learned so far directly to practical switchgear and protection design. ⚡🔥