Cable Sizing in EPC Electrical Projects

Cable sizing is one of the most practical calculations an electrical engineer performs in an EPC project.

The basic question is:

How do I select the correct cable for a load?

In the previous lessons, we followed this electrical design sequence:

Load
  ↓
Load Calculation
  ↓
Transformer Selection
  ↓
Switchgear Selection

Now we go one step further:

Switchgear
     ↓
   CABLE
     ↓
    LOAD

Cable selection may look simple at first. For example:

“The motor current is 100 A, so I’ll select a 100 A cable.”

❌ That is not enough for proper EPC electrical design.

A practical cable sizing calculation considers several factors, including:

• Current-carrying capacity
• Cable derating
• Voltage drop
• Short-circuit withstand
• Installation method
• Cable grouping
• Number of cable runs
• Protection requirements

Let’s understand cable sizing step by step.

1. What Does a Cable Have to Do?

A cable safely transfers electrical power from the source to the load.

For example:

       415 V MCC
           │
           │
        CABLE
           │
           ▼
         MOTOR

A properly selected cable must be capable of:

① Carrying the Load Current

The cable must safely carry the required operating current without excessive heating.

② Maintaining Acceptable Voltage

The voltage at the load must remain within the allowable voltage-drop limit specified by the project.

③ Withstanding Fault Conditions

During a short circuit, the cable must withstand the thermal stress until the protective device clears the fault.

Therefore:

Cable sizing is not simply an ampere-rating calculation.

2. Step One — Calculate Load Current

Before selecting a cable, we first need to determine the design current.

For a three-phase motor:

I = P / (√3 × V × PF × η)

Where:

• P = motor output power in watts
• V = line voltage
• PF = power factor
• η = motor efficiency

For motors, efficiency is considered because the electrical input power is higher than the mechanical output power.

3. Example — 100 kW Motor

Suppose we have:

• Motor power = 100 kW
• Voltage = 415 V
• Power factor = 0.85
• Efficiency = 95%
• System = 3-phase

Then:

I = 100000 / (√3 × 415 × 0.85 × 0.95)

The approximate full-load current is:

163 A

So the motor’s calculated full-load current is approximately:

163 A

Is cable selection finished?

No.

This is only the starting point.

4. Step Two — Apply Cable Derating

One of the most important concepts in cable sizing is derating.

A cable’s catalogue current-carrying capacity does not necessarily represent the capacity available under every installation condition.

Cable ampacity can be affected by:

• Ambient temperature
• Number of loaded cables
• Cable grouping
• Installation method
• Soil thermal conditions for underground cables
• Enclosure or duct conditions
• Cable arrangement

Therefore, correction or derating factors may need to be applied.

The basic concept is:

Catalogue Current Capacity
          ↓
Temperature Factor
          ↓
Grouping Factor
          ↓
Installation Factor
          ↓
Actual Allowable Capacity

5. Simple Derating Example

Suppose a cable has a tabulated current-carrying capacity of:

240 A

The applicable combined derating factor is:

0.80

Then:

Allowable Current = 240 × 0.80

Allowable Current = 192 A

Our motor current is:

163 A

Therefore:

Cable allowable capacity = 192 A
Motor design current      = 163 A

192 A > 163 A
       ✓

The cable passes the basic current-carrying check.

⚠️ However, this does not mean the cable is finally selected. Other checks are still required.

6. The Basic Cable Selection Rule

A useful engineering relationship is:

Ib ≤ Iz

Where:

• Ib = design or load current
• Iz = permissible cable current-carrying capacity after applicable correction factors

In simple words:

The cable’s allowable current-carrying capacity must be greater than or equal to the required design current.

For our example:

Motor current
     ↓
    163 A

Cable allowable capacity
     ↓
    192 A

192 A > 163 A
       ✓

7. Step Three — Check Voltage Drop

Now imagine the cable is very long.

The cable may have sufficient current-carrying capacity but still cause excessive voltage drop.

For example:

415 V
 │
 ▼
SOURCE
 │
 │──────────── LONG CABLE ────────────│
 │                                    │
 ▼                                    ▼
415 V                                390 V
SOURCE                                MOTOR

The motor terminal voltage is lower than the source voltage.

This difference is called:

Voltage drop

8. Why Is Voltage Drop Important?

Excessive voltage drop can cause problems such as:

• Poor motor performance
• Difficulty starting motors
• Reduced equipment performance
• Possible equipment malfunction
• Increased electrical losses

Therefore, voltage drop must be checked during cable sizing.

9. Cable Size Can Increase Because of Voltage Drop

This is a very important EPC design concept.

Suppose the current calculation suggests:

70 mm² cable

But the cable run is very long.

The 70 mm² cable may pass the current-carrying capacity check but fail the voltage-drop requirement.

The engineer may therefore need to increase the cable size, for example to:

95 mm²

or

120 mm²

depending on the actual calculation.

Therefore:

Cable size is not always determined by current capacity alone.

10. Basic Voltage-Drop Calculation

For a three-phase circuit, voltage drop can be approximately represented by:

ΔV = √3 × I × (R cosφ + X sinφ) × L

Where:

• I = load current
• R = cable resistance
• X = cable reactance
• cosφ = power factor
• L = cable length

The exact calculation method depends on the cable data, installation arrangement and applicable design standard.

For now, remember:

Longer cable + higher current + smaller conductor generally means greater voltage drop.

11. Voltage Drop Percentage

Voltage drop is normally expressed as a percentage of the system voltage.

% Voltage Drop = (ΔV / Vsystem) × 100

For example:

• System voltage = 415 V
• Voltage drop = 8 V

Then:

% Voltage Drop = (8 / 415) × 100

Approximately:

1.93%

Whether this value is acceptable depends on the project’s specified limits and applicable design criteria.

12. Step Four — Short-Circuit Withstand

Now we come to another important cable-sizing check.

Imagine a short circuit occurs:

SOURCE
  │
 CABLE
  │
  X ← SHORT CIRCUIT

During the fault, a very large current may flow through the cable for a short period.

This can cause a rapid increase in conductor temperature.

Therefore, the cable must be capable of withstanding the fault until the protective device clears it.

13. Short-Circuit Cable Check

A commonly used simplified adiabatic relationship is:

S ≥ I√t / k

Where:

• S = conductor cross-sectional area
• I = short-circuit current
• t = fault-clearing time
• k = material- and insulation-dependent constant

This gives us an important engineering insight:

Higher fault current → larger required conductor

Longer fault duration → larger required conductor

Therefore:

Cable sizing must also consider the short-circuit study and protective-device clearing time.

14. Three Major Cable Sizing Checks

At this stage, remember the three major checks:

             CABLE SIZING
                  │
       ┌──────────┼──────────┐
       ▼          ▼          ▼
Current Capacity  Voltage Drop  Short Circuit
       │          │          │
       ▼          ▼          ▼
   Heating      Load Voltage   Fault
    Check         Check       Withstand

A cable that passes only one of these checks is not necessarily acceptable.

15. Cable Installation Method

The same cable can have different current-carrying capacities depending on its installation method.

For example:

Cable Tray

────────────────
 │ │ │ │ │ │ │
 C C C C C C C
────────────────

Conduit

┌──────────────────┐
│ C   C   C        │
└──────────────────┘

Underground

GROUND
────────────────
      ↓
   ───────
    CABLE
   ───────

The surrounding thermal conditions affect how effectively heat can escape.

Therefore, the installation method must be known before finalizing cable ampacity.

16. Cable Grouping

Consider one cable installed by itself:

C

Now compare it with multiple loaded cables installed together:

C C C C C C C C

Grouped cables can generate more heat and have less effective cooling.

Therefore, a grouping or bunching correction factor may be required.

This is why cable schedules and installation layouts need information such as:

• Number of cables
• Number of circuits
• Tray arrangement
• Cable spacing
• Installation configuration

17. Single-Core vs Multicore Cable

In EPC projects, you will encounter both single-core and multicore cables.

Multicore Cable

Several conductors are contained within one cable.

┌───────────────┐
│ ○  ○  ○  ○    │
└───────────────┘

Single-Core Cables

Each conductor is installed as an individual cable.

○       ○       ○
R       Y       B

The choice depends on factors such as:

• System voltage
• Load current
• Installation method
• Cable size
• Fault level
• Project requirements
• Manufacturer data

For large LV currents, multiple single-core cables per phase may be required.

18. Parallel Cables

Sometimes one cable cannot carry the required current economically or practically.

Multiple cables may then be installed in parallel.

For example:

Phase R:
[Cable] [Cable]

Phase Y:
[Cable] [Cable]

Phase B:
[Cable] [Cable]

This can increase the total current-carrying capacity.

However, parallel cables must be properly designed to ensure acceptable:

• Current sharing
• Thermal performance
• Installation arrangement
• Impedance balance

You cannot simply add cables randomly.

19. Cable and Breaker Must Work Together

In Lesson 12, we discussed switchgear and circuit breakers.

Now connect that concept with cable sizing:

       SOURCE
          │
        ACB
          │
        CABLE
          │
        MCCB
          │
         LOAD

The breaker protects the circuit, while the cable carries the load current.

The overall design must ensure:

• Cable can carry the design current
• Thermal limits are respected
• Protective device is suitable
• Fault protection is adequate
• Protection coordination is considered

Therefore:

Cable selection and protection-device selection are interconnected.

20. Complete Cable Selection Example

Let’s take our 100 kW motor again.

Load

100 kW motor

System

415 V, 3-phase

Power Factor

0.85

Efficiency

95%

Calculated current:

≈163 A

Suppose a preliminary cable has a catalogue current capacity of:

240 A

After applying a combined derating factor of 0.80:

240 × 0.80 = 192 A

Current check:

192 A > 163 A

Check 1 — Current Capacity

PASS ✓

Now check the cable length and voltage drop.

If the cable run is long and the voltage drop exceeds the project requirement:

FAIL ✗

The cable size must be increased.

Then perform the short-circuit withstand calculation.

If the selected cable withstands the applicable fault current for the required clearing time:

PASS ✓

Now the cable is a much stronger candidate for final selection.

21. Final Cable Selection Is a Decision

The cable-sizing workflow can be summarized as:

Load
 ↓
Calculate Current
 ↓
Select Preliminary Cable
 ↓
Apply Derating
 ↓
Check Current Capacity
 ↓
Check Voltage Drop
 ↓
Check Short-Circuit Withstand
 ↓
Check Installation Conditions
 ↓
Check Protection
 ↓
Select Cable Size
 ↓
Determine Number of Runs

This is the engineering process behind practical cable selection.

22. Cable Schedule in an EPC Project

In a real EPC project, you will eventually work with a Cable Schedule.

A simplified example is:

A real project cable schedule will contain considerably more information.

Typical information includes:

• Cable tag
• Source
• Destination
• Cable type
• Number of cores
• Cross-sectional area
• Number of runs
• Cable length
• Installation method
• Voltage drop
• Current capacity
• Cable route
• Cable destination
• Drum information

23. Cable Tagging

EPC projects use cable tags to identify individual cables.

For example:

PCC-MCC-001

or:

CBL-001

The exact naming convention depends on the project.

The important principle is that every cable should be traceable:

SOURCE
  ↓
CABLE
  ↓
DESTINATION
  ↓
LOAD

This becomes extremely important when a large EPC project contains thousands of cables.

24. Don’t Forget Cable Termination

Selecting the cable size is not the end of the engineering work.

Suppose you select:

3 × 1C × 240 mm²

You also need to consider:

• Cable glands
• Cable lugs
• Termination space
• Cable bending radius
• Panel entry
• Equipment terminal size
• Earthing arrangement
• Installation accessibility

A cable can be electrically suitable but physically difficult or impossible to terminate.

Therefore:

A good EPC design must be electrically correct and physically practical.

25. Cable Routing Matters

Consider:

PCC
 │
 │ 50 m
 ▼
MCC
 │
 │ 120 m
 ▼
Motor

The actual cable route affects:

• Cable length
• Voltage drop
• Installation cost
• Cable tray requirements
• Cable losses
• Installation complexity

Therefore, cable sizing and electrical layout are closely connected.

26. EPC Cable Engineering Workflow

The complete workflow can be remembered as:

LOAD LIST
    ↓
LOAD CURRENT
    ↓
CABLE TYPE
    ↓
INSTALLATION METHOD
    ↓
CURRENT-CARRYING CAPACITY
    ↓
DERATING FACTORS
    ↓
VOLTAGE DROP
    ↓
SHORT-CIRCUIT WITHSTAND
    ↓
PROTECTION CHECK
    ↓
CABLE SIZE
    ↓
NUMBER OF RUNS
    ↓
CABLE ROUTE / LENGTH
    ↓
CABLE SCHEDULE
    ↓
VENDOR DATA
    ↓
FINAL ENGINEERING REVIEW

🔥 This is how cable selection becomes a real EPC electrical engineering activity.

🧠 What You Should Know After Lesson 13

If someone gives you:

100 kW, 415 V motor

You should not immediately answer with a cable size.

Instead, your engineering thought process should be:

What is the current?

↓

What is the installation method?

↓

What are the applicable derating factors?

↓

What cable capacity is available?

↓

What is the cable length?

↓

What is the allowable voltage drop?

↓

What is the short-circuit level and clearing time?

↓

What protection device is being used?

↓

What cable size and number of runs are required?

That is the mindset of an EPC electrical engineer. ⚡

🎯 EPC Engineer Exercise

A motor has:

• Power = 75 kW
• Voltage = 415 V
• Power factor = 0.85
• Efficiency = 95%
• 3-phase

Questions

1. Calculate the approximate motor full-load current.

Use:

I = P / (√3 × V × PF × η)

2. Why can’t you select the cable immediately after calculating the current?

3. What happens to cable ampacity when several cables are grouped together?

4. Why do we perform a voltage-drop calculation?

5. Why do we perform a short-circuit withstand calculation?

6. If a 70 mm² cable passes current capacity but fails voltage drop, what can you do?

7. What is the relationship between the cable and the MCCB protecting it?

🚀 Your EPC Learning Path So Far

You are now building the complete electrical design chain:

LESSON 10
LOAD CALCULATION
      ↓
kW → kVA
      ↓
LESSON 11
TRANSFORMER SELECTION
      ↓
Technical Specification
      ↓
LESSON 12
SWITCHGEAR
      ↓
ACB / MCCB / VCB
      ↓
LESSON 13
CABLE SIZING
      ↓
Current + Derating
+ Voltage Drop
+ Short Circuit
      ↓
LOAD

Next Lesson

⚡ Lesson 14 — Motor Electrical Design

In the next lesson, we will take a motor from:

Motor nameplate → FLC → starting current → starter selection → MCCB/MPCB → contactor → overload relay → cable → voltage drop → motor feeder

and build a complete motor feeder design similar to what you would encounter in a practical EPC project.