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. 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+A2:2022 Regulation 525 / Appendix 4 Section 6.4.

2. Overload Protection (In & Cc)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 3036 fuses require a 0.725 factor due to their 1.45 fusing factor (Reg 433.1.201).

3. Installation Derating FactorsAmbient 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 ParametersTabulated 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.

Calculation Summary

Live First-Principles Sizing

Suggested cable size (Table 4D5 Method C, clipped T&E)-First-pass only — confirm against the Appendix 4 table for your installation method.

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.

Full 7-Step Design Working

Step 1

Design Current (Ib)

Ib = 32.00 A

Continuous load operating current determined from load demand.

Step 2

Protective Device Rating & Coordination (In)

In = 32.00 A ≥ Ib (32.00 A)

Under BS 7671 Regulation 433.1.1 (Condition 1), the protective device nominal rating In must be ≥ Ib. The circuit conductor capacity is sized to protect against sustained overloads up to In.

Step 3

Installation Correction Factors Considered

Total derating factor (C) = Ca × Cg × Ci × Cc = 1.000

Step 4

Required Tabulated Current-Carrying Capacity (It)

It ≥ In / (Ca × Cg × Ci × Cc)

It ≥ 32.00 / 1.000 = 32.00 A

The minimum continuous current rating the uninstalled cable must possess at 30°C in free air to ensure effective capacity Iz ≥ In under actual site conditions.

Step 5

Conductor Selection from BS 7671 Appendix 4

Look up the required It ≥ - in the appropriate Appendix 4 table matching your cable type and Installation Reference Method:

Table 4D5: 70°C thermoplastic flat twin-and-earth (Method C: Clipped direct, Method 100–103 in insulation).
Table 4D1A / 4D2A: 70°C single-core or multicore PVC in conduit or trunking (Method A / B).
Table 4E4A: 90°C XLPE insulated steel wire armoured (SWA) cables.
Table 4F1A / 4F2A: Flexible cords and industrial rubber cables.

Select the smallest standard conductor cross-section (e.g. 2.5 mm², 4 mm², 6 mm², 10 mm², 16 mm²) whose tabulated capacity satisfies It.

Step 6

Statutory Voltage Drop Verification (Reg 525)

Vd = (mV/A/m × Ib × L) / 1000

Calculated Drop-

Percentage Drop-

Statutory Limit-

Step 7

What This Calculation Has NOT Considered

Current capacity and voltage drop are only two legs of a safe circuit design. The certifying engineer must separately verify:

Earth fault loop impedance (Zs) & Disconnection Times: Verify Zs ≤ Zs_max under BS 7671 Tables 41.2–41.4 to ensure 0.4 s (TN) or 0.2 s (TT) automatic disconnection (Reg 411.3.2).
Thermal fault withstand (Adiabatic Equation): Verify conductor and CPC size satisfy k²S² ≥ I²t under fault currents (Reg 434.5.2 & 543.1.3).
Harmonic neutral currents: Sizing for non-linear loads (LED drivers, IT, VFDs) with >15% or >33% third harmonic content per Appendix 4 Section 5.5.
Environmental conditions: Direct solar radiation derating (Cs), depth of burial (Cd), or corrosive atmospheres.

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+A2:2022 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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