Verification

BS 7671 cable-sizing verification

Fixed worked examples run through MEPCalc's cable-sizing and circuit-design calculations, compared line by line with the values printed in BS 7671:2018+A2:2022 and the IET On-Site Guide.

Result: 52 of 52 checks pass
Cases: 14 worked examples
Version / published: v1.0 · 2026-09-03
Engine fingerprint: 8241c1ea83c0

Why we publish this

No formal certification scheme covers BS 7671 cable sizing. CIBSE TM33 tests building-services thermal software; nothing equivalent exists for the IET wiring regulations. So we publish our own evidence: a fixed set of worked examples, each with its expected value cited to the table, regulation and printed page it comes from, run through the same calculations our users run.

How to read the tables

Expected is the value printed in the standard, or a hand calculation from printed values. The working is shown under each quantity.
Tolerance is exact for tabulated values and selected conductor sizes, and ±2 % for calculated quantities such as voltage drop, earth-fault loop impedance and adiabatic conductor size.
Every check re-runs automatically whenever the calculation code changes, and this page is regenerated from those results. The engine fingerprint above identifies the calculation code the results came from.

Sources

BS 7671:2018+A2:2022
IET On-Site Guide, 18th Edition, Amendment 2:2022

Printed page numbers refer to the editions above. Amendment 3:2024 to BS 7671 adds two definitions and one regulation on bidirectional protective devices and does not change any table cited here.

Worked examples

CS-01

Flat twin-and-earth clipped direct — 21 A radial on a 25 A Type B breaker, 25 m

Inputs

Design current Ib = 21 A, single-phase 230 V
70 °C thermoplastic flat twin-and-earth, copper
Reference Method C (clipped direct), 25 m, ambient 30 °C, single circuit
Protective device In = 25 A (Type B circuit-breaker)

Checks for case CS-01
Quantity	Expected	Source	MEPCalc	Tolerance	Result
Selected conductor size 1.5 mm² carries 20 A < 25 A; 2.5 mm² carries 27 A ≥ 25 A	2.5mm²	BS 7671:2018+A2:2022 Table 4D5, column 8, printed p456	2.5mm²	exact	Pass
Tabulated current-carrying capacity, It	27A	BS 7671:2018+A2:2022 Table 4D5, column 8 (2.5 mm²), printed p456	27A	exact	Pass
Tabulated voltage drop	18mV/A/m	BS 7671:2018+A2:2022 Table 4D5, column 9 (2.5 mm²), printed p456	18mV/A/m	exact	Pass
Voltage drop 18 mV/A/m × 21 A × 25 m ÷ 1000 = 9.45 V	9.45V	BS 7671:2018+A2:2022 Appendix 4 section 6, printed p429	9.45V	±2 %	Pass
Voltage-drop limit 5 % of 230 V = 11.5 V	11.5V	BS 7671:2018+A2:2022 Table 4Ab row (i), other uses, printed p430	11.5V	exact	Pass

CS-02

Two-core 70 °C thermoplastic clipped direct — 40 A on a 40 A breaker, 10 m

Inputs

Design current Ib = 40 A, single-phase 230 V
Multicore 70 °C thermoplastic, non-armoured, copper (two-core)
Reference Method C (clipped direct), 10 m, ambient 30 °C, single circuit
Protective device In = 40 A

Checks for case CS-02
Quantity	Expected	Source	MEPCalc	Tolerance	Result
Selected conductor size 4 mm² carries 36 A < 40 A; 6 mm² carries 46 A ≥ 40 A	6mm²	BS 7671:2018+A2:2022 Table 4D2A, column 6, printed p450	6mm²	exact	Pass
Tabulated current-carrying capacity, It	46A	BS 7671:2018+A2:2022 Table 4D2A, column 6 (6 mm²), printed p450	46A	exact	Pass
Tabulated voltage drop	7.3mV/A/m	BS 7671:2018+A2:2022 Table 4D2B, column 3 (6 mm²), printed p451	7.3mV/A/m	exact	Pass
Voltage drop 7.3 mV/A/m × 40 A × 10 m ÷ 1000 = 2.92 V	2.92V	BS 7671:2018+A2:2022 Appendix 4 section 6, printed p429	2.92V	±2 %	Pass

CS-03

Four-core 90 °C thermosetting SWA on perforated tray — 90 A three-phase on a 100 A device, 20 m

Inputs

Design current Ib = 90 A, three-phase 400 V
Multicore armoured 90 °C thermosetting, copper (three- or four-core)
Reference Method E (perforated cable tray), 20 m, ambient 30 °C, single circuit
Protective device In = 100 A

Checks for case CS-03
Quantity	Expected	Source	MEPCalc	Tolerance	Result
Selected conductor size 16 mm² carries 99 A < 100 A; 25 mm² carries 131 A ≥ 100 A	25mm²	BS 7671:2018+A2:2022 Table 4E4A, column 5, printed p464	25mm²	exact	Pass
Tabulated current-carrying capacity, It	131A	BS 7671:2018+A2:2022 Table 4E4A, column 5 (25 mm²), printed p464	131A	exact	Pass
Tabulated voltage drop	1.65mV/A/m	BS 7671:2018+A2:2022 Table 4E4B, column 4, z (25 mm²), printed p465	1.65mV/A/m	exact	Pass
Voltage drop 1.65 mV/A/m × 90 A × 20 m ÷ 1000 = 2.97 V	2.97V	BS 7671:2018+A2:2022 Appendix 4 section 6, printed p429	2.97V	±2 %	Pass
Voltage-drop limit 5 % of 400 V = 20 V	20V	BS 7671:2018+A2:2022 Table 4Ab row (i), other uses, printed p430	20V	exact	Pass

CS-04

Ambient 40 °C — 20 A two-core 70 °C thermoplastic clipped direct on a 20 A device, 10 m

Inputs

Design current Ib = 20 A, single-phase 230 V
Multicore 70 °C thermoplastic, non-armoured, copper (two-core)
Reference Method C (clipped direct), 10 m, ambient 40 °C, single circuit
Protective device In = 20 A

Checks for case CS-04
Quantity	Expected	Source	MEPCalc	Tolerance	Result
Ambient-temperature rating factor, Ca	0.87	BS 7671:2018+A2:2022 Table 4B1, 70 °C thermoplastic at 40 °C, printed p441	0.87	exact	Pass
Required tabulated capacity, Ib ÷ (Ca·Cg·Cs·Ci) Ib ÷ Ca = 20 ÷ 0.87 = 22.99 A (Ib = In here, so Equations 1 and 3 coincide)	22.99A	BS 7671:2018+A2:2022 Appendix 4 section 5.1, Equations 1 and 3, printed p425	22.99A	±2 %	Pass
Selected conductor size 1.5 mm² carries 19.5 A < 22.99 A; 2.5 mm² carries 27 A	2.5mm²	BS 7671:2018+A2:2022 Table 4D2A, column 6, printed p450	2.5mm²	exact	Pass
Tabulated current-carrying capacity, It	27A	BS 7671:2018+A2:2022 Table 4D2A, column 6 (2.5 mm²), printed p450	27A	exact	Pass

CS-05

Three circuits bunched — 20 A two-core 70 °C thermoplastic clipped direct on a 20 A device, 10 m

Inputs

Design current Ib = 20 A, single-phase 230 V
Multicore 70 °C thermoplastic, non-armoured, copper (two-core)
Reference Method C (clipped direct), 10 m, ambient 30 °C, three circuits bunched
Protective device In = 20 A

Checks for case CS-05
Quantity	Expected	Source	MEPCalc	Tolerance	Result
Grouping rating factor, Cg	0.7	BS 7671:2018+A2:2022 Table 4C1, item 1, three circuits, printed p443	0.7	exact	Pass
Required tabulated capacity, Ib ÷ (Ca·Cg·Cs·Ci) Ib ÷ Cg = 20 ÷ 0.70 = 28.57 A (Ib = In here, so Equations 2 and 3 coincide)	28.57A	BS 7671:2018+A2:2022 Appendix 4 section 5.1.2, Equations 2 and 3, printed p425	28.57A	±2 %	Pass
Selected conductor size 2.5 mm² carries 27 A < 28.57 A; 4 mm² carries 36 A	4mm²	BS 7671:2018+A2:2022 Table 4D2A, column 6, printed p450	4mm²	exact	Pass
Tabulated current-carrying capacity, It	36A	BS 7671:2018+A2:2022 Table 4D2A, column 6 (4 mm²), printed p450	36A	exact	Pass

CS-06

Buried direct in soil of 1.0 K·m/W at 25 °C — 32 A two-core 70 °C thermoplastic SWA on a 32 A device

Inputs

Design current Ib = 32 A, single-phase 230 V
Multicore armoured 70 °C thermoplastic, copper (two-core)
Reference Method D (buried direct), 30 m, soil thermal resistivity 1.0 K·m/W, ground 25 °C
Protective device In = 32 A

Checks for case CS-06
Quantity	Expected	Source	MEPCalc	Δ	Tolerance	Result
Soil thermal-resistivity rating factor, Cs	1.5	BS 7671:2018+A2:2022 Table 4B3, direct buried cables at 1.0 K·m/W, printed p442	1.5	0.00 %	exact	Pass
Ambient-temperature rating factor, Ca	0.95	BS 7671:2018+A2:2022 Table 4B2, 70 °C thermoplastic at 25 °C ground, printed p441	0.95	0.00 %	exact	Pass

CS-07

500 mm run in thermal insulation — 20 A two-core 70 °C thermoplastic on a 20 A device, 10 m

Inputs

Design current Ib = 20 A, single-phase 230 V
Multicore 70 °C thermoplastic, non-armoured, copper (two-core)
Reference Method C (clipped direct), 10 m, ambient 30 °C, 500 mm totally surrounded by thermal insulation
Protective device In = 20 A

Checks for case CS-07
Quantity	Expected	Source	MEPCalc	Tolerance	Result
Thermal-insulation rating factor, Ci	0.5	BS 7671:2018+A2:2022 Appendix 4 section 2.6, length in insulation ≥ 500 mm (conductors up to 10 mm²), printed p423	0.5	exact	Pass
Required tabulated capacity, Ib ÷ (Ca·Cg·Cs·Ci) Ib ÷ Ci = 20 ÷ 0.50 = 40 A (Ib = In here, so Equations 1 and 3 coincide)	40A	BS 7671:2018+A2:2022 Appendix 4 section 5.1, Equations 1 and 3, printed p425	40A	±2 %	Pass
Selected conductor size 4 mm² carries 36 A < 40 A; 6 mm² carries 46 A	6mm²	BS 7671:2018+A2:2022 Table 4D2A, column 6, printed p450	6mm²	exact	Pass
Tabulated current-carrying capacity, It	46A	BS 7671:2018+A2:2022 Table 4D2A, column 6 (6 mm²), printed p450	46A	exact	Pass

CS-08

Four-core 70 °C thermoplastic clipped direct — 50 A three-phase on a 50 A device, 40 m

Inputs

Design current Ib = 50 A, three-phase 400 V
Multicore 70 °C thermoplastic, non-armoured, copper (three- or four-core)
Reference Method C (clipped direct), 40 m, ambient 30 °C, single circuit
Protective device In = 50 A

Checks for case CS-08
Quantity	Expected	Source	MEPCalc	Tolerance	Result
Selected conductor size 6 mm² carries 41 A < 50 A; 10 mm² carries 57 A	10mm²	BS 7671:2018+A2:2022 Table 4D2A, column 7, printed p450	10mm²	exact	Pass
Tabulated current-carrying capacity, It	57A	BS 7671:2018+A2:2022 Table 4D2A, column 7 (10 mm²), printed p450	57A	exact	Pass
Tabulated voltage drop	3.8mV/A/m	BS 7671:2018+A2:2022 Table 4D2B, column 4 (10 mm²), printed p451	3.8mV/A/m	exact	Pass
Voltage drop 3.8 mV/A/m × 50 A × 40 m ÷ 1000 = 7.6 V	7.6V	BS 7671:2018+A2:2022 Appendix 4 section 6, printed p429	7.6V	±2 %	Pass
Voltage-drop limit 5 % of 400 V = 20 V	20V	BS 7671:2018+A2:2022 Table 4Ab row (i), other uses, printed p430	20V	exact	Pass

CS-09

Lighting circuit — 5 A on a 6 A Type B breaker, 1.0 mm² flat twin-and-earth clipped direct, 30 m

Inputs

Design current Ib = 5 A (the assumed load for a 6 A lighting circuit), single-phase 230 V
70 °C thermoplastic flat twin-and-earth, copper
Reference Method C (clipped direct), 30 m, ambient 30 °C, single circuit
Protective device In = 6 A (Type B circuit-breaker); lighting voltage-drop limit

The 5 A load for a 6 A lighting circuit is the On-Site Guide Table 7.1(i) assumption, printed p74.

Checks for case CS-09
Quantity	Expected	Source	MEPCalc	Tolerance	Result
Selected conductor size 1.0 mm² carries 16 A ≥ 6 A	1mm²	BS 7671:2018+A2:2022 Table 4D5, column 8, printed p456	1mm²	exact	Pass
Tabulated voltage drop	44mV/A/m	BS 7671:2018+A2:2022 Table 4D5, column 9 (1.0 mm²), printed p456	44mV/A/m	exact	Pass
Voltage drop 44 mV/A/m × 5 A × 30 m ÷ 1000 = 6.6 V	6.6V	BS 7671:2018+A2:2022 Appendix 4 section 6, printed p429	6.6V	±2 %	Pass
Voltage-drop limit 3 % of 230 V = 6.9 V	6.9V	BS 7671:2018+A2:2022 Table 4Ab row (i), lighting, printed p430	6.9V	exact	Pass

CS-10

Ring final circuit (On-Site Guide circuit A1) — 32 A Type B, 2.5/1.5 mm² flat twin-and-earth, 60 m loop, TN-C-S

Inputs

Ring final circuit, load 26 A (20 A at the far point plus the balance evenly distributed)
70 °C thermoplastic flat twin-and-earth, copper, 2.5 mm² line with 1.5 mm² protective conductor
Reference Method C (clipped direct), ring loop 60 m, ambient 30 °C
32 A Type B circuit-breaker, TN-C-S with Ze = 0.35 Ω, 0.4 s disconnection

Checks for case CS-10
Quantity	Expected	Source	MEPCalc	Δ	Tolerance	Result
Selected conductor size	2.5mm²	IET On-Site Guide, 18th Edition, Amendment 2:2022 Table H2.1, circuit A1, printed p210	2.5mm²	0.00 %	exact	Pass
Voltage drop 26 A over two parallel legs of L/2 is equivalent to L/4 = 15 m: 18 mV/A/m × 26 A × 15 m ÷ 1000 = 7.02 V	7.02V	IET On-Site Guide, 18th Edition, Amendment 2:2022 Section 7.2.2 load model, printed p86; Table 7.1(i), printed p74	7.02V	0.00 %	±2 %	Pass
Earth-fault loop impedance, Zs Zs = Ze + (R1 + R2) × L/4 = 0.35 + 19.51 × 1.20 × 15 ÷ 1000 = 0.701 Ω	0.701Ω	IET On-Site Guide, 18th Edition, Amendment 2:2022 Table I1 (2.5/1.5 mm² = 19.51 mΩ/m), printed p218; Table I3 (×1.20), printed p220	0.7012Ω	0.03 %	±2 %	Pass
Maximum Zs for the device	1.37Ω	BS 7671:2018+A2:2022 Table 41.3(a), Type B 32 A, printed p68	1.37Ω	0.00 %	exact	Pass

CS-11

Radial socket circuit (On-Site Guide circuit A2) — 32 A Type B, 26 A load, 4/1.5 mm² flat twin-and-earth, 20 m, TN-C-S

Inputs

Radial circuit, design current 26 A (the assumed load for a 32 A radial), single-phase 230 V
70 °C thermoplastic flat twin-and-earth, copper
Reference Method C (clipped direct), 20 m, ambient 30 °C
32 A Type B circuit-breaker, TN-C-S with Ze = 0.35 Ω, 0.4 s disconnection

The 26 A load for a 32 A radial is the On-Site Guide Table 7.1(i) assumption, printed p74.

Checks for case CS-11
Quantity	Expected	Source	MEPCalc	Tolerance	Result
Selected conductor size BS 7671 Table 4D5 column 8: 2.5 mm² carries 27 A < 32 A; 4 mm² carries 37 A	4mm²	IET On-Site Guide, 18th Edition, Amendment 2:2022 Table H2.1, circuit A2, printed p210	4mm²	exact	Pass
Voltage drop 11 mV/A/m × 26 A × 20 m ÷ 1000 = 5.72 V	5.72V	BS 7671:2018+A2:2022 Table 4D5, column 9 (4 mm² = 11 mV/A/m), printed p456	5.72V	±2 %	Pass
Earth-fault loop impedance, Zs Zs = 0.35 + 16.71 × 1.20 × 20 ÷ 1000 = 0.751 Ω	0.751Ω	IET On-Site Guide, 18th Edition, Amendment 2:2022 Table I1 (4/1.5 mm² = 16.71 mΩ/m), printed p218; Table I3 (×1.20), printed p220	0.751Ω	±2 %	Pass
Maximum Zs for the device	1.37Ω	BS 7671:2018+A2:2022 Table 41.3(a), Type B 32 A, printed p68	1.37Ω	exact	Pass

CS-12

The same radial on a 32 A Type C breaker — earth-fault loop impedance governs the size

Inputs

Radial circuit, design current 26 A, single-phase 230 V
70 °C thermoplastic flat twin-and-earth, copper
Reference Method C (clipped direct), 20 m, ambient 30 °C
32 A Type C circuit-breaker, TN-C-S with Ze = 0.35 Ω, 0.4 s disconnection

Checks for case CS-12
Quantity	Expected	Source	MEPCalc	Δ	Tolerance	Result
Maximum Zs for the device	0.68Ω	BS 7671:2018+A2:2022 Table 41.3(b), Type C 32 A, printed p68	0.68Ω	0.00 %	exact	Pass
Selected conductor size 4/1.5 mm² gives Zs = 0.751 Ω > 0.68 Ω, so the next size, 6/2.5 mm², is needed	6mm²	IET On-Site Guide, 18th Edition, Amendment 2:2022 Table I1 (6/2.5 mm² = 10.49 mΩ/m), printed p218	6mm²	0.00 %	exact	Pass
Earth-fault loop impedance, Zs Zs = 0.35 + 10.49 × 1.20 × 20 ÷ 1000 = 0.602 Ω ≤ 0.68 Ω	0.602Ω	IET On-Site Guide, 18th Edition, Amendment 2:2022 Table I1 (6/2.5 mm² = 10.49 mΩ/m), printed p218; Table I3 (×1.20), printed p220	0.6018Ω	0.03 %	±2 %	Pass

CS-13

Adiabatic check — copper protective conductor in a 70 °C thermoplastic cable, 6 kA for 0.1 s

Inputs

Fault current I = 6000 A, disconnection time t = 0.1 s
Copper protective conductor incorporated in a 70 °C thermoplastic cable
Installed conductor 25 mm² (used only for the pass/fail comparison)

Checks for case CS-13
Quantity	Expected	Source	MEPCalc	Δ	Tolerance	Result
Adiabatic constant, k	115	BS 7671:2018+A2:2022 Table 54.3, copper, 70 °C thermoplastic, printed p200	115	0.00 %	exact	Pass
Minimum conductor size, S S = √(I²t) ÷ k = √(6000² × 0.1) ÷ 115 = 1897.4 ÷ 115 = 16.5 mm²	16.5mm²	BS 7671:2018+A2:2022 Regulation 543.1.3, printed p199	16.5mm²	0.00 %	±2 %	Pass

CS-14

Adiabatic check — steel wire armour of a 90 °C thermosetting cable as the protective conductor, 10 kA for 0.4 s

Inputs

Fault current I = 10 000 A, disconnection time t = 0.4 s
Steel wire armour of a 90 °C thermosetting cable used as the protective conductor
Installed armour 150 mm² (used only for the pass/fail comparison)

Checks for case CS-14
Quantity	Expected	Source	MEPCalc	Δ	Tolerance	Result
Adiabatic constant, k	46	BS 7671:2018+A2:2022 Table 54.4, steel, 90 °C thermosetting, printed p200	46	0.00 %	exact	Pass
Minimum conductor size, S S = √(10 000² × 0.4) ÷ 46 = 6324.6 ÷ 46 = 137.5 mm²	137.5mm²	BS 7671:2018+A2:2022 Regulation 543.1.3, printed p199	137.49mm²	0.01 %	±2 %	Pass