What is BS7671 cable sizing and why does Appendix 4 matter?

BS7671:2018 (18th Edition) Appendix 4 contains the current-carrying capacity tables (It) for cables under standard installation conditions. These tables form the basis of all cable sizing calculations in the UK. You start with the tabulated It for your reference method and cable type, then apply correction factors for ambient temperature (Ca), grouping (Cg), and thermal insulation (Ci) to obtain the actual current-carrying capacity (Iz) of the installed cable.

What are the BS7671 reference installation methods for cable sizing?

BS7671 Appendix 4 defines reference methods A to F: Method A — enclosed in conduit within a thermally insulating wall (lowest rating); Method B — in conduit on wall or in enclosed trunking; Method C — clipped direct to a surface or on unperforated tray; Method D — buried in the ground; Method E — on a perforated cable tray in free air; Method F — supported by cable cleats or hangers in free air (highest rating). Each method gives different tabulated current-carrying capacities.

How do I use this BS7671 cable sizing calculator?

Select single-phase or three-phase, enter your design current (Ib) in amps or as a kW load, choose your reference installation method and cable insulation type, then set ambient temperature, number of grouped circuits, and thermal insulation condition. The calculator applies BS7671 Appendix 4 correction factors (Ca, Cg, Ci), finds the minimum cable size, and checks voltage drop against the Regulation 525.1 limits of 3% for lighting and 5% for other circuits.

What is the voltage drop limit per BS7671 Regulation 525.1?

BS7671:2018+A2:2022 Regulation 525.1 sets maximum voltage drop limits from the origin of installation to any fixed current-using equipment or socket outlet: 3% for lighting circuits (6.9 V on a 230 V supply) and 5% for all other circuits (11.5 V on a 230 V supply). For supplies that serve both lighting and other loads, the stricter 3% limit applies to lighting circuits and 5% to the rest.

How do BS7671 correction factors Ca, Cg and Ci affect cable sizing?

Ca (ambient temperature, Table 4B1) reduces cable rating when ambient exceeds 30°C: at 40°C a 70°C PVC cable is derated to 87% of its tabulated value. Cg (grouping, Table 4C1) reduces rating when cables are bundled: 3 circuits bunched = 0.70 factor, meaning the cable can only carry 70% of its single-circuit rating. Ci (thermal insulation, Regulation 523.9) applies when a cable is enclosed in insulation — completely surrounded by thermal insulation reduces rating to 50% (Ci = 0.5).

What is the BS7671 cable sizing formula from Appendix 4?

The BS7671 cable sizing formula: Iz = It × Ca × Cg × Ci ≥ In ≥ Ib. Where Ib = design current, In = OCPD rating (In ≥ Ib), It = tabulated current-carrying capacity from Appendix 4, Ca = ambient temperature correction (Table 4B1), Cg = grouping correction (Table 4C1), Ci = thermal insulation correction (Regulation 523.9). The minimum required It = In ÷ (Ca × Cg × Ci). Select the smallest cable size from Appendix 4 where the tabulated It equals or exceeds this value.

Frequently asked questions about BS7671 cable sizing

Common BS7671 cable sizing questions: always start with the OCPD rating In for the minimum required It calculation, not the design current Ib. Voltage drop must be checked separately after selecting the cable for current-carrying capacity — passing the thermal check does not guarantee the voltage drop is acceptable. For ring final circuits, Ib is half the total circuit load. A 1.5 mm² cable in Method C (clipped) can carry 20 A before correction factors — always verify conditions for your specific installation.

🇬🇧 BS7671 Calculator

BS7671 Cable Sizing Calculator

18th Edition · Appendix 4 · Reference Methods A–F · Ca × Cg × Ci Correction Factors · Voltage Drop per Regulation 525.1

⚡ Circuit Parameters

Design Current Ib (A)

The actual current the circuit will carry under normal operation.

OCPD Rating In (A) — optional

In ≥ Ib required. Cable must carry In: Iz ≥ In.

🏗️ Installation Conditions

Reference Installation Method (BS7671 Appendix 4)

Method B: In conduit on wall, enclosed trunking, or cable duct — standard for most UK domestic installations.

Cable Insulation

70°C PVC is standard T&E, SWA. 90°C XLPE has ~10–15% higher rating.

Ambient Temperature (°C)

Standard is 30°C in air, 15°C in ground. Lofts and boiler rooms often exceed 40°C.

Grouped Circuits (Cg — Table 4C1)

Thermal Insulation (Ci — Reg 523.9)

Cables touching insulation or buried within it require Ci derating.

📏 Voltage Drop Check (BS7671 Reg 525.1)

Circuit Length (metres)

Circuit Type

Power Factor (VD calc)

Please fill in all required fields.

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Min Required It (A)

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Cable Size

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Actual Iz (A)

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Volt Drop (%)

📊 Step-by-Step — BS7671 Appendix 4

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Current ratings from BS7671:2018+A2:2022 Appendix 4 Tables 4D2A / 4E2A (Cu, PVC/XLPE multicore, in air). Buried values from Table 4D5A (soil resistivity 2.5 K·m/W).
Formula: Iz = It × Ca × Cg × Ci ≥ In ≥ Ib. Minimum required It = In ÷ (Ca × Cg × Ci). Voltage drop per Appendix 4 mV/A/m tables.
90°C XLPE ratings approximate (Table 4E2A × ~1.08–1.17 vs 70°C PVC). For Method D, single-circuit values — no Cg applies. Results are indicative — always verify calculations with a qualified electrical engineer.

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