What are the BS 7671 correction factors and how do I apply them?

Correction factors reduce the current-carrying capacity of a cable to account for the actual installation conditions, which may be less favourable than the reference conditions used to derive the tabulated values. The main correction factors are: Ca (ambient temperature) — if the ambient temperature exceeds the reference temperature (typically 30 degrees C for PVC cables in air), the cable can carry less current. Ca values are found in BS 7671 Table 4B1. Cg (grouping) — when cables are installed together, they generate heat that reduces each cable's capacity. Cg values depend on the number of circuits grouped together, the installation method, and whether the cables are touching. Values are in Table 4C1 to 4C5. Ci (thermal insulation) — if a cable passes through or is enclosed in thermal insulation, its ability to dissipate heat is reduced. Ci values range from 0.5 (fully enclosed) to 1.0 (no insulation contact). Values are in Table 52.2. Cf (semi-enclosed fuse factor) — if a semi-enclosed (rewirable) fuse is used for overcurrent protection, a factor of 0.725 is applied. The formula is: It = In / (Ca x Cg x Ci x Cf), where It is the minimum tabulated current-carrying capacity required.

How do I calculate voltage drop for a circuit?

Voltage drop is calculated using the mV/A/m values from the cable tables in BS 7671 Appendix 4. The formula is: Voltage drop (V) = (mV/A/m x Ib x L) / 1000, where mV/A/m is the voltage drop per ampere per metre for the cable type and size (from the relevant table), Ib is the design current in amperes, and L is the cable route length in metres. For three-phase circuits, use the three-phase mV/A/m values. The maximum permitted voltage drop from the origin of the installation to the load is 3% of the nominal supply voltage for lighting circuits (6.9V for a 230V supply) and 5% for other circuits (11.5V for a 230V supply). These limits are specified in BS 7671 Appendix 4, Table 4Ab. If the calculated voltage drop exceeds the limit, you need to either increase the cable size (lower mV/A/m) or reduce the circuit length (shorter cable route). For circuits with varying loads (such as a ring circuit), the voltage drop calculation considers the distribution of load around the ring.

What is protection coordination and why is it important?

Protection coordination ensures that protective devices (MCBs, fuses, RCDs, RCBOs) will disconnect the supply quickly enough to prevent danger in both overload and fault conditions. There are two key aspects: Overload protection — the protective device must satisfy two conditions: (1) In >= Ib (the device rated current must be equal to or greater than the design current), and (2) I2 <= 1.45 x Iz (the device operating current must not exceed 1.45 times the cable current-carrying capacity). I2 is the current at which the device will definitely operate within the conventional time — for MCBs, I2 = 1.45 x In, so the second condition simplifies to In <= Iz. Fault protection — the protective device must disconnect the circuit within the maximum disconnection time specified in BS 7671 Table 41.1: 0.4 seconds for final circuits up to 63A in a TN system, 0.2 seconds for final circuits up to 32A in a TT system, and 5 seconds for distribution circuits. To verify this, you must check that the earth fault loop impedance (Zs) at the most remote point of the circuit does not exceed the maximum value for the device type, rating, and disconnection time.

How do I assess the general characteristics of an installation?

Part 3 of BS 7671 requires the designer to assess the general characteristics of the installation before detailed circuit design begins. This assessment covers five key areas: (1) Purpose, supplies, and structure (Chapter 31) — determine the maximum demand, the number and type of circuits required, the supply characteristics (voltage, frequency, prospective fault current at the origin), and the type of earthing arrangement (TN-S, TN-C-S, or TT). (2) External influences (Chapter 32) — assess the environmental conditions that will affect the installation: ambient temperature, presence of water, mechanical stress, presence of corrosive substances, impact, vibration, flora, fauna, electromagnetic influences, and solar radiation. Each external influence is classified using the coding system in Appendix 5. (3) Compatibility (Chapter 33) — ensure that equipment is compatible with the characteristics of the supply and the environment. For example, electronic equipment may need protection against voltage transients, and installations with harmonic-generating loads may need special consideration for neutral sizing. (4) Maintainability (Chapter 34) — design the installation so it can be safely maintained, tested, and inspected throughout its service life. Provide adequate labelling, access to equipment, and isolation facilities.

What earthing arrangement should I use for a domestic installation?

The earthing arrangement for a domestic installation in the UK is determined by the type of earth provided by the Distribution Network Operator (DNO). The three main types are: TN-S (separate earth) — the DNO provides a separate earth conductor, typically the metallic sheath of the supply cable. This is the most reliable earthing arrangement with a low earth fault loop impedance, typically found in older urban areas with lead-sheathed cables. TN-C-S (PME / Protective Multiple Earthing) — the DNO provides an earth connection derived from the combined neutral and earth conductor of the supply. This is the most common earthing arrangement for new domestic installations. It provides a reliable low-impedance earth but has specific requirements for bonding to gas, water, and other metallic services. BS 7671 imposes additional requirements for PME installations, particularly regarding bathroom installations, EV charging, and outdoor circuits. TT (earth rod) — no earth is provided by the DNO. The installation relies on a local earth electrode (typically a driven earth rod). Common in rural areas. TT systems have a higher earth fault loop impedance and require RCD protection on all circuits to achieve adequate disconnection times. The designer must verify the earthing arrangement with the DNO and design the installation accordingly.

How do I determine the design current for a circuit?

The design current (Ib) is the expected current that the circuit will carry under normal operating conditions. For fixed loads (such as an electric shower, immersion heater, or cooker), the design current is calculated from the rated power and the supply voltage using the formula: Ib = P / V for single-phase, or Ib = P / (sqrt(3) x VL) for three-phase balanced loads. For socket outlet circuits, the design current is based on the assumed maximum demand with diversity applied. BS 7671 does not prescribe diversity factors — these are typically taken from the IET On-Site Guide or the Electrician's Guide to the Building Regulations. Common diversity assumptions include: ring circuit (socket outlets) — 100% of the first 10A plus 40% of the remainder, which gives a design current of approximately 20A for a typical domestic ring; lighting circuit — 66% of the total connected load; cooker circuit — 10A plus 30% of the rated power above 10A, plus 5A if the cooker control unit has a socket outlet. The design current must be determined before the protective device can be selected and the cable can be sized.

DESIGN GUIDE

Electrical Design Guide: The BS 7671 Design Process

Good electrical design is the foundation of a safe, compliant installation. This guide walks through the entire BS 7671 design process — from assessing general characteristics and determining design current, through cable selection and correction factors, to protection coordination and voltage drop verification.

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15 min readUpdated 2026-06-10Andrew Moore, Founder of Elec-Mate

Written and reviewed by Andrew Moore, founder of Elec-Mate, against BS 7671:2018+A4:2026, IET Guidance Note 3 and the IET On-Site Guide.

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Key Takeaways

1Every electrical installation must be designed in accordance with BS 7671 Part 3 (Assessment of General Characteristics) before any work begins — covering supply characteristics, load assessment, earthing arrangement, and external influences.
2Cable selection involves a systematic process: determine design current (Ib), select protective device rating (In), apply correction factors (Ca, Cg, Ci, Cf), calculate tabulated current-carrying capacity (It), and verify from the tables in BS 7671 Appendix 4.
3Protection coordination ensures that protective devices disconnect the supply within the required time for both overload (In >= Ib and I2 <= 1.45 x Iz) and fault conditions (the device operates within the maximum disconnection time for the circuit type).
4Voltage drop must not exceed 3% for lighting circuits and 5% for other circuits (from the origin of the installation to the load) under BS 7671 Appendix 4.
5Elec-Mate's AI Circuit Designer helps electricians with cable sizing, voltage drop calculations, and protection coordination — producing BS 7671 compliant designs on your phone.

01 · Design Guide

The BS 7671 Electrical Design Process

Every electrical installation — from a single additional socket to a full house rewire — requires a design. The design process is set out in BS 7671:2018+A4:2026 (the IET Wiring Regulations, 18th Edition with Amendment 4) and follows a structured sequence that ensures the completed installation is safe, functional, and compliant.

The design process is not just for large commercial installations. Even a domestic consumer unit upgrade or an additional circuit for an EV charger requires the designer to work through the same logical steps: assess the supply, determine the load, select the protection, size the cable, verify voltage drop, and confirm fault protection.

The design is documented on the Electrical Installation Certificate (EIC), which includes a section for design information. The designer signs the EIC to confirm that the installation has been designed in accordance with BS 7671.

The Design Sequence

Assessment of general characteristics (Part 3)
Determination of design current (Ib) for each circuit
Selection of protective device type and rating (In)
Cable selection — type, size, and installation method
Verification of voltage drop
Verification of earth fault loop impedance and disconnection time
Verification of overload protection coordination
Documentation on the EIC

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02 · Design Guide

Assessment of General Characteristics (Part 3)

Part 3 of BS 7671 requires the designer to assess the general characteristics of the installation before any detailed circuit design begins. This is the foundation of the entire design — if you get this wrong, everything that follows is compromised.

Supply characteristics (Chapter 31). Determine the nominal voltage (typically 230V single-phase or 400V three-phase in the UK), frequency (50Hz), prospective fault current at the origin (obtained from the DNO or measured), and the maximum demand of the installation.
Earthing arrangement. Identify the type of earthing system — TN-S, TN-C-S (PME), or TT. This is determined by the DNO and fundamentally affects the design of the earthing and bonding, the selection of protective devices, and the maximum earth fault loop impedance values.
External influences (Chapter 32). Assess environmental conditions: ambient temperature, presence of water, mechanical impacts, corrosive substances, vibration, and any other factors that affect the choice of equipment, cables, and installation methods.
Compatibility (Chapter 33). Consider electromagnetic compatibility, harmonic distortion, voltage fluctuations, and the compatibility of equipment with the supply characteristics.
Maintainability (Chapter 34). Design the installation to allow safe maintenance, inspection, and testing throughout its service life. Provide isolation and switching devices, clear labelling, and adequate access.

The assessment of general characteristics should be recorded and is part of the design documentation. It informs every subsequent design decision, from cable type to protective device selection.

03 · Design Guide

Determining Design Current (Ib)

The design current (Ib) is the current that the circuit is expected to carry under normal operating conditions. It is the starting point for selecting the protective device and sizing the cable.

Fixed loads: Ib = P / V (single-phase) or Ib = P / (1.732 x VL) (three-phase balanced). For example, a 9.5kW electric shower on a 230V supply: Ib = 9500 / 230 = 41.3A.
Socket circuits: Use diversity factors from the IET On-Site Guide. For a domestic ring circuit: 100% of the first 10A plus 40% of the remainder. A typical domestic ring has a design current of approximately 20A.
Lighting circuits: Sum the wattage of all connected light fittings and apply 66% diversity. For a circuit with 1,200W total connected load: Ib = (1200 x 0.66) / 230 = 3.4A.
Cooker circuits: 10A plus 30% of the remaining load plus 5A if a socket outlet is included. For a 12kW cooker: Ib = 10 + (0.3 x ((12000/230) - 10)) + 5 = 10 + 12.7 + 5 = 27.7A.

Getting the design current right is critical. An underestimated Ib could result in an undersized cable that overheats under normal load. An overestimated Ib wastes money on unnecessarily large cables and protective devices.

04 · Design Guide

Cable Selection and Sizing

Cable selection is the core of electrical design. The process involves determining the minimum cable size that can carry the design current safely, taking into account the installation conditions that reduce the cable's capacity.

The Cable Sizing Formula

It >= In / (Ca x Cg x Ci x Cf)

It — minimum tabulated current-carrying capacity required
In — rated current of the protective device
Ca — ambient temperature correction factor (Table 4B1)
Cg — grouping correction factor (Tables 4C1 to 4C5)
Ci — thermal insulation correction factor (Table 52.2)
Cf — semi-enclosed fuse factor (0.725 for BS 3036 fuses, 1.0 otherwise)

Once you have calculated It, refer to the current-carrying capacity tables in Appendix 4 of BS 7671 to find the cable size with a tabulated capacity equal to or greater than It. The table you use depends on the cable type (PVC, XLPE, MICC) and the installation method (clipped direct, in conduit, in trunking, in thermal insulation).

After selecting the cable size for current-carrying capacity, you must also verify that it meets the voltage drop limits and that the earth fault loop impedance at the extremity of the circuit is low enough for the protective device to disconnect within the required time.

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05 · Design Guide

Protection Coordination: Overload and Fault

Protection coordination ensures that the protective devices will operate correctly in both overload and fault conditions. There are two separate requirements that must both be satisfied:

Overload Protection

Two conditions must be met:

Condition 1: In >= Ib

The device rated current must be equal to or greater than the design current.

Condition 2: I2 <= 1.45 x Iz

The device effective operating current must not exceed 1.45 times the cable's current-carrying capacity. For MCBs where I2 = 1.45 x In, this simplifies to In <= Iz.

Fault Protection

The device must disconnect within:

0.4 seconds for final circuits up to 63A (TN systems)

0.2 seconds for final circuits up to 32A (TT systems)

5 seconds for distribution circuits

This is verified by checking the earth fault loop impedance (Zs) at the most remote point does not exceed the maximum value for the device.

The maximum earth fault loop impedance values for each device type (BS EN 60898, BS EN 61009, BS 88, BS 3036) and rating are given in the tables in BS 7671 Chapter 41. The measured or calculated Zs at the extremity of the circuit must not exceed these values. If it does, you need to either increase the cable size (to reduce the circuit impedance) or select a device with a higher fault current sensitivity.

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06 · Design Guide

Voltage Drop Calculations

Voltage drop occurs because the cable has resistance — as current flows through the cable, some voltage is "lost" across the cable impedance. Excessive voltage drop can cause equipment to malfunction (lights dimming, motors overheating) and is limited by BS 7671.

Maximum voltage drop: 3% for lighting circuits (6.9V on 230V) and 5% for all other circuits (11.5V on 230V), measured from the origin of the installation to the load.
Formula: Voltage drop (V) = (mV/A/m x Ib x L) / 1000, where mV/A/m is from the cable tables, Ib is the design current, and L is the cable route length in metres.
If voltage drop exceeds the limit: increase the cable size (lower mV/A/m value) or reduce the cable route length. Cable size may need to be increased beyond what current-carrying capacity alone requires.
Worked example: A 32A radial circuit using 4mm2 T&E (clipped direct, mV/A/m = 11), design current 28A, cable length 25m. Voltage drop = (11 x 28 x 25) / 1000 = 7.7V. As a percentage: 7.7 / 230 x 100 = 3.35%. This is within the 5% limit for a socket circuit but would exceed the 3% limit if this were a lighting circuit.

Voltage drop is often the factor that determines cable size for long cable runs — even when the cable has adequate current-carrying capacity, it may not meet the voltage drop limit. Always check both criteria.

07 · Design Guide

Earthing and Bonding Design

The earthing and bonding arrangement is a fundamental part of the design. It provides the path for earth fault current to flow, enabling protective devices to disconnect the supply and preventing dangerous touch voltages on exposed metalwork.

Main earthing terminal — connects the installation earth to the means of earthing (DNO earth for TN systems, earth electrode for TT systems).
Main protective bonding conductors — connect incoming metallic services (gas, water, oil) to the main earthing terminal. Minimum size: 10mm2 for TN-S and TN-C-S, 6mm2 for TT (subject to the requirements of Table 54.8).
Circuit protective conductors (CPCs) — the earth conductor in each circuit cable. Size determined by the adiabatic equation or by Table 54.7 (minimum CPC size relative to the line conductor size).
Supplementary bonding — additional bonding between simultaneously accessible exposed-conductive-parts and extraneous-conductive-parts in specific locations (e.g., bathrooms under certain conditions).

The earthing arrangement must be designed to ensure that the earth fault loop impedance at every point in the installation is low enough for the protective devices to operate within the required disconnection times. For TT systems, where the earth fault loop impedance is inherently higher, RCD protection is essential to achieve adequate disconnection.

08 · Design Guide

Discrimination Between Protective Devices

Discrimination (also called selectivity) means that only the protective device nearest to the fault operates, leaving the rest of the installation energised. This is important for availability — a fault on one circuit should not disconnect the entire installation.

Between MCBs: discrimination is achieved when the upstream device has a higher rating and slower time-current characteristic than the downstream device. Generally, a ratio of 2:1 or higher in rated current provides adequate discrimination for most overcurrent conditions.
Between RCDs: discrimination requires the upstream RCD to have both a higher rated residual operating current (e.g., 100mA vs 30mA) and a time delay (Type S or selective RCD). Without a time delay, both RCDs will trip simultaneously.
RCBO boards: provide inherent discrimination for earth faults — only the RCBO on the faulty circuit trips, leaving all other circuits energised. This is the main advantage of a full RCBO consumer unit over a split-load RCD arrangement.

Good discrimination is essential for commercial and industrial installations where loss of power can have serious consequences. In domestic installations, a full RCBO board provides the best discrimination for the consumer — a single faulty circuit does not plunge the entire house into darkness.

09 · Design Guide

For Electricians: Design Tools in Elec-Mate

Electrical design requires calculations that are time-consuming to do by hand but essential for compliance. Elec-Mate puts these design tools on your phone, so you can complete the design on site and produce the certificate immediately.

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Electrical Design Guide: The BS 7671 Design Process

Key Takeaways

The BS 7671 Electrical Design Process

The Design Sequence

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Assessment of General Characteristics (Part 3)

Determining Design Current (Ib)

Cable Selection and Sizing

The Cable Sizing Formula

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Protection Coordination: Overload and Fault

Overload Protection

Fault Protection

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Voltage Drop Calculations

Earthing and Bonding Design

Discrimination Between Protective Devices

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AI Circuit Designer

Calculators On Site

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Frequently Asked Questions About Electrical Design

What is the design process for an electrical installation under BS 7671?

What are the BS 7671 correction factors and how do I apply them?

How do I calculate voltage drop for a circuit?

What is protection coordination and why is it important?

How do I assess the general characteristics of an installation?

What earthing arrangement should I use for a domestic installation?

How do I determine the design current for a circuit?

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Correction Factors Guide

Earthing Arrangements

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