Switchgear Selection in EPC Projects: A Practical Guide for Electrical Engineers

Introduction

Switchgear selection in EPC projects is an important step in electrical system design.

After preparing the load list and developing the Single Line Diagram (SLD), the electrical engineer needs to determine what type of switchgear is required, its voltage rating, current rating, short-circuit rating, protection requirements and construction.

In a typical EPC project, switchgear may include:

• LV Switchboard
• PCC — Power Control Centre
• MCC — Motor Control Centre
• MDB — Main Distribution Board
• SMDB — Sub-Main Distribution Board
• ACDB — AC Distribution Board
• MV Switchgear
• ACB — Air Circuit Breaker
• MCCB — Molded Case Circuit Breaker
• VCB — Vacuum Circuit Breaker
• Isolators and disconnectors
• Protection relays
• CTs and VTs

Switchgear selection should not be based only on the connected load. Voltage, continuous current, short-circuit level, protection philosophy, installation conditions and project specifications all need to be considered. (ZY POWER)

1. What Is Switchgear?

Switchgear is a combination of electrical equipment used to switch, control, protect, isolate and distribute electrical power.

A simple power flow can be represented as:

      TRANSFORMER
           |
           |
      +----+----+
      | INCOMER |
      |  ACB    |
      +----+----+
           |
           |
      =================
          BUSBAR
      =================
        |      |      |
        |      |      |
       MCCB   MCCB   MCCB
        |      |      |
       LOAD   LOAD   MOTOR

The switchgear provides a controlled point between the power source and downstream loads.

If a fault occurs downstream, the protective device should operate and disconnect the affected circuit.

2. Where Does Switchgear Selection Come in an EPC Project?

This is important for an electrical designer to understand.

A simplified EPC electrical design sequence is:

                    LOAD LIST
                       |
                       ↓
              LOAD ESTIMATION
                       |
                       ↓
             TRANSFORMER SIZING
                       |
                       ↓
                 SLD DEVELOPMENT
                       |
                       ↓
             SWITCHGEAR SELECTION
                       |
             +---------+---------+
             |                   |
             ↓                   ↓
       BREAKER SELECTION    BUSBAR SIZING
             |
             ↓
        CABLE SIZING
             |
             ↓
        CABLE SCHEDULE
             |
             ↓
        CABLE ROUTING
             |
             ↓
          CABLE TRAY

So, switchgear selection is not an isolated activity.

It is connected to the transformer, SLD, short-circuit calculation, protection coordination and cable system.

3. First Step — Determine the System Voltage

The first question is:

What voltage level will the switchgear operate at?

For example:

        UTILITY / GRID
             |
          11 kV
             |
      +--------------+
      | 11 kV SWITCH |
      |    GEAR      |
      +------+-------+
             |
        TRANSFORMER
       11 kV / 415 V
             |
      +------+-------+
      | LV SWITCHGEAR|
      |    415 V     |
      +------+-------+
             |
          LOADS

The voltage level determines the switchgear family and applicable standards.

For example:

For LV assemblies, IEC 61439 is a key standard family. For MV metal-enclosed switchgear, IEC 62271 is commonly applicable. The exact project specification and local requirements must always be checked. (Electrical Installation)

4. Second Step — Calculate the Load Current

Once the voltage is known, determine the expected operating current.

For a three-phase load:$$I = \frac{P}{\sqrt{3} \times V \times PF}$$

Where:

• I = Current in amperes
• P = Power in watts
• V = Line-to-line voltage
• PF = Power factor

Example

Suppose a plant load is:

• Power = 500 kW
• Voltage = 415 V
• Power factor = 0.9

Then:$$I = \frac{500000}{1.732 \times 415 \times 0.9}$$

Approximately:$$I \approx 773 A$$

The switchgear and incomer therefore need to be selected to safely carry the required continuous current, with the actual project design accounting for demand, diversity, temperature and other applicable derating factors.

5. Connected Load Is Not the Same as Maximum Demand

This is a very important EPC concept.

Suppose a panel has:

Motor 1       100 kW
Motor 2       100 kW
Motor 3       100 kW
Heater         50 kW
Lighting       20 kW
---------------------
Connected Load = 370 kW

It does not automatically mean that all loads operate simultaneously at their full rating.

The electrical engineer therefore considers:

• Demand factor
• Diversity factor
• Load factor
• Operating conditions
• Future loads
• Standby equipment
• Motor starting requirements

The resulting design load is used for downstream sizing calculations.

6. Third Step — Determine the Short-Circuit Level

This is one of the most important parts of switchgear selection.

A switchboard may carry only 800 A during normal operation, but during a fault, the current can become many times higher.

For example:

NORMAL OPERATION

Transformer
    |
    | 800 A
    ↓
+-----------+
| SWITCHGEAR|
+-----------+
    |
   LOAD

During a short circuit:

TRANSFORMER
     |
     | VERY HIGH FAULT CURRENT
     ↓
+----------------+
|   SWITCHGEAR   |
|                |
|   SHORT CIRCUIT|
|      FAULT     |
+----------------+

Therefore, the switchgear must have suitable short-circuit withstand and switching capabilities.

The fault level is obtained from the short-circuit study.

7. Why Fault Level Is Important

Consider two panels.

Panel A

Load Current = 800 A
Fault Level  = 20 kA

Panel B

Load Current = 800 A
Fault Level  = 50 kA

Both panels carry the same normal current.

But their short-circuit requirements are different.

Therefore:

You cannot select switchgear only from the load current.

The short-circuit level at the installation point must also be checked.

For LV systems, parameters such as Icu, Ics and Icw may be relevant depending on the equipment and application. For MV circuit breakers, rated short-circuit breaking current and short-time withstand characteristics are among the parameters considered. (ZY POWER)

8. Understanding Icu and Ics

For an LV circuit breaker, two important terms are:

Icu — Ultimate Breaking Capacity

Icu represents the maximum prospective short-circuit current that the circuit breaker is rated to interrupt under the applicable standard conditions.

Ics — Service Breaking Capacity

Ics relates to the short-circuit current the breaker can interrupt while maintaining the specified service capability according to the standard.

A simple way to remember:

Icu
 ↓
Maximum ultimate fault interruption capability

Ics
 ↓
Service-level fault interruption capability

These values should be checked against the calculated fault level and the project protection philosophy.

9. Fourth Step — Select the Incomer Breaker

After determining the required current and fault level, the incomer breaker can be selected.

For an LV main switchboard, an ACB may be used as the incomer.

Example:

        TRANSFORMER
          1000 kVA
             |
             |
        +----+----+
        |   ACB   |
        | INCOMER |
        +----+----+
             |
       =================
            BUSBAR
       =================
          |    |    |
         ACB  MCCB  MCCB
          |    |    |
        MCC  DB    MOTOR

The ACB selection may include:

• Rated current
• Breaking capacity
• Short-time withstand
• Trip unit
• Long-time protection
• Short-time protection
• Instantaneous protection
• Earth-fault protection
• Under-voltage protection
• Over-voltage protection
• Communication
• Motorized operation

The actual functions depend on the project protection philosophy.

10. MCCB Selection for Outgoing Feeders

MCCBs are commonly used for outgoing LV feeders.

For example:

                 LV SWITCHBOARD
                       |
                 +-----+-----+
                 |    BUS    |
                 +-----+-----+
                       |
          +------------+------------+
          |            |            |
         MCCB         MCCB         MCCB
          |            |            |
        Motor        Panel       Heater

The MCCB rating should be coordinated with:

• Load current
• Cable ampacity
• Fault level
• Starting current
• Protection requirements
• Selectivity
• Coordination with upstream/downstream devices

11. Cable and Breaker Selection Are Connected

This is extremely important for EPC designers.

Consider:

SWITCHBOARD
    |
   MCCB
    |
    |
   CABLE
    |
    |
   MOTOR

The breaker cannot simply be selected independently from the cable.

Suppose:

Load current = 180 A
Cable capacity = 220 A

A suitable protective device needs to be selected so that the cable is properly protected while the load can operate normally.

Therefore:

LOAD
 ↓
LOAD CURRENT
 ↓
CABLE SIZE
 ↓
BREAKER RATING
 ↓
PROTECTION SETTING

The exact selection must follow the applicable installation standard, project requirements and coordination study.

12. Busbar Selection

Switchgear selection also involves busbar sizing.

For example:

             INCOMER
                |
                |
        =================
             BUSBAR
        =================
          |      |      |
          |      |      |
        MCCB    MCCB   MCCB

The busbar must be capable of carrying the required continuous current.

Typical considerations include:

• Rated current
• Temperature rise
• Short-time withstand current
• Peak withstand current
• Ambient temperature
• Enclosure arrangement
• Busbar material
• Installation configuration

For LV assemblies, IEC 61439 includes requirements relating to current-carrying capability, temperature rise and short-circuit performance. (Electrical Installation)

13. Fixed or Withdrawable Switchgear?

Another important EPC decision is construction type.

Fixed Type

The switching device remains installed in its operating position.

+----------------+
|    SWITCHGEAR  |
|                |
|     BREAKER    |
|      FIXED     |
|                |
+----------------+

Withdrawable Type

The breaker or functional unit can be moved between positions.

+----------------------+
|      SWITCHGEAR      |
|                      |
|  [ BREAKER ]  ←→     |
|                      |
+----------------------+

Withdrawable construction can provide operational and maintenance advantages where the project requires reduced downtime or easier isolation.

The choice depends on:

• Criticality
• Maintenance philosophy
• Availability requirements
• Space
• Cost
• Project specifications

14. Form of Separation

For LV switchboards, the internal separation arrangement is another important specification.

A simplified concept is:

FORM 1
-----------------
Common enclosure


FORM 2
-----------------
Busbar separated


FORM 3
-----------------
Busbar + functional units
separated


FORM 4
-----------------
Busbar + functional units
+ terminals separated

The required form depends on the project’s safety, maintenance and operational requirements.

IEC 61439 addresses functional units and forms of separation in LV assemblies. (Electrical Installation)

15. IP Rating

The switchgear enclosure also needs an appropriate IP rating.

IP rating indicates the degree of protection provided by the enclosure against ingress of solids and water.

For example:

Indoor Electrical Room
        ↓
   Suitable IP rating

Outdoor Installation
        ↓
   Higher environmental
   protection may be required

The selection depends on:

• Indoor/outdoor installation
• Dust
• Water
• Humidity
• Process environment
• Corrosive atmosphere
• Project specification

Do not simply select an IP rating from a standard template. It should match the actual installation environment.

16. Environmental Conditions

Switchgear may be installed in:

• Indoor electrical rooms
• Outdoor substations
• Process areas
• Coastal environments
• Dusty industrial areas
• High-temperature locations

Therefore, check:

Ambient temperature
       ↓
Altitude
       ↓
Humidity
       ↓
Dust
       ↓
Corrosion
       ↓
Installation location
       ↓
Enclosure / derating requirements

Environmental conditions can affect enclosure design, thermal performance, insulation and material selection. (Cneeg)

17. Protection Coordination

Switchgear selection cannot be separated from protection coordination.

Consider:

        INCOMER ACB
             |
             |
          MCCB-1
             |
          MCCB-2
             |
           MOTOR

If a fault occurs at the motor:

MOTOR FAULT
     ↓
MCCB-2 SHOULD TRIP
     ↓
MCCB-1 SHOULD REMAIN ON
     ↓
INCOMER SHOULD REMAIN ON

The objective is generally to disconnect the affected portion while maintaining supply to healthy portions of the system, subject to the project’s protection philosophy.

This is called selectivity/discrimination.

18. Protection Relay Selection for MV Switchgear

For MV switchgear, protection relays are commonly used.

A simplified arrangement:

          11 kV INCOMING
                |
              VCB
                |
          +-----+-----+
          |   CT / VT |
          +-----+-----+
                |
          PROTECTION
             RELAY
                |
          TRIP SIGNAL
                |
              VCB

The relay may provide functions such as:

• Overcurrent protection
• Earth-fault protection
• Under-voltage
• Over-voltage
• Transformer protection
• Motor protection
• Directional protection

The exact protection functions depend on the electrical system and protection study.

19. CT and VT Selection

Switchgear can also include:

CT — Current Transformer

Used for:

• Protection
• Metering
• Monitoring

VT/PT — Voltage Transformer

Used for:

• Voltage measurement
• Protection
• Metering
• Synchronization where required

Example:

             11 kV BUS
                |
       +--------+--------+
       |                 |
      CT                VT
       |                 |
       ↓                 ↓
   PROTECTION         METERING
      RELAY            /RELAY

CT and VT specifications should be coordinated with the protection and metering requirements.

20. Switchgear Selection Data Sheet

In an EPC project, the engineer may prepare a switchgear technical specification/datasheet containing parameters such as:

The values above are illustrative only. Actual values must come from the project’s electrical studies and specifications.

21. Documents Used for Switchgear Selection

As an EPC electrical designer, you will normally work with several documents.

1. Load List

Provides:

• Equipment loads
• Motor ratings
• Operating loads
• Voltage
• Power factor
• Starting information

2. Single Line Diagram

Shows:

SOURCE
  ↓
TRANSFORMER
  ↓
SWITCHGEAR
  ↓
BREAKERS
  ↓
CABLES
  ↓
LOADS

3. Short-Circuit Study

Provides the fault level required for equipment selection.

4. Protection Coordination Study

Helps determine:

• Breaker settings
• Relay settings
• Selectivity
• Protection philosophy

5. Cable Schedule

Provides:

• Cable size
• Number of runs
• Cable type
• From/To information

6. Project Electrical Design Criteria

Defines project-specific requirements.

22. Practical EPC Example

Let’s take a simple example.

Suppose an industrial plant has:

UTILITY
  |
  | 11 kV
  ↓
11 kV SWITCHGEAR
  |
  ↓
11/0.415 kV TRANSFORMER
  |
  ↓
415 V LV SWITCHBOARD
  |
  +------------+-------------+
  |            |             |
 MCC           DB            MCC
  |            |             |
Motors       Lighting       Motors

The electrical engineer follows this sequence:

Step 1 — Load List

Determine all connected loads.

Step 2 — Maximum Demand

Determine the expected operating demand.

Step 3 — Transformer

Select transformer rating.

Step 4 — Short-Circuit Calculation

Determine fault current at the relevant switchboards.

Step 5 — Switchgear

Select:

• Voltage rating
• Current rating
• Short-circuit rating
• Construction
• Busbar rating
• IP rating
• Protection

Step 6 — Breakers

Select ACB/MCCB/VCB as applicable.

Step 7 — Cable Sizing

Select cables based on:

• Current capacity
• Voltage drop
• Short-circuit withstand
• Installation conditions

Step 8 — Cable Schedule

Prepare the cable schedule.

Step 9 — Cable Routing

Route cables through trays.

This is how the different electrical engineering activities connect together in an EPC project.

23. Simple Switchgear Selection Flowchart

             LOAD LIST
                 |
                 ↓
        SYSTEM VOLTAGE
                 |
                 ↓
          LOAD CURRENT
                 |
                 ↓
       MAXIMUM DEMAND
                 |
                 ↓
       SHORT-CIRCUIT STUDY
                 |
                 ↓
       ┌─────────────────┐
       │ SWITCHGEAR TYPE │
       └────────┬────────┘
                |
        +-------+-------+
        |               |
       LV              MV
        |               |
      ACB/MCCB        VCB + RELAY
        |               |
        +-------+-------+
                |
                ↓
          BUSBAR RATING
                |
                ↓
       PROTECTION SYSTEM
                |
                ↓
       ENVIRONMENT / IP
                |
                ↓
       CONSTRUCTION TYPE
                |
                ↓
      PROJECT SPECIFICATION
                |
                ↓
       FINAL SWITCHGEAR

24. Common Mistakes in Switchgear Selection

Mistake 1 — Selecting only based on load current

❌ “Load is 500 A, so select a 500 A panel.”

You must also consider fault level, environmental conditions, protection and project requirements.

Mistake 2 — Ignoring short-circuit current

A panel with adequate normal current rating can still be unsuitable if its short-circuit capability is inadequate.

Mistake 3 — Ignoring cable coordination

The breaker, cable and load must work together.

Breaker
   ↓
Cable
   ↓
Load

Mistake 4 — Ignoring protection coordination

Selecting breakers without checking upstream/downstream coordination can cause unnecessary shutdown of larger parts of the plant.

Mistake 5 — Ignoring future expansion

Some EPC projects require spare feeders or additional capacity.

Future requirements should be identified from the project specification rather than adding an arbitrary percentage.

Mistake 6 — Ignoring environmental conditions

A switchboard installed inside a clean electrical room and one installed outdoors in a harsh environment may require very different enclosure and environmental considerations.

25. Important Standards

Some commonly referenced standards include:

The applicable edition, project specification, local regulations and client requirements should always be checked before final engineering. (Electrical Installation)

26. Switchgear Selection — The Easy Way to Remember

As an EPC electrical engineer, remember these 8 questions:

1. WHAT VOLTAGE?
       ↓
2. WHAT LOAD CURRENT?
       ↓
3. WHAT FAULT LEVEL?
       ↓
4. WHAT BREAKER?
       ↓
5. WHAT BUSBAR RATING?
       ↓
6. WHAT PROTECTION?
       ↓
7. WHAT ENVIRONMENT / IP?
       ↓
8. WHAT PROJECT REQUIREMENTS?

If you can answer these questions, you have the foundation for understanding switchgear selection.

27. How Switchgear Connects to the Next EPC Design Activities

The overall relationship can be remembered like this:

             LOAD LIST
                 |
                 ↓
          LOAD ESTIMATION
                 |
                 ↓
          TRANSFORMER SIZE
                 |
                 ↓
               SLD
                 |
                 ↓
        SWITCHGEAR SELECTION
                 |
                 ↓
         BREAKER SELECTION
                 |
                 ↓
            CABLE SIZING
                 |
                 ↓
          CABLE SCHEDULE
                 |
                 ↓
          CABLE ROUTING
                 |
                 ↓
          CABLE TRAY DESIGN
                 |
                 ↓
         TRAY SUPPORT DESIGN

This sequence is especially useful for a designer who wants to understand how an EPC electrical design develops from the load list into the final 3D model.

Conclusion

Switchgear selection in EPC projects is much more than choosing a panel or circuit breaker from a catalogue.

The engineer must consider:

• System voltage
• Load current
• Maximum demand
• Short-circuit level
• Breaker rating
• Busbar rating
• Protection coordination
• Cable coordination
• Construction type
• Form of separation
• IP rating
• Environmental conditions
• Future requirements
• Project specifications
• Applicable IEC standards

The most important concept to remember is:

Load current tells you what the switchgear normally carries; fault-level calculations tell you what it may need to withstand or interrupt during a fault.

NEXT LESSON IS CABLE SIZING IN EPC