Part 5 - Selection and Erection of Equipment

Overview

Part 5 of BS 7671 is the largest and most practically focused section of the Wiring Regulations, dealing with the selection and erection of all electrical equipment within an installation. With an exam weighting of 14%, it carries the highest number of questions of any topic area and demands thorough preparation.

This part translates the protective principles set out in Part 4 into concrete requirements for choosing cables, wiring systems, switchgear, earthing conductors, and switching devices. Candidates must understand not only the individual rules but also how they interact — cable selection, for example, requires simultaneous consideration of current-carrying capacity, voltage drop, fault current withstand, grouping factors, and thermal insulation.

Key Sections

Chapter 51 - Common Rules (Regulations 510-515)

Chapter 51 establishes the overarching principles for selecting all equipment:

• All equipment must comply with applicable product standards
• Equipment must be selected and erected for the voltage, current, and frequency of the installation
• Every item must be suitable for the external influences it will encounter

Regulation 512 requires suitability for the conditions of the location, including:

• Ambient temperature
• Presence of water (IPX ratings)
• Mechanical impact
• Corrosive or polluting substances
• Flora, fauna, and electromagnetic influences

The external influences classification system (Appendix 5) is referenced frequently in exam questions.

Chapter 52 - Wiring Systems (Regulations 521-529)

This chapter covers the selection and installation of wiring systems. It is one of the most detail-rich areas of the exam.

Cable Types and Installation Methods (Regulation 521)

The type of wiring system is selected based on the nature of the location and external influences. Common systems include:

• Cables in conduit (metallic or non-metallic)
• Cables in trunking (metallic or non-metallic)
• Cables on cable tray or cable ladder
• Steel wire armoured (SWA) cables
• Thermoplastic (PVC) and thermosetting (XLPE) insulated cables

Cable Current-Carrying Capacity (Regulation 523)

The tabulated current-carrying capacity must satisfy the relationship:

It ≥ In / (Ca × Cg × Ci × Cc)

Where the correction factors are:

• Ca — Ambient temperature correction factor
• Cg — Grouping correction factor (for cables installed together)
• Ci — Thermal insulation correction factor
• Cc — Correction factor for the type of protective device

The actual current-carrying capacity values are found in the tables in Appendix 4.

Voltage Drop Limits (Regulation 525)

Voltage drop must not exceed:

• 3% of nominal voltage for lighting circuits
• 5% of nominal voltage for all other circuits

These percentage figures are among the most commonly tested values in the exam. The total drop from the origin of the installation must not exceed the values specified in Appendix 12.

Other Key Requirements

• Regulation 526 — All electrical connections must be accessible for inspection, testing, and maintenance (unless specifically designed to be buried, such as underground cable joints)
• Regulation 527 — Cables must be selected and erected to minimise the spread of fire, including fire-stopping and sealing where cables pass through fire barriers

Chapter 53 - Switchgear and Controlgear (Regulation 530)

Switchgear must be selected with regard to:

• Its duty (making and breaking capacity)
• The conditions of its location
• The prospective fault current at the point of installation

The making and breaking capacity links directly to the overcurrent protection requirements of Part 4, Chapter 43.

Chapter 54 - Earthing Arrangements and Protective Conductors (Regulations 541-544)

This chapter sets out the requirements for all protective conductors. It contains some of the most frequently tested tables in the exam.

Circuit Protective Conductor (CPC) Sizing — Table 54.7

Line Conductor Size	Minimum CPC Size
Up to 16mm²	Equal to the line conductor
16mm² to 35mm²	16mm²
Above 35mm²	Half the line conductor size

Main Protective Bonding Conductors (Regulation 544.1)

Minimum sizes for main protective bonding conductors:

• 6mm² copper (or 10mm² aluminium)
• Not less than half the cross-sectional area of the earthing conductor
• Subject to a maximum of 25mm² for copper

Supplementary Bonding Conductors

Where required, supplementary bonding conductors must be:

• Not less than 4mm² copper if mechanically protected
• Not less than 2.5mm² if not mechanically protected

Chapter 55 - Other Equipment (Regulations 551-559)

This chapter addresses specific types of equipment:

• Section 551 — Generators and low-voltage generating sets
• Section 559 — Luminaires and lighting installations (particularly relevant for emergency lighting questions)
• Safety services and standby supply requirements

Important Regulation Numbers

• 512 - Equipment suitability for external influences
• 521-522 - Selection and erection of wiring systems; external influences on cables
• 523 - Current-carrying capacities of cables (references Appendix 4)
• 525 - Voltage drop limits (3% lighting, 5% other)
• 526 - Electrical connections (accessibility requirements)
• 527 - Minimising spread of fire
• 530 - Switchgear selection and breaking capacity
• 541-543 - Earthing arrangements and earthing conductors
• 544.1 - Main protective bonding conductor sizing (minimum 6mm² Cu)
• Table 54.7 - Minimum cross-sectional area of protective conductors
• 559 - Luminaires and lighting installations

Common Exam Topics

• Cable sizing calculations using correction factors (Ca, Cg, Ci, Cc) and Appendix 4 tables
• Voltage drop calculations and the 3%/5% limits
• Protective conductor sizing using Table 54.7
• Main protective bonding conductor minimum sizes
• External influences classification and its effect on equipment selection
• IP ratings and their meaning for enclosure protection
• Wiring system selection for different environments (conduit, trunking, SWA)
• Requirements for accessibility of connections (Regulation 526)
• Fire-stopping requirements where cables penetrate fire barriers

Study Tips

• Table 54.7 is essential to memorise. The three size bands (up to 16mm², 16-35mm², above 35mm²) and corresponding CPC requirements appear in multiple exam questions.
• Practice voltage drop calculations using Appendix 4 data. Know the formula: voltage drop = mV/A/m × design current × route length / 1000.
• Understand how to apply multiple correction factors to determine the required tabulated current rating. Work through several cable sizing examples to build confidence.
• Remember the bonding conductor minimums: 6mm² copper, 10mm² aluminium. These are frequently tested.
• Link Part 5 requirements back to Part 4 principles. For instance, the selection of an overcurrent device in Chapter 53 must satisfy the coordination rules in Chapter 43.

Related Topics

• Part 4 - Protection for Safety
• Part 6 - Inspection and Testing
• Part 2 - Definitions
• Part 7 - Special Installations or Locations