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Cable Sizing Calculator — BS 7671 🇬🇧

United KingdomEdition BS 7671:2018+A2:2022Free Online Tool

BS 7671:2018+A2:2022, commonly known as the IET Wiring Regulations or the 18th Edition, is the national standard for electrical installations in the United Kingdom. Published by the Institution of Engineering and Technology (IET) and the British Standards Institution (BSI), it sets out the rules for the design, erection, and verification of electrical installations operating at up to 1000V AC.

Cable sizing under BS 7671 follows the methodology in Section 523 (current-carrying capacities) and Appendix 4 (tables of correction factors and cable data). The standard uses a 30°C reference ambient temperature for cables in air and 20°C for cables in the ground. Four correction factors govern the design: Ca for ambient temperature (Table B1), Cg for grouping (Table C1), Ci for thermal insulation (Regulation 523.7), and Cc for the type of overcurrent protective device (Regulation 433.1.1).

This calculator implements the complete BS 7671 cable sizing procedure with real-time Appendix 4 table lookups, automatic factor application, and voltage drop verification against Regulation 525.1. Every result includes specific regulation and table references for audit and compliance purposes.

Open Cable Sizing Calculator

How Cable Sizing Works Under BS 7671

Step 1 — Establish the Design Current (Ib)

Determine the circuit’s maximum design current from the connected load. For a single-phase resistive load at 230V: Ib = P / V. For a three-phase balanced load at 400V: Ib = P / (√3 × VL × cosφ). BS 7671 Regulation 311.1 requires the designer to assess the maximum demand taking into account diversity factors where appropriate (Appendix 1).

Step 2 — Select the Rated Current of the Protective Device (In)

Choose a protective device (MCB, MCCB, or fuse) such that In ≥ Ib per Regulation 433.1. For circuits protected by BS EN 60898 MCBs, the standard ratings are 6, 10, 16, 20, 25, 32, 40, 50, 63, 80, 100, and 125A. The protective device type also affects the Cc factor: for MCBs (or fuses where I2 = 1.45 × In), Cc = 1.0; for semi-enclosed (rewirable) fuses to BS 3036, Cc = 0.725 per Regulation 433.1.1.

Step 3 — Identify the Installation Method

BS 7671 Appendix 4 Table 4A1 defines reference installation methods using letter-number codes. Key methods include: Method A (enclosed in thermally insulating wall), Method B (enclosed in conduit or trunking), Method C (clipped direct to surface), Method E (on unperforated cable tray), and Method D (direct in ground). Each method has associated current-rating tables: Table 4D1A for single-core 90°C thermosetting cables, Table 4D2A for multicore 90°C thermosetting, Table 4D5A for 70°C thermoplastic, etc.

Step 4 — Determine Correction Factors

BS 7671 applies up to four correction factors to find the tabulated current-carrying capacity required:

  • Ca (ambient temperature): From Appendix 4 Table B1. At 30°C (reference) Ca = 1.00. At 40°C, Ca = 0.87 for 70°C thermoplastic, or 0.91 for 90°C thermosetting insulation.
  • Cg (grouping): From Table C1 for cables touching or Table C2 for cables spaced. Two circuits in conduit gives Cg = 0.80; six circuits gives Cg = 0.57.
  • Ci (thermal insulation): Regulation 523.7 defines factors for cables in contact with thermal insulation. Totally surrounded for over 0.5 m: Ci = 0.50. One side touching: Ci = 0.75.
  • Cc (protective device): 0.725 for BS 3036 semi-enclosed fuses; 1.0 for MCBs and HBC fuses per Regulation 433.1.1.

Step 5 — Calculate the Tabulated Current (It)

The cable must have a tabulated current-carrying capacity of at least: It ≥ In / (Ca × Cg × Ci × Cc). Select the smallest standard cable size from the appropriate table whose It meets this requirement. Ensure the effective current-carrying capacity Iz = It × Ca × Cg × Ci × Cc satisfies Iz ≥ In per Regulation 433.1.

Step 6 — Verify Voltage Drop

BS 7671 Appendix 4 Table 4Ab provides voltage drop values in mV/A/m. Calculate: Vdrop = (mV/A/m × Ib × L) / 1000. Regulation 525.1 limits voltage drop to 3% for lighting circuits and 5% for other circuits (from the origin of the installation). If the voltage drop exceeds the limit, upsize the cable and recalculate.

Step 7 — Verify Fault Loop Impedance and Disconnection Time

Regulation 411.3.2.1 requires that the earth fault loop impedance Zs is low enough to ensure the protective device operates within 0.4s for final circuits or 5s for distribution circuits. Tables 41.2–41.6 provide maximum Zs values for each device rating. The calculator checks that the selected cable, combined with the external loop impedance Ze, satisfies this requirement.

Key Reference Tables

Table 4A1 — Installation Methods

Defines reference installation methods using codes A1, A2, B1, B2, C, D1, D2, E, F, G. Each method describes a specific physical arrangement (e.g. B1 = single-core cables in conduit on wall, C = clipped direct).

Select the method matching the physical installation to determine which current-rating table column to use. Method C (clipped direct) gives the highest ratings for cables in air; Method A1 (insulated wall) gives the lowest.

Table 4D1A — Single-Core 90°C Thermosetting (XLPE/EPR) Copper

Current-carrying capacities for single-core copper cables with 90°C-rated insulation (XLPE or EPR), from 1 mm² to 300 mm², across installation methods A1 to F.

Use for single-core XLPE or EPR cables. A 10 mm² cable clipped direct (Method C) is rated at 68A; the same cable in conduit (Method B1) is rated at 57A.

Table 4D2A — Multicore 90°C Thermosetting (XLPE/EPR) Copper

Current ratings for multicore copper cables with thermosetting insulation. Ratings are lower than Table 4D1A due to mutual heating of conductors within the common sheath.

Use for multicore XLPE/EPR cables such as SWA (steel wire armoured). A 10 mm² two-core cable clipped direct is rated at 60A vs 68A for the equivalent single-core.

Table B1 (Appendix 4) — Ambient Temperature Correction Factor (C<sub>a</sub>)

Correction factors for ambient temperatures from 25°C to 80°C for 70°C and 90°C rated insulation. Reference temperature is 30°C (factor = 1.00).

At 35°C ambient, C_a = 0.94 for 70°C PVC insulation. At 50°C, C_a = 0.71 for 70°C PVC. The factor is multiplied into the derating calculation to reduce the allowable current.

Table C1 (Appendix 4) — Grouping Correction Factor (C<sub>g</sub>)

Correction factors for grouped cables based on the number of circuits and the installation arrangement (bunched, single layer on wall, single layer on tray, etc.).

For 4 circuits bunched together in conduit, C_g = 0.65. For 4 circuits in a single layer on a perforated tray, C_g = 0.77. Spacing cables apart can significantly improve the grouping factor.

Table 4Ab (Appendix 4) — Voltage Drop

Millivolt-per-ampere-per-metre values for single-phase and three-phase circuits, for copper and aluminium conductors, split into resistive (r) and reactive (x) components.

Calculate V_drop = mV/A/m × I_b × L / 1000. For a more precise result at non-unity power factor, use the (r cosφ + x sinφ) formula with the separate r and x values from the table.

Worked Example — BS 7671 Cable Sizing

Scenario

A 40A electric shower circuit in a UK house. The cable runs 15 metres from the consumer unit, clipped direct along the wall and through floor joists. Ambient temperature is 30°C (UK reference). No grouping with other circuits. Supply is 230V single-phase. Cable type: 90°C thermosetting (XLPE) twin and earth copper.

1

Determine design current

The shower is rated at 9.5 kW on a 230V single-phase supply (resistive load, power factor = 1.0).

I_b = P / V = 9500 / 230 = 41.3A

I_b = 41.3A — but the shower has a 40A maximum load by design, so I_b = 40A governs.

2

Select protective device

Choose a 40A Type B MCB. Per Regulation 433.1, I_n ≥ I_b: 40A ≥ 40A is satisfactory. As an MCB, C_c = 1.0 per Regulation 433.1.1.

I_n = 40A MCB, C_c = 1.0

3

Identify installation method

Cable is clipped directly to the wall surface. This is BS 7671 Reference Method C per Table 4A1. Use Table 4D2A for multicore 90°C thermosetting copper cable.

Reference Method C, Table 4D2A

4

Determine correction factors

Ambient temperature: 30°C is the reference, so C_a = 1.00 (Table B1). No grouping with other circuits: C_g = 1.00 (Table C1). No thermal insulation contact: C_i = 1.00. MCB protective device: C_c = 1.00.

C_total = C_a × C_g × C_i × C_c = 1.00 × 1.00 × 1.00 × 1.00 = 1.00

C_total = 1.00 (no derating required)

5

Calculate required tabulated current

With all correction factors at unity, the required tabulated current equals the protective device rating.

I_t ≥ I_n / C_total = 40 / 1.00 = 40.0A

I_t ≥ 40.0A

6

Select cable size from Table 4D2A

From BS 7671 Appendix 4 Table 4D2A, Method C, multicore 90°C thermosetting copper: 6 mm² = 47A (satisfactory, 47A ≥ 40A). 4 mm² = 36A would be insufficient.

Selected cable: 6 mm² XLPE twin and earth (47A tabulated rating)

7

Verify voltage drop

From Table 4Ab, the mV/A/m for 6 mm² two-core copper at unity power factor is 7.3 mV/A/m.

V_drop = (mV/A/m × I_b × L) / 1000 = (7.3 × 40 × 15) / 1000 = 4.38V

V_drop = 4.38V (1.90% of 230V) — within the 5% limit for power circuits (Regulation 525.1). PASS.

A 6 mm² XLPE twin and earth cable is adequate for this 40A shower circuit at 15 metres. With no adverse installation conditions (standard ambient, no grouping, no insulation contact), no derating was required and the cable’s 47A rating provides 17.5% headroom above the 40A MCB. The voltage drop of 1.90% is comfortably within the BS 7671 5% limit. If the cable were to pass through thermal insulation in the loft (C_i = 0.50), a 16 mm² cable would be needed instead.

Common Mistakes When Using BS 7671

  1. 1

    Forgetting the C_c factor for BS 3036 semi-enclosed (rewirable) fuses. When protecting a circuit with a rewirable fuse, C_c = 0.725 must be applied per Regulation 433.1.1. This effectively increases the required cable rating by 38%, often necessitating a cable two sizes larger than for MCB protection. Many older UK installations still use BS 3036 fuses.

  2. 2

    Using the wrong current-rating table for the insulation type. BS 7671 Appendix 4 has separate tables for 70°C thermoplastic (PVC, Tables 4D5A/4D5B) and 90°C thermosetting (XLPE/EPR, Tables 4D1A/4D2A). Using the 90°C table for PVC cable overestimates the permitted current by approximately 15–20%.

  3. 3

    Not applying the thermal insulation factor C_i. Regulation 523.7 requires derating when cables are in contact with or surrounded by thermal insulation. With modern energy efficiency requirements, loft insulation depths of 300 mm+ are standard in UK homes. A cable laid on top of and covered by insulation must use C_i = 0.50, halving its effective current-carrying capacity.

  4. 4

    Applying the lighting voltage drop limit (3%) to power circuits. Regulation 525.1 allows 5% voltage drop for power circuits but only 3% for lighting. Misapplying the 3% limit to a ring final or radial power circuit forces unnecessary cable upsizing.

  5. 5

    Ignoring Regulation 433.1 requirement that I_z ≥ I_n. After applying all correction factors, some designers only verify that I_t is sufficient without checking that the effective current-carrying capacity I_z = I_t × C_a × C_g × C_i × C_c is at least equal to I_n. Both conditions must be satisfied simultaneously.

How Does BS 7671 Compare?

BS 7671 is the UK national implementation of IEC 60364, so the general methodology is similar, but table numbering and regulatory references differ entirely. BS 7671 uses explicit correction factor notation (C_a, C_g, C_i, C_c) while AS/NZS 3008 uses derating factor terminology. The critical difference from AS/NZS 3008 is the 30°C reference ambient (vs 40°C), meaning cables rated identically in their respective standards will differ in real-world capacity. Compared to NEC, BS 7671 uses metric (mm²) conductor sizes, mV/A/m voltage drop values, and a fundamentally different approach to conductor temperature rating interaction with termination ratings.

Frequently Asked Questions

BS 7671 Regulation 525.1 sets two voltage drop limits: 3% for lighting circuits and 5% for all other circuits, measured from the origin of the installation to the furthest point of utilisation. For a 230V single-phase supply, this means a maximum of 6.9V for lighting and 11.5V for power circuits. These percentages apply to the nominal supply voltage. Where the installation is supplied via a private LV supply or a long service cable from the DNO transformer, the voltage at the origin may already be lower than nominal, making the effective available drop even less. The IET On-Site Guide provides additional guidance on accounting for supply impedance in these cases.
The C_c factor of 0.725 applies only when the circuit is protected by a semi-enclosed (rewirable) fuse to BS 3036, per Regulation 433.1.1. This is because BS 3036 fuses have a fusing factor of approximately 2 (i.e. they may not blow until the current reaches twice the rated value), compared to 1.45 for MCBs and HBC fuses. The lower C_c factor ensures the cable can safely carry the higher potential let-through current. For circuits protected by MCBs (BS EN 60898), MCCBs, or HBC (cartridge) fuses to BS 88, C_c = 1.0. Rewirable fuses are still found in many older UK properties and must be accounted for in circuit design or modification work.
Regulation 527.2 requires that where cables pass through elements of building construction (walls, floors, partitions), the penetration must be internally sealed to maintain the fire resistance rating of the building element. While this is primarily a fire safety requirement rather than a cable sizing issue, the sealing material can affect thermal dissipation. If a fire barrier material creates a thermal insulation effect, the C_i factor from Regulation 523.7 should be considered. Additionally, Regulation 521.6.2 requires that cables crossing thermal insulation in accessible roof spaces must be suitably supported and positioned to avoid mechanical damage.

Cable Sizing Calculator for Other Standards

🇦🇺AS/NZS 3008
🌍IEC 60364
🇺🇸NEC/NFPA 70

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