Title Page


About the Authors

Preface to the Fourth Edition


Chapter 1: Introduction

1.1 Layout of Chapters

1.2 Wiring Regulations

1.3 Terminology

1.4 Competence and Responsibility

1.5 Procedures

1.6 Inspection and Test

1.7 Completion

1.8 Working Methods and Materials

1.9 Operatives

1.10 Materials

1.11 Amendments to BS 7671: 2008

1.12 Voltages

1.13 Voltage Drop

Chapter 2: Three Bedroom House

2.1 The Bare Minimum

2.2 Standards

2.3 Building Regulations

2.4 Load Assessment

2.5 A Typical Domestic Supply

2.6 Project Specification

2.7 Wiring Systems and Cable Sizes

2.8 Lighting

2.9 13 A Socket-Outlets

2.10 Cable Sizes

2.11 Circuit Protection

2.12 Additional Protection for Socket-Outlets

2.13 Arrangement of Circuits

2.14 Arrangement of Consumer Unit

2.15 Main Switch

2.16 Earthing and Bonding

2.17 Gas Services Bonding and External Meters

2.18 Supplementary Bonding

Chapter 3: A Block of Retirement Flatlets

3.1 Two Schemes

3.2 Early Considerations

3.3 Other Interested Parties

3.4 Building Details

3.5 Part 1 – Flats

3.6 Part 2 – Landlord's Areas

Chapter 4: Overcurrent Protection

4.1 Overload

4.2 Overload Protection

4.3 Overload Protective Devices

4.4 Fault Current

4.5 Fault Current Protection

4.6 Omission of Fault Current Protection

4.7 Short-Circuit Rating

4.8 Disconnection Times

4.9 Earth Loop Impedance

4.10 Summary of cb Specification

4.11 Conclusion

Chapter 5: An Architect's Office

5.1 Other Interested Parties

5.2 Building Structure and Finishes

5.3 Electrical Requirements

5.4 Skirting System

5.5 Underfloor System

5.6 Socket-Outlets

5.7 Lighting Circuits

5.8 Battened Out Ceilings

5.9 Extra-Low Voltage Lighting (elv)

5.10 Group Transformers

5.11 Individual Transformers

5.12 Fire Prevention

5.13 Arrangement of Circuits

5.14 Distribution Boards

5.15 Cable Sizes

5.16 Switchgear

5.17 Print Machine

5.18 Wall Heaters in Toilets

5.19 Storage Heaters

5.20 Presence of 400 Volts

5.21 Access to Switchgear

5.22 Earthing and Bonding

5.23 Main Earthing Terminal

5.24 False Ceiling Grid

5.25 Computer Installations

5.26 High Protective Conductor Currents

5.27 Mains Filters

5.28 Uninterruptible Power Supplies (UPS)

Chapter 6: A High Street Shop

6.1 Special Considerations

6.2 Other Interested Parties

6.3 Building Structure and Finishes

6.4 Electrical Requirements

6.5 Loading and Diversity

6.6 Lighting

6.7 Socket-Outlets

6.8 Other Appliances

6.9 Phase Balance

6.10 Wiring Systems

6.11 Start by Considering Cost

6.12 Shop Area

6.13 Bakery Area

6.14 Temperature Limit of 70 °C

6.15 Temperature Limit of 90 °C

6.16 Final Selection and Cable Sizes

6.17 Bakery Wiring

6.18 Shop Wiring

6.19 Distribution Board

6.20 Cable Sizes

6.21 Switchgear

6.22 Isolation and Switching

6.23 Earthing and Bonding

6.24 Main Earthing Terminal (MET)

6.25 False-Ceiling Grid

6.26 Steel Tables in the Bakery

Chapter 7: Earthing and Bonding

7.1 Terminology

7.2 Definitions

7.3 Green-and-Yellow Conductors

7.4 Protective Earthing and Protective Equipotential Bonding

7.5 Protective Multiple Earthing (PME)

7.6 Reliability of the Earth-Neutral Path

7.7 Main Bonding

7.8 Single Fault Condition

7.9 Supplementary Bonding

7.10 Circuit Protective Conductors (CPCs)

7.11 Steel Conduit and Trunking

7.12 Steel Wire Armoured Cable

7.13 Comparison of Thermoplastic (PVC) and Thermosetting (XLPE) Armoured Cable

7.14 Continuity of Cable Glands

7.15 Equipment Having High Protective Conductor Currents

7.16 Protective Conductor Currents

7.17 ‘High Integrity’ Earthing

7.18 Earth Monitoring and Isolated Supplies

7.19 Socket-Outlets for Desktop Computers

7.20 Connections of Protective Conductors

7.21 Residual Current Devices

Chapter 8: Car Service Workshop

8.1 Standards and Recommendations

8.2 An Adaptable Design

8.3 Motor Vehicle Repair Premises

8.4 Other Interested Parties

8.5 Building Structure and Finishes

8.6 Construction

8.7 Electrical Requirements

8.8 Health and Safety Executive Guidance and Regulations

8.9 Health and Safety Guidance Note HSG 261

8.10 Wiring Regulations

8.11 Load Assessment and Maximum Demand

8.12 Maximum Demand Load and Diversity

8.13 Lighting

8.14 Welder

8.15 Compressor

8.16 Gas Blowers

8.17 Phase Balance

8.18 Estimate of Maximum Demand

8.19 What about a Distribution Circuit (Sub-Main)?

8.20 Wiring Systems

8.21 Workshop

8.22 Office

8.23 Arrangement of Circuits

8.24 Distribution Boards

8.25 Cable Sizes

8.26 Isolation and Switching

8.27 Machinery

8.28 Cooker

8.29 Gas Boiler

8.30 110 V Transformer

8.31 Earthing and Bonding

8.32 Main Earthing Terminal

8.33 Protective Conductors at Distribution Board B

8.34 Armoured Cable Glands

8.35 Steel Conduit and Trunking

Chapter 9: Circuits

9.1 Terminology

9.2 Colours of Three Phases

9.3 Conventional Circuits

9.4 Lighting Circuits

9.5 Induction

9.6 Socket-Outlet Circuits

9.7 Changing Methods

9.8 Ring Main Obsolescence

9.9 History of the Ring Final Circuit

9.10 Times have Changed

9.11 Alternative Methods

9.12 Radial Circuits

9.13 Introducing the Tree

9.14 20 A Tree

9.15 32 A Tree

9.16 Switching and Control

9.17 Comparison of Systems

9.18 32 A Ring Final Circuit

9.19 20 A Tree

9.20 Composite Circuits

Chapter 10: Farming and Horticulture

10.1 Why Farms are Different

10.2 Special Earthing Requirements on Farms with TT Systems

10.3 Earth Electrodes

10.4 Alternative Electrodes

10.5 Bonding

10.6 Supplementary Bonding

10.7 Residual Current Devices

10.8 Shock Protection

10.9 General Requirements for Automatic Disconnection of Supply (ADS)

10.10 Fire Protection

10.11 Automatic Life Support for High Density Livestock Rearing

10.12 Switchgear

10.13 Wiring Systems

10.14 Overhead or Underground Wiring

10.15 Non-Metallic Wiring Systems

10.16 Steel Wire Armoured (SWA) Cable

10.17 Twin and Earth Cable

10.18 General Rules Regarding Farm Electrical Installations

Chapter 11: Isolation and Switching

11.1 Isolation and Switching

11.2 Isolation

11.3 Mechanical Maintenance

11.4 Emergency Switching

11.5 Labelling and Notices

Chapter 12: A Village Sports Centre

12.1 Special Conditions

12.2 Codes of Practice

12.3 Other Interested Parties

12.4 Building Details

12.5 Structure and Finishes

12.6 Electricity Supply and Requirements

12.7 Off-Peak Tariff

12.8 Normal Tariff

12.9 Load Assessment and Diversity

12.10 Off-Peak Heating

12.11 Normal Tariff

12.12 Total Estimated Maximum Current Demand

12.13 Wiring Systems

12.14 Circuitry and Cable Sizing

12.15 Cable Grouping Factors

12.16 Arrangement of Circuits

12.17 Switchgear

12.18 Shock Protection

12.19 Earthing

12.20 Bonding

12.21 An Occasional Problem

12.22 Solutions

12.23 Requirements for a TT Installation

Chapter 13: An Indoor Swimming Pool

13.1 Special Conditions

13.2 Other Interested Parties

13.3 Building Details

13.4 Application of Zoning to this Project

13.5 Dehumidifiers

13.6 Changing Room/Shower Area

13.7 Loading and Diversity for the Swimming Pool Project

13.8 Wiring Systems

13.9 Cable Sizes

13.10 Distribution Board

13.11 Isolation

13.12 110 V System

13.13 Earthing

13.14 Local Supplementary Bonding

13.15 Floor Grid

Chapter 14: Cables and Wiring Systems

14.1 External Influences

14.2 Cost Considerations

14.3 Choosing Suitable Cable Routes

14.4 Is Armouring Always Necessary?

14.5 Fire Barriers

14.6 Holes through Fire Barriers

14.7 Sealing the Wiring System

14.8 Work in Progress

14.9 Records

14.10 Hidden Cables

14.11 Cables within a Floor

14.12 Cables above False Ceilings

14.13 Cables in Walls

14.14 Mechanically Protected Cables

14.15 Fire and Smoke

14.16 Thermoplastic (PVC) Insulation

14.17 Thermosetting (XLPE)

14.18 Silicone Rubber

14.19 Low Smoke Zero Halogen (LS0H)

14.20 Mineral Insulated Copper Sheathed (MICS) Cables

14.21 Heat Transference from Cables

14.22 Wiring Systems and Cable Management

14.23 Emergency Systems

14.24 Care with Wiring Systems

14.25 Thermoplastic (PVC) Insulated and Sheathed Cables

14.26 Thermosetting (PVC) Insulated Conduit Cables

14.27 Steel Conduit Systems and Trunking

14.28 Plastic Conduit Systems and Trunking

14.29 MICS Cables

14.30 Steel Wire Armoured Cables

14.31 Silicone Insulated PVC Sheathed Cables

Chapter 15: Inspection, Testing and Certification

15.1 Labelling and Documentation

15.2 Specification and Manual

15.3 Regulations

15.4 Electrical Installation Certificate (EIC)

15.5 Signatories

15.6 Alterations and Additions

15.7 Limits of Responsibility

15.8 Deviations and Departures

15.9 New Materials and Inventions

15.10 Particulars of the Installation

15.11 Inspections and Test Schedules

15.12 Inspection Procedures

15.13 Testing

15.14 Continuity Testing

15.15 Polarity

15.16 Continuity of Protective Conductors

15.17 Continuity of Ring Circuit Conductors

15.18 Insulation Resistance

15.19 Earth Fault Loop Impedance

15.20 Supply Impedance Ze

15.21 Earth Loop Impedance of Circuits Zs

15.22 Prospective Fault Current

15.23 Operation of Residual Current Devices

Chapter 16: A Caravan Park

16.1 Measures for Protection against Electric Shock

16.2 Earthing Arrangements

16.3 PME Must Not be Used for Caravans

16.4 Electrical Equipment (External Influences)

16.5 Wiring Systems

16.6 Cables Buried in the Ground

16.7 Overhead Cables

16.8 Caravan Pitch Electrical Supply Equipment

16.9 Plugs and Socket-Outlets

Chapter 17: Residual Current Devices

17.1 How does an RCD Work?

17.2 Fault Protection

17.3 Additional Protection

17.4 Requirements to Provide Additional Protection by RCDs

17.5 RCDs Incorporated Into a Consumer Unit, to Meet the Requirements for Additional Protection

17.6 Protection against Fire

17.7 Avoiding a Hazard and/or Minimising an Inconvenience due to the Tripping of an RCD

17.8 Reducing the Possibility of Unwanted Tripping of RCDs

17.9 Use of a ‘Front-End’ 30 mA RCD is Generally Considered Unacceptable Practice

17.10 Installations Forming Part of a TT System

17.11 RCDs Connected in Series

17.12 Labelling

Chapter 18: Flood Lighting (Outdoor Lighting) Project

18.1 Lighting Arrangement

18.2 General Requirements

18.3 Wiring System

18.4 Protective Measures

18.5 Load Assessment

18.6 Rating of the Overcurrent Protective Device

18.7 Circuit Design

18.8 Voltage Drop Consideration

18.9 Switchgear

Chapter 19: Circuit Design Calculations

19.1 Design Process

19.2 Protective Conductors

19.3 Worked Example

19.4 Solution


Title Page

About the Authors

Roger Lovegrove's extensive experience in the electrical contracting industry spans over 50 years. He served an apprenticeship, worked as an electrician and contracts engineer and then managed his own business for 18 years. Having left electrical contracting, he became a consultant specialising in inspecting, testing and surveying electrical installations. For a number of years he delivered lectures for ECA and IIE on various topics associated with BS 7671, the IEE Wiring Regulations. He is a Fellow of the IET and serves on the joint IET/BSI Wiring Regulations Committee. For many years an ECA member, he has served on the association's technical committee. Also, he has represented UK electrical contracting interests on International and European electrical installation standards working groups.

Gary Gundry is one of the UK's leading electrical safety trainers and presenters. Working at the Electrical Safety Council (ESC), he primarily produces technical guidance material for the electrical industry and safety literature for consumers. He also serves on one of the four committees responsible for the technical content of the Wiring Regulations (BS 7671); accordingly, he is able to speak authoritatively (to audiences of any number) on the requirements of the Seventeenth Edition of the Wiring Regulations.

Prior to working at the ESC, Gary was a Senior Engineer at NICEIC Training delivering bespoke training courses all over the United Kingdom, and before that he worked in the Standards division on, among other things, the NICEIC's Technical Manual.

Before joining the NICEIC in 1999, he was a Director and Qualified Supervisor of an electrical contracting business enrolled as an NICEIC Approved Contractor.

Gary began his career as an apprentice with SEEBoard, and later joined Eastern Electricity.

Preface to the Fourth Edition

There are many books on electrical installation practice where the focus is on calculations and regulations. Electrical Installation Designs has been written from a different viewpoint. Typical projects are examined to produce designs that will fit current standards.

Most electrical contractors have an understanding of requirements related to their own regular everyday activities, where work is carried out using rule-of-thumb methods. Repetitive designs are used. Many installers claim that they are not designers and show concern that they are now required to certify the adequacy of an installation design.

In practice, problems only arise when an unusual project is undertaken or there is a change in regulations.

There is no harm in using a standardised design, rather in the way that an experienced cook uses a published recipe for a cake. Electrical Installation Designs is a book of recipes. The installer may select a design that corresponds as near as possible to the contract in hand and take up such technical and regulatory advice as is required. This will reduce the need for lengthy calculations and detailed study of BS 7671, Requirements for Electrical Installations (IET Wiring Regulations).

Most basic electrical installations may be completed by a competent person, with appropriate guidance to avoid serious problems and hazards.

Project chapters illustrate methods that could be used for particular types of installation ranging from a house to an industrial workshop. The ideas are by no means exclusive. Alternative solutions are always possible. In many instances carrying out detailed calculations and utilising different circuitry will be more profitable. By their very nature, simplified examples of fictional projects can only produce generalised results.

The book contains special chapters on earthing, isolation and switching and overcurrent protection, which give a down to earth interpretation of the regulations.

Electrical installation students and non-electrical associates in the construction industry will appreciate the user-friendly approach. Nevertheless, this is not a do-it-yourself book for the untrained person. Warnings are therefore given where more specialised study is necessary. For example, readers are advised not to embark on installations in hazardous areas (such as petrol filling stations) without further training. Apart from moral implications and contractual risks, statutory requirements are such that incompetent work may carry criminal penalties.

Although the emphasis is on tried and tested methods, some new techniques are introduced. The most significant is the option for tree circuitry as an alternative to the ring final circuit. This is the first book to give designers the opportunity to compare the advantages of the tree system for both domestic and commercial installations. In recent years, consumer requirements have changed. It is essential that the industry keeps an open mind on changes in traditional wiring practice. Introduced in this fourth edition are new chapters on Residual Current Devices and, for those readers who require a basic understanding of circuit design, a chapter explaining the process for carrying out simple cable design calculations.


The Institution of Electrical Engineers (IEE) joint wiring regulations committee amended BS 7671: 1992 (16th edition) in 1994, 1997 and 1999. In 2001, the standard was renamed BS 7671: 2001 with further amendments, and minor amendments were made again in 2002.

In 2008, a complete review of the 16th edition saw the publication of the 17th edition: BS 7671: 2008. The first amendment to the 17th edition was published in 2011 and came fully into force on 1 January 2012. This latest amendment also introduced the rebranded name of the IEE, namely the IET (Institution of Engineering and Technology). The most recent edition of BS 7671 incorporates all alterations and additions to the European HD 384 series of standards.

As is often the case, many of the changes are of a minor nature in order to harmonise with other standards. However, significant changes were introduced in BS 7671: 2008 (17th edition) with several other significant changes being introduced in its first amendment, in 2011. These are summarised in the following tables – Table A.1 summarises the changes introduced by BS 7671: 2008 (17th edition) and Table A.2 summarises the significant changes introduced by Amendment 1 to BS 7671: 2008:

Table A.1 Summary of changes introduced by BS 7671: 2008 (17th edition).

BS 7671 reference Subject Summary of notable change/new content
General Regulation numbering system The 17th edition introduced the adoption of the IEC numbering system, in which the Regulation numbers are separated by a decimal point, rather than a hyphen as was previously the case.
Chapter 41 Protection against electric shock Chapter was rewritten.
Protection against direct contact – was replaced by basic protection.
Protection against indirect contact – was replaced by fault protection.
Introduction of term ‘Additional protection’. Socket-outlets with a rated current not exceeding 20 A and intended for general use by ordinary persons must be protected by 30 mA RCDs.
Mobile equipment having a current rating of 32 A or less for use outdoors must also have 30 mA RCD protection.
Chapter 52 Selection and erection of wiring systems For installations not under the supervision of a skilled or instructed person, such as domestic or similar installations, cables that are buried in a wall or partition at a depth of 50 mm or less and are not enclosed in earthed metallic covering (metallic armouring), earthed conduit, earthed trunking or have mechanical protection capable of resisting nails, screws or the like, are required to be protected by a 30 mA RCD as well as being installed in the so-called ‘safe zones’.
Similarly cables that are installed in metal framed walls require 30 mA RCD protection if not otherwise protected by earthed metallic covering, earthed conduit, earthed trunking or have mechanical protection capable of resisting nails, screws or the like, to be protected by a 30 mA RCD, unless the installation is under the control of skilled or instructed persons, such as office buildings and industrial premises.
Section 559 Luminaires and lighting A new section applicable to all general lighting installations as appropriate to particular locations and structures. Requirements for fixed outdoor lighting, highway power supplies and street furniture are also included, which were previously in Part 6.
Part 6 Inspection and testing Was previously Part 7.
Part 7 Special installations or locations Was previously Part 6.
701 – Locations containing a bath or a shower.
Zones 0, 1 and 2 as defined in the 16th edition were retained. Zone 3 was removed. All circuits supplying equipment in bathrooms and shower rooms are required to be protected by 30 mA RCDs.
Supplementary bonding is no longer required, provided all of the following three conditions are met:
• all final circuits of the location are protected by a 30 mA RCD;
• all final circuits of the location meet the required disconnection times;
• main bonding of services within the property is correctly installed.
SELV (separated extra-low voltage) socket-outlets and shaver socket-outlets are permitted outside Zone 1
230 V socket-outlets are permitted, provided they are more than 3 metres horizontally from the boundary of zone 1.
708 – Electrical installations in caravan/camping parks and similar locations. Formerly caravans, motor caravans and caravan parks in the 16th edition.
New Sections were added as follows:
709 – Marinas and similar locations;
711 – Exhibitions, shows and stands;
712 – Solar photovoltaic (PV) power supply systems;
717 – Mobile or transportable units;
721 – Electrical installations in caravans and motor caravans;
740 – Temporary electrical installations for structures, amusement devices and booths at fairgrounds, amusement parks and circuses.

Table A.2 Summary of changes introduced by Amendment 1 to BS 7671: 2008.

BS 7671 reference Subject Summary of notable change/new content
General Regulation numbering system In order to further implement changes to the requirements given in the International (IEC) and European (CENELEC) base documents, a number of the requirements contained in BS 7671 are particular to the United Kingdom. Those Regulations are identified via a ‘10X’ suffix. For example, Regulation 522.6.100.
General References to ESQCR 2002 Amendments have been made throughout BS 7671 to clarify the requirements of the Electricity Safety, Quality and Continuity Regulations 2002 with regard to the suitability for use of Protective Multiple Earthing (PME).
Tables 41.2; 41.4; and 41.6 BS 88 fuses Amended to reflect changes in product standards for cartridge fuses.
Table 41.5 Simplification of table Scope of the table revised to cover only circuits at a nominal a.c. rms line-to-earth voltage (U0) of 230 V.
Section 444 Measures against electromagnetic disturbances New section added, Measures against electromagnetic disturbances.
514.10.1 Nominal voltages Simplification of wording regarding enclosures within which nominal voltages exceeding 230 V to earth are present but may not be expected.
Requirement relating to nominal voltage exceeding 230 V between simultaneously accessible enclosures has been deleted.
522.6.100 Cables in floor or ceiling voids Clarification provided by adding indent relating to SELV and PELV circuits.
522.6.101 Cables in walls or partitions Clarification provided by adding indent relating to SELV and PELV circuits.
522.6.103 Cables in partitions having metallic parts Clarification provided by adding indent relating to SELV and PELV circuits.
526.3 Connections Maintenance-free accessories complying with BS 5733 recognised as an option for not needing to be accessible for inspection and maintenance.
Section 534 Surge protection devices New section added, Devices for protection against overvoltage.
Section 710 Medical locations New section added, Medical locations.
Section 729 Operating and maintenance gangways New section added, Operating and maintenance gangways.
Appendix 4 Current-carrying capacity and voltage drop for cables New clauses added: 5.5, Rating factors for triple harmonic currents in four-core and five-core cables with four cores carrying current and 5.6 Harmonic currents in line conductors. (New clauses based on text that was previously in Appendix 11).
New clause added: 6.4, Voltage drop in consumers' installations. (Text was previously in Appendix 12).
Appendix 6 Model forms for certification and reporting Introduction of Electrical Installation Condition Report to replace Periodic Inspection Report.
New Schedule added: Condition report inspection schedule for domestic and similar premises with up to 100 A supply.
New item added for other types of installation: Examples of items requiring inspection for an electrical installation condition report.
Appendix 16 Protection against overvoltage New Appendix added: Devices for protection against overvoltage.

This book takes account of all amendments published since the 17th edition of the Wiring Regulations was first issued in 2008, including those introduced by Amendment 1, which was first issued in 2011 and which came into effect on 1 January 2012.

Roger Lovegrove and Gary Gundry


Gary Gundry would like to thank Roger Lovegrove for the opportunity to update this publication and for his encouragement and support during the reviewing and commenting stage of the project.

Gary would also like to thank his employer, The Electrical Safety Council (ESC), for granting permission to update this publication, and for allowing him to reproduce any copyright material, where applicable. In recognition of this, and as a gesture of good faith, he has requested his share of the royalties be sent directly to the Electrical and Electronics Industries Benevolent Association (EEIBA), for as long as he remains employed at the ESC. The EEIBA is a charity dedicated to helping those in hardship and distress who work or have worked in the Electrical and Electronics Industry.

Special thanks goes to Peter, Liz, Laura and their colleagues at Wiley for the book design concept and typesetting, and to the Institution of Engineering and Technology for its permission to reproduce some of the model certificates and associated forms shown in Appendix 6 of the Seventeenth Edition of the IET Wiring Regulations.

Gary would also like to express thanks to his mentors and family for their endless patience, support and encouragement along the way.

Finally, any views expressed within this publication are those of the authors’ so they should not be associated with their employers, where applicable.



This book contains designs for electrical installations that have been prepared with reference to the Wiring Regulations and includes interpretations of particular technicalities.

This is not a do-it-yourself book for the amateur or untrained person. It is a guidance manual for competent electrical designers and students of installation practice.

As far as possible, all information accords with the requirements of BS 7671: 2008, incorporating Amendment 1, 2011 Requirements for Electrical Installations, the IET Wiring Regulations (17th edition), which is issued jointly by the British Standards Institution (BSI) and the Institution of Engineering and Technology (IET) as BS 7671. Relevant Regulation numbers and other references are shown in the margins. (Because of the space restrictions, the following abbreviations have been used: Ch. – Chapter; Sec. – Section; Defs – Definitions; App. – Appendix.) Reference is also made to various other British and European Standards and related Health and Safety documentation.

1.1 Layout of Chapters

Interspersed throughout the book are two types of chapters, giving information in different formats.

1. Project chapters: These may be compared with a selection of recipes for an experienced chef. The recipes give ideas for the design of typical electrical installations. Each project is dealt with on a stand-alone basis. Cross-references between these chapters are avoided, wherever possible, and similar information may be found for more than one scheme.
2. Topic chapters: These supplement the project chapters with in-depth discussion of generalised technicalities. They also provide study information on regulatory subjects. It may be necessary to refer to these details to finalise a design with particular problems.

1.2 Wiring Regulations

Throughout this book the terms Wiring Regulations (or Regulations) refer to BS 7671:2008 Requirements for Electrical Installations, the IET Wiring Regulations, issued jointly by the BSI and the IET. The Standard therefore represents a code of acceptable safety for electrical installations to protect:


against electrical hazards, which are described as:


The Regulations are not a statutory document, but are quoted as a means of compliance with certain statutory instruments. Criminal charges could not be brought for failure to comply with the Wiring Regulations, and such non-compliance could be used in evidence if there were a charge for breach of the forthcoming Electrical Safety Quality and Continuity Regulations (ESQCR) or the Electricity at Work Regulations.

It would be most unwise to ignore any of the requirements of the Regulations. They must be considered in their entirety and are a pass or fail test. An installation cannot partially comply.

1.3 Terminology

In order to understand technicalities, the importance of correct terminology is stressed throughout this book. In general, however, the use of overcomplicated expressions and trade jargon has been avoided.

The Wiring Regulations contain a list of definitions for words and expressions, which may not accord with standard dictionary definitions. Wherever there is any doubt, the Wiring Regulations definition should be applied.

1.4 Competence and Responsibility

Any person involved with the installation of wiring in buildings takes on both legal and moral responsibilities for safety. A high level of technical and practical competence is essential. This can only be achieved with appropriate study.

There are always three components to an electrical installation project:

1. Design;
2. Installation;
3. Inspection and testing.

Often one person or company takes on all three responsibilities, especially for simple repetitive jobs such as house wiring. On larger schemes, specialist companies may be contractually involved for each aspect and in turn use a team of operators. As the work progresses from planning to completion, there must always be one or more supervising individuals, who will eventually certify that the three aspects of the contract have been carried out in accordance with the Wiring Regulations and any other statutory or specification requirements.

1.5 Procedures

1.5.1 Design

It is sometimes thought that the use of tried and tested methods removes the design aspect from a scheme. This is not the case. Every project involves electrotechnical design decisions, which should not to be confused with architectural or customer instructions for the physical location of electrical equipment. Thus, a self-employed electrical contractor, who makes a decision on the selection and connection of an electrical accessory, is a designer. The same applies to an electrician who makes a similar decision on behalf of an employer.


All technical design information must be recorded. This is a requirement of the Wiring Regulations. IET guidance stresses that it is essential to prepare a full specification prior to commencement or alteration of an electrical installation. The size and content of the specification will correspond with the complexity of the work. For simple installations, a schedule of circuit details and test results may suffice.

The designs shown in the following chapters are for guidance only and each one includes a suggestion for a suitable design specification. A person selecting this guidance makes a design decision and therefore becomes the person responsible.


Upon completion of a contract, the person or persons responsible for the design, construction, and inspection and testing of the installation must, as appropriate, provide the person who ordered the work with an Electrical Installation Certificate, stating the works that were carried out, together with schedules for inspection and for test results.

1.5.2 Installation

Where a technical design is drawn up by an electrical engineer or other competent person, it should not be the installer's job to check design details, unless this is one of the contract requirements. The installer is always under obligation to point out to the designer any obvious conflict with regulations or standards and the installer should always refuse to carry out substandard work. There would be no defence in law against creating an unsafe installation on the basis of inherently bad instructions.

The installer will use the designer's specification document as required by the Wiring Regulations. This may only cover performance requirements or may give full technical details for the selection and erection of equipment. Once again it must be emphasised that a non-technical instruction to take an electrical supply to a particular appliance or location does not constitute design information.

The installer has the responsibility to ensure that equipment is installed correctly and in accordance with the specification, supplemented by manufacturers' information. The installer is often delegated other tasks, such as that of negotiating with the electricity supply company and verifying local licensing requirements.

Upon completion of the project, the installer certifies that the installation work has been carried out in accordance with the Wiring Regulations.

1.6 Inspection and Test


No matter how simple or straightforward a job is, the work should always be inspected, both during the course of the work, and upon completion, and tested before the installation is put into service. This applies equally to work carried out by a self-employed operator. Self-certification is normally acceptable, provided that the contractor has the competence and equipment to test correctly. The customer or an insurer may require specialist certification. This particularly applies in the case of safety alarm systems or work in hazardous areas.

Whether an in-house or independent specialist, the inspector must be given the full design documentation, with amendments showing any relevant on-site modifications. On larger projects, this will include ‘as fitted’ drawings.

Certain parts of the installation may be hidden from view upon completion. In such cases the inspector must arrange for inspection during the course of erection or receive certified confirmation that the work is satisfactory.


Upon completion of the project, the inspector certifies that the inspection and test have been carried out in accordance with the Wiring Regulations.

1.7 Completion


The signatures of the designer, installer and inspector are required for the Electrical Installation Certificate. This cannot be issued until the work has been completed in accordance with the Wiring Regulations. Where there are acceptable departures from the Regulations, these must be shown on the certificate. See Chapter 15 for inspection and test procedures.

1.8 Working Methods and Materials


The Wiring Regulations require good workmanship by competent persons and that proper materials are used.

1.9 Operatives

Any person carrying out electrical work must be competent, trained and skilled in the type of installation work being carried out. Where trainees or unskilled operatives are employed for electrical work, they must be appropriately supervised.

Workmanship must be of a quality appropriate to the location. A working knowledge of the building structure is necessary where holes and fixings are made to carry cables. Decor should be disturbed as little as possible, with prearranged responsibility for making good.

1.10 Materials


The Regulations require that every item of equipment complies with a British or harmonised Standard. Alternatively, equipment complying with a relevant foreign standard may be used, provided that the designer confirms that the equipment provides a degree of safety acceptable to the Regulations. This may mean product certification by an approvals organisation.

1.11 Amendments to BS 7671: 2008

There is one amendment to BS 7671: 2008, published in 2011. Many of the changes are minor, resulting from European harmonisation, and the numbering of product standards where these have also been harmonised. Where changes have been made that affect the technical or practical aspects of this book, the text has been changed accordingly. If an amendment noticeably alters conventional procedures, details of the change have been included. This particularly applies in the case of voltage harmonisation.

1.12 Voltages

On 1 January 1995, the Electricity Supply Regulations 1988 were changed to take into account a change in the standard UK supply voltage (Table 1.1). All specifications, designs and calculations should now use the 230/400 V figures. The change in voltage is minimal, therefore load assessments given in this book have not been recalculated, but there are other consequences to be considered.

Table 1.1 Changes in nominal voltage.


Taking all of the above factors and tolerances into account, it will be seen that under the most extreme circumstances, equipment rated at 230 V may be supplied at anything between 216 and 253 V.

1.13 Voltage Drop

The Regulations require the voltage at the terminals of fixed current-using equipment under normal service conditions to be greater than the lower limit corresponding to the product standard for that equipment.

For an installation supplied from a public supply, a voltage drop of 3% for lighting and 5% for other uses is deemed to satisfy this requirement. At 230 V, this is 6.9 V and 11.5 V, respectively. Care therefore needs to be taken in the selection of cables, particularly where equipment is voltage-dependent:

All of the designs shown in this book use cables that are capable of handling the prospective loadings with a reasonable margin of safety.