Use of Appendices

Overview

The Appendices of BS 7671 contain the reference data, tables, and supplementary information that support the main body of the regulations. While the regulations themselves set out the requirements that an installation must meet, the appendices provide the practical data needed to demonstrate compliance through design calculations.

Being able to navigate the appendices efficiently is a skill that is tested both:

• Directly — through questions requiring table look-ups
• Indirectly — through questions that require candidates to know where specific data is found

Although the exam weighting for appendices as a standalone topic is modest, the data contained within them underpins questions across many other topic areas, particularly cable sizing, earth fault loop impedance verification, and the selection of protective devices. Familiarity with the structure and content of the key appendices will save valuable time in the exam.

Key Sections

Appendix 4 - Current-Carrying Capacity and Voltage Drop

Appendix 4 is the most heavily used appendix in both practice and examinations. It contains:

• Current-carrying capacity tables for cables installed using various reference methods:

  • Clipped direct
  • In trunking
  • In conduit
  • On cable tray
  • Enclosed in thermal insulation

• Voltage drop data per ampere per metre, essential for verifying that the voltage at the furthest point of a circuit remains within permitted limits

How to Use Appendix 4

When sizing a cable, follow this process:

1. Identify the cable type (e.g., thermoplastic twin and earth, SWA)
2. Determine the installation method and its reference method designation
3. Find the correct table based on cable type and installation method
4. Apply correction factors for:
   • Ambient temperature (Ca)
   • Grouping (Cg)
   • Thermal insulation (Ci)
5. Identify the minimum conductor size that can carry the design current after corrections

Appendix 3 - CPC Sizing and the Adiabatic Equation

Appendix 3 provides information relevant to the selection of circuit protective conductors (CPCs).

The Adiabatic Equation

The key formula is:

S = √(I²t) / k

Where:

• S = minimum cross-sectional area of the CPC (mm²)
• I = fault current (A)
• t = disconnection time (s)
• k = a factor depending on the conductor material and insulation type

Values of k

The values of k are tabulated within Appendix 3 for different combinations of:

• Conductor material — copper or aluminium
• Insulation type — thermoplastic (PVC) or thermosetting (XLPE)
• Whether the conductor is incorporated in a cable or bare

Candidates must understand how to apply the adiabatic equation and where to find the k values.

Appendix 5 - Classification of External Influences

Appendix 5 classifies the external influences that must be considered when designing an installation. Influences are categorised under three headings:

Category	Code	Meaning
Environment	A	Physical conditions (temperature, water, dust, etc.)
Utilisation	B	How the space is used and by whom
Construction	C	Building construction type and materials

Commonly Tested Codes

• AA — Ambient temperature range
• AD — Presence of water (AD1 = negligible through to AD8 = submersion)
• AE — Presence of foreign bodies (dust, small objects)
• BA — Competence of persons (BA1 = ordinary, BA4 = instructed, BA5 = skilled)

This classification system is used to determine the suitability of equipment for its intended environment and is referenced by Part 5 when selecting and erecting equipment.

Appendix 15 - Ring Final Circuit Arrangement

Appendix 15 provides guidance on the design and arrangement of ring final circuits, which remain common in UK domestic installations.

Key content includes:

• Maximum floor area served by a ring circuit
• Assumed current demand for standard ring circuits
• Requirements for spurs:
  • Fused spurs — permitted from a fused connection unit
  • Non-fused spurs — maximum of one socket outlet per spur
• Number of socket outlets permitted on a ring circuit

Other Important Appendices

Appendix	Content
Appendix 1	British Standards to which equipment and accessories should conform
Appendix 2	Statutory regulations and associated memoranda relevant to electrical installations
Appendix 6	Time/current characteristics of protective devices (fuses and circuit breakers) — essential for verifying disconnection times
Appendix 14	Earth fault loop impedance (Zs) tables — maximum Zs values that must not be exceeded to achieve disconnection within the required time

Appendices 6 and 14 are particularly important for exam questions involving:

• Verification of disconnection times
• Checking whether a protective device will operate fast enough for a given Zs value

Key Appendices

• Appendix 3 - CPC sizing using the adiabatic equation and k values for conductor materials
• Appendix 4 - Current-carrying capacity tables and voltage drop data for all cable types and installation methods
• Appendix 5 - Classification of external influences (environment, utilisation, construction)
• Appendix 6 - Time/current characteristics of overcurrent protective devices
• Appendix 14 - Maximum earth fault loop impedance (Zs) tables for protective devices
• Appendix 15 - Ring final circuit design, floor area limits, and spur arrangements
• Appendix 1 - British Standards for equipment and accessories
• Appendix 2 - Statutory regulations and associated memoranda

Common Exam Topics

Exam questions on the appendices typically require candidates to locate specific data from the tables. Common tasks include:

• Finding the current-carrying capacity of a given cable type installed by a specified reference method
• Determining the maximum Zs value for a particular protective device and circuit
• Applying correction factors for grouping or ambient temperature
• Calculating the minimum CPC size using the adiabatic equation for a given fault current and disconnection time
• Identifying the correct external influence code from Appendix 5
• Identifying the correct reference method designation for a particular installation arrangement

Study Tips

• Spend time becoming familiar with the layout of Appendix 4. Know how to navigate from cable type → installation method → correct table. Speed in finding the right table is critical in a timed exam.
• Practise using the adiabatic equation from Appendix 3. Work through several examples with different conductor materials and disconnection times until the process becomes second nature.
• Learn the alphanumeric coding system for external influences in Appendix 5. Focus on the most commonly tested codes: AA (ambient temperature), AD (presence of water), and BA (competence of persons).
• For Appendix 14, understand the relationship between Zs values and disconnection times. A lower Zs value means faster disconnection under fault conditions.
• Use tabbed page markers or bookmarks in your physical copy of BS 7671 so you can turn to the key appendices quickly during the exam.
• Remember that correction factors reduce the effective current-carrying capacity of a cable. When multiple correction factors apply, they are multiplied together.

Related Topics

• Part 4 - Protection for Safety
• Part 5 - Selection and Erection of Equipment
• Part 6 - Inspection and Testing
• Part 3 - Assessment of General Characteristics