What is the maximum permitted statutory voltage drop under BS 7671?

Under BS 7671 Appendix 4 Section 6.4 for low-voltage public distribution supplies, standard statutory limits are 3% for lighting circuits (6.9 V at 230 V) and 5% for general power, sockets, and machinery (11.5 V at 230 V).

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BS 7671 Cable Sizing & Derating Calculator

This tool calculates the required tabulated current-carrying capacity (It) and verifies statutory voltage drop from first principles. Rather than acting as a black box, it shows every step of the design calculation, details each derating factor applied or dismissed, and references the exact tables in BS 7671 Appendix 4 for conductor selection.

Plant-Room Ready: This calculator caches to your phone and works 100% offline with zero mobile signal. Add to your home screen for quick access on site.

1-Click Mobile Presets:Tap for instant on-site sizing

Select a preset above to auto-fill common circuit values

1. Circuit Load & SupplyDesign current (Ib) [A] Circuit design load operating current in Amperes.Nominal supply voltage

230 V (1Φ)400 V (3Φ)

Circuit application & volt-drop limitBS 7671:2018+A4:2026 Regulation 525 / Appendix 4 Section 6.4.

2. Overload & Short-Circuit Protection

Motor Feeder Mode (BS 7671 Reg 435.2)Decouple overload from upstream device: cable continuous capacity (It) sizes for motor Ib (protected by starter overload relay Ib ≤ I_OL ≤ Iz). Upstream device (In) acts solely as short-circuit protection against motor inrush.

Protective device nominal rating (In) [A] Nominal rating of fuse or circuit breaker. Per Reg 433.1.1: In ≥ Ib.Protective device type (Cc factor) BS 7671 standard protective devices with conventional operating multiplier ≤ 1.45.

3. Installation Derating Factors

Installation Method (BS 7671 Table 4A2)Tap a card to auto-apply standard derating

Method CClipped DirectSurface mounted on wall or timber joists

Method BIn ConduitEnclosed in trunking or wall conduit

Method 103In InsulationTotally surrounded in loft batt (Ci = 0.50)

Method DUndergroundDirect buried or in duct (20°C ground)

Ambient temperature factor (Ca)Table 4B1. 1.00 for 30°C in air / 20°C in ground (e.g. 0.87 for 40°C 70°C PVC).Grouping factor (Cg)Table 4C1. 1.00 for single isolated circuit (e.g. 0.80 for 2 bunched circuits).Thermal insulation factor (Ci)Table 52.2 / Reg 523.9. 1.00 if uninsulated (e.g. 0.50 if totally surrounded >500mm).

4. Voltage Drop ParametersCircuit topology (route impedance model)

Radial Circuit (Full L)Ring Final Circuit (Midpoint L/4)

Standard radial circuit with full length route impedance.Tabulated mV/A/m valueVoltage drop per ampere per metre from BS 7671 Appendix 4 cable tables.Length of circuit run (L) [m]One-way route distance in metres from distribution board to load.

ENGINEERING CALCULATION RECORDDESIGN VERIFICATION AID · NOT AN EIC

UK ELECTRICAL INSTALLATION DESIGN & COMPLIANCE RECORD

BS 7671:2018+A4:2026 Step-by-Step Cable Sizing & Derating Calculation

Project / Site: Installation Design
Circuit Reference: Distribution Circuit
Standard Reference: BS 7671:2018+A4:2026 Section 433 · Appendix 4
Calculated By: Adrian King (Design Verification)
Date & Time: Generated On-Site
Design Status: PASS / COMPLIANT

Calculation Summary

Standard Basis: BS 7671:2018+A4:2026 · Reg 433.1.1 (In ≥ Ib, Iz ≥ In) · Appendix 4Topology: Radial (Full L)

Suggested Conductor CSA (Method C Clipped)BS 7671 Table 4D5

First-pass sizing: verify against Appendix 4 for exact run route.

Required Tabulated Capacity (It): -Estimated Voltage Drop: -

This calculation provides a first-pass starting point; the person signing the Electrical Installation Certificate owns the final design decision.

Show Full Mathematical Derivation (AM2 Working Standard)

BS 7671:2018+A4:2026

AM2 PORTFOLIO & DESIGN AUDIT FORMATTRANSPARENT FIRST-PRINCIPLES MATHS

Step 1 · Stated Design Target

Required Tabulated Current Capacity (It)

It ≥ 32.00 A

BS 7671 Reg 433.1.1 (Condition 1: In ≥ Ib, Iz ≥ In)

Step 2 · Statutory Derating Formula

Effective Thermal Capacity Derating

It ≥ In / (Ca × Cg × Ci × Cc)

It ≥ 32.00 / (1.000 × 1.000 × 1.000 × 1.000) = 32.00 A

Ca (Ambient Temp): 1.000 (BS 7671 Table 4B1 / 30°C in air)
Cg (Grouping): 1.000 (BS 7671 Table 4C1 / Single circuit)
Ci (Insulation): 1.000 (BS 7671 Table 52.2 / Not surrounded)
Cc (Protective Device): 1.000 (Standard MCB/RCBO/BS 88)

Step 3 · Conductor Selection & Verification

Conductor Cross-Sectional Area

Selected: 4 mm² (Table 4D5 Method C, Tabulated It = 37 A ≥ 32.00 A)

BS 7671 Appendix 4 Current-Carrying Capacity

Step 4 · Voltage Drop Compliance (Reg 525)

Route Impedance & Drop Verification

Vd = (mV/A/m × Ib × L) / 1000

Vd = (7.30 × 32.00 × 25.0) / 1000 = 5.84 V (2.54%)

PASS · Within statutory limit

Statutory Compliance Note:Worked example to BS 7671:2018+A4:2026. To be verified by a competent person against current standards and manufacturer data before use (EAWR 1989).

STATUTORY COMPLIANCE & VERIFICATION STATEMENT:Calculated in accordance with BS 7671:2018+A4:2026 (Requirements for Electrical Installations). This document is an engineering design verification calculation aid for review by a qualified assessor. The signatory takes full responsibility for verifying all design parameters, installation methods, and on-site environmental conditions under the Electricity at Work Regulations 1989. Not valid as a statutory Electrical Installation Certificate (EIC).

Designer / Assessor Name:____________________________________

Scheme / Reg No (NICEIC / NAPIT / ECA):____________________________________

Signature:____________________________________

Date of Verification:____________________________________

Worked Example & Practical Reference

Example Scenario: Sizing a dedicated 7.4 kW (32.17 A) single-phase domestic radial circuit (e.g. electric vehicle charger or instantaneous shower) run 25 m in a loft.

Design Current: Ib = 7400 W / 230 V = 32.17 A.
Protective Device: In = 40 A Type B MCB (next standard size ≥ 32.17 A).
Correction Factors: Loft summer ambient 40°C → Ca = 0.87 (Table 4B1); grouped with 1 other circuit → Cg = 0.80 (Table 4C1); clipped outside thermal insulation → Ci = 1.00; MCB protection → Cc = 1.00. Total C = 0.87 × 0.80 × 1.00 × 1.00 = 0.696.
Required Tabulated Capacity: It ≥ 40 / 0.696 = 57.47 A.
Table Lookup: Per Table 4D5 Reference Method C (Clipped direct), 6 mm² flat Twin & Earth carries 47 A (fails It), so 10 mm² is selected (tabulated It = 64 A ≥ 57.47 A).
Voltage Drop Check: 10 mm² has 4.4 mV/A/m. Vd = (4.4 × 32.17 × 25) / 1000 = 3.54 V (1.54% of 230 V ≤ 5.0% limit → PASS).

Frequently Asked Questions

What is the difference between Design Current (Ib), Device Rating (In), and Cable Capacity (Iz)?

Under BS 7671 Regulation 433.1.1, circuit coordination must satisfy Ib ≤ In ≤ Iz. Ib is design current (operating load). In is nominal protective device rating. Iz is the effective current-carrying capacity of the installed cable after applying site derating factors (Iz = It × Ca × Cg × Ci × Cc).

When must I apply derating factors for thermal insulation (Ci)?

Under BS 7671 Table 52.2 and Regulation 523.9, if a cable passes through or is surrounded by thermal insulation, derating factor Ci applies: 0.89 for 50 mm, 0.81 for 100 mm, 0.68 for 200 mm, 0.63 for 400 mm, and 0.50 if totally enclosed on all sides.

What is the maximum statutory voltage drop allowed?

Under BS 7671 Appendix 4 (Section 6.4) for low-voltage supplies from a public distributor, standard limits are 3% for lighting circuits (6.9 V at 230 V) and 5% for general power, sockets, and machinery (11.5 V at 230 V).

Regulation reference: BS 7671:2018+A4:2026 Requirements for Electrical Installations, Chapter 43 (Protection against overcurrent), Chapter 52 (Selection and erection of wiring systems), and Appendix 4 (Current-carrying capacity and voltage drop).

Suggested:-It: -

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