Cable Sizing to BS 7671: A Practical Guide

Why cable sizing matters

Cable sizing is one of the most fundamental calculations in electrical design. An undersized cable overheats under load, degrading insulation, creating fire risk, and potentially causing protective devices to trip nuisance. An oversized cable wastes money — copper is expensive — and takes up unnecessary space in containment. The larger the project, the more these costs compound.

BS 7671 (the IET Wiring Regulations) provides the methodology for sizing cables safely and correctly for UK installations. This guide walks through the process step by step, covering current capacity, correction factors, voltage drop, earth fault loop impedance, and a worked example.

The BS 7671 approach

The cable sizing process in BS 7671 is fundamentally about ensuring three things:

1. The cable can carry the design current safely without exceeding its temperature rating.
2. The voltage drop across the cable is within acceptable limits.
3. The protective device can disconnect the circuit fast enough in a fault condition to prevent electric shock.

The key relationship that governs current capacity is:

Where:

• I_b = design current of the circuit (the actual current the cable must carry)
• I_n = nominal rating of the protective device (MCB, MCCB, or fuse)
• I_z = effective current-carrying capacity of the cable (after applying correction factors for the installation conditions)

This relationship ensures the protective device is rated above the design current (so it does not trip in normal operation) but below the cable’s effective capacity (so the cable is protected against overload).

Step 1: Design current (I_b)

The design current is the maximum sustained current the circuit will carry in normal operation. How to calculate it depends on the type of load:

Single-phase loads

Where: P = power (W), V = supply voltage (230 V for UK single phase), pf = power factor.

Three-phase loads

Where: V = line voltage (400 V for UK three phase), pf = power factor.

Step 2: Protective device rating (I_n)

Select the next standard protective device rating at or above the design current I_b. The relationship I_n ≥ I_b must be satisfied.

Standard MCB ratings (BS EN 60898)

6, 10, 16, 20, 25, 32, 40, 50, 63 A

Common MCCB ratings

These vary by manufacturer but typically include: 16, 20, 25, 32, 40, 50, 63, 80, 100, 125, 160, 200, 250 A [VERIFY: standard MCCB rating steps].

The choice between MCB types (B, C, D) depends on the load characteristics — type B for resistive loads, type C for motors and circuits with moderate inrush, type D for high-inrush loads like transformers and large motors.

Step 3: Tabulated current capacity (I_t)

The tabulated current capacity is the base current-carrying capacity of a cable under standard reference conditions. Look this up from BS 7671 Appendix 4, based on three parameters:

• Cable type: Thermoplastic (PVC/PVC) or thermosetting (XLPE/SWA), single-core or multicore.
• Installation method: Reference methods from Table 4A1 — clipped direct to a surface, in trunking, in conduit, on cable tray, in free air, etc. Each method has different thermal dissipation characteristics.
• Conductor cross-sectional area: Standard sizes in mm² — 1.0, 1.5, 2.5, 4, 6, 10, 16, 25, 35, 50, 70, 95, 120, 150, 185, 240, 300 mm².

The tabulated values assume standard conditions: 30°C ambient temperature, no grouping with other circuits, and no thermal insulation contact. Real installations rarely match these ideal conditions, which is why correction factors are needed.

Step 4: Correction factors

Correction factors account for the difference between standard reference conditions and actual installation conditions. Each factor reduces the effective current-carrying capacity of the cable.

Source: BS 7671, 18th Edition [VERIFY: confirm table references against current edition].

The effective current-carrying capacity of the cable is:

The relationship I_z ≥ I_n ≥ I_b must be satisfied. If I_z is less than I_n after applying correction factors, you need to select the next larger cable size and recalculate.

Alternatively, you can rearrange to find the minimum tabulated capacity required:

Then select the smallest cable size from the tables where I_t ≥ I_t (min).

Step 5: Voltage drop check

After sizing the cable for current capacity, you must check that the voltage drop across the cable does not exceed the limits set by BS 7671.

Voltage drop limits

BS 7671 Appendix 12 recommends the following maximum voltage drops from the origin of the installation to any load point [VERIFY: confirm values against current edition]:

• Lighting circuits: 3% of nominal voltage (i.e. 6.9 V for 230 V single phase)
• Other circuits: 5% of nominal voltage (i.e. 11.5 V for 230 V single phase, or 20 V for 400 V three phase)

Calculating voltage drop

Where:

• mV/A/m = voltage drop per amp per metre (from BS 7671, Tables 4D1B onwards, or cable manufacturer data)
• I_b = design current (A)
• L = cable route length (m) — note this is the one-way route length, not a loop

Step 6: Earth fault loop impedance

The final check ensures that in a fault condition (phase to earth), the total earth fault loop impedance is low enough for the protective device to disconnect within the required time. This is critical for electric shock protection.

The total earth fault loop impedance:

Where:

• Z_s = total earth fault loop impedance (Ω)
• Z_e = external earth fault loop impedance (from the supply authority or measured at the origin)
• R₁ = resistance of the phase conductor (Ω)
• R₂ = resistance of the circuit protective conductor (CPC) (Ω)

The calculated Z_s must not exceed the maximum values given in BS 7671, Table 41.3 [VERIFY: confirm table reference] for the type and rating of the protective device. If it does, you may need to increase the cable size (which reduces R₁ and R₂) or upgrade the CPC.

Temperature correction

The (R₁ + R₂) values from BS 7671 tables are given at 20°C. At the cable’s normal operating temperature, resistance is higher. Apply a correction factor:

• For thermoplastic (PVC) cables: multiply (R₁ + R₂) by 1.20 [VERIFY: confirm factor for current edition]
• For thermosetting (XLPE) cables: multiply (R₁ + R₂) by 1.28 [VERIFY: confirm factor for current edition]

Worked example

Size a cable for a three-phase motor with the following parameters:

• Motor: 15 kW, power factor 0.85, 400 V three phase
• Cable run: 50 m, multicore XLPE/SWA, clipped direct to wall
• Grouping: 3 circuits grouped together
• Ambient temperature: 35°C
• Protective device: 32 A MCB, Type C

Step 1: Design current

Step 2: Protective device

I_n = 32 A (the next standard MCB rating above 25.5 A). Check: 32 A ≥ 25.5 A. Satisfied.

Step 3: Correction factors

• C_a for 35°C ambient with XLPE insulation: 0.96 [VERIFY: from BS 7671, Table 4B1]
• C_g for 3 circuits grouped, clipped direct: 0.79 [VERIFY: from BS 7671, Table 4C1]
• C_i = 1.0 (no thermal insulation contact)
• C_c = 1.0 (MCB, not semi-enclosed fuse)

Step 4: Required tabulated capacity

Step 5: Select cable size

From BS 7671 Appendix 4, for multicore XLPE/SWA cable clipped direct (Reference Method C [VERIFY]), the tabulated current capacities are [VERIFY: exact values from current tables]:

• 4 mm²: 36 A — too low (36 < 42.2)
• 6 mm²: 46 A — adequate (46 ≥ 42.2)

Select 6 mm² cable.

Step 6: Voltage drop check

For 6 mm² XLPE/SWA three-phase, mV/A/m = 6.4 [VERIFY: from BS 7671, Table 4D4B or manufacturer data].

5% of 400 V = 20 V. The voltage drop of 8.16 V is well within the limit.

Step 7: Earth fault loop impedance

This requires the external earth fault loop impedance (Z_e) and the (R₁ + R₂) values for the cable. For 6 mm² cable with a 6 mm² CPC over 50 m [VERIFY: look up resistance values from BS 7671 tables], calculate Z_s and check against Table 41.3 for a 32 A Type C MCB [VERIFY: maximum Z_s for Type C 32A].

Common mistakes

• Forgetting correction factors. The most common error. Grouping factors in particular can significantly reduce cable capacity — a factor of 0.65 for 6 grouped circuits is a 35% reduction. Always check the installation conditions.
• Using voltage drop values at the wrong temperature. The mV/A/m values in BS 7671 are given at the conductor operating temperature, not ambient. Check the table notes to ensure you are using the correct column.
• Not checking earth fault loop impedance. Sizing for current and voltage drop but skipping the Z_s check is a compliance failure. This check is particularly important for long cable runs where conductor resistance is significant.
• Sizing only for current and ignoring voltage drop. Long cable runs (above roughly 30–40 m) commonly fail on voltage drop before they fail on current capacity. Always check both.
• Using American wire gauge (AWG). UK installations use metric conductor sizes (mm²). AWG and mm² are not interchangeable — using the wrong sizing system will result in incorrect cable selection.
• Not accounting for future load growth. While you should not massively oversize cables, allowing some margin (typically 10–20% on the protective device rating) for foreseeable load increases is good practice, especially for distribution cables.
• Forgetting that diversity applies to distribution, not final circuits. Individual final circuits should be sized for the full connected load. Diversity factors apply at the distribution board and submain level to reduce overall cable and switchgear sizes.