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BS 7671:2018+A4:2026 · On-Site Guide Table A1

BS 7671 Earth Fault Loop Impedance (Zs)

Select device and rating to see both the BS 7671 tabulated limit (Table 41.2 to 41.4) and the measured on-site limit with the mandatory 0.8 temperature correction multiplier applied.

Protective DeviceRating (In)

Max Measured Zs (80%)1.10 ΩCompare against site test reading

Tabulated 100% Limit1.37 ΩTable 41.3 at conductor operating temp

Show Full Mathematical Derivation (AM2 Working Standard)

BS 7671:2018+A4:2026

AM2 PORTFOLIO & DESIGN AUDIT FORMATTRANSPARENT FIRST-PRINCIPLES MATHS

Step 1 · Stated Test Limits

Maximum Allowable Measured Zs (80% Cold Rule)

Zs(test) ≤ 1.10 Ω (Operating Tabulated Limit: 1.37 Ω)

On-Site Guide Table B6 · BS 7671 Table 41.3

Step 2 · Regulation Formula

Earth Fault Loop Impedance (Reg 411.4.4)

Zs(tab) ≤ (U0 × Cmin) / Ia

U0 = 230 V, Cmin = 0.95 (BS 7671 Table 41.1)

Step 3 · Conductor Temperature Correction (80% Rule)

Cold Circuit Testing Adjustment

Zs(test) ≤ 0.8 × Zs(tab)

BS 7671 Appendix 3 · Rule of thumb for testing at ambient temperature

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).

Quick Cable Current-Carrying Capacity (Iz)

Standard BS 7671 Appendix 4 capacity ratings for standard 70°C thermoplastic (Table 4D5) and 90°C XLPE SWA (Table 4E4A).

Size	T&E Method C (Clipped)	T&E Method A (Conduit)	SWA XLPE (Clipped)
1.5 mm²	20 A	14.5 A	27 A
2.5 mm²	27 A	20 A	36 A
4.0 mm²	37 A	26 A	47 A
6.0 mm²	47 A	32 A	58 A
10 mm²	64 A	44 A	77 A
16 mm²	85 A	57 A	102 A

⚠ Statutory Compliance Notice: This reference is an advisory guide strictly aligned with Electrical Safety First Best Practice Guide 4 (Issue 5). Defect classification remains the sole statutory responsibility of the qualified certifying inspector under the Electricity at Work Regulations 1989.

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12-Way Consumer Unit & Distribution Board Schedule

PINNED BOARD CONSTANTS ACTIVE

Set supply constants once. They are retained across all 12 circuit ways. Calculates maximum demand with On-Site Guide Table A1 diversity, checks 80% Max Zs limits, and evaluates cut-out capacity.

Earthing System

Ze Value (Ω)

Supply Voltage

Supply Cut-Out Fuse

1-Tap Rapid Circuit Presets (Adds Next Way):

Way	Circuit Description	Device	Rating	Cable	Max Zs (80%)	Diversified	Action

Total Connected Load0.0 ADirect arithmetic sum of circuit protective devices

Diversified Maximum Demand0.0 AOn-Site Guide Table A1 diversity rules applied

Cut-Out Fuse StatusCOMPLIANT (Within Fuse)Headroom: 100.0 A remaining

Statutory Design Notice: This schedule is a design working aid calculated in accordance with BS 7671:2018+A4:2026 and IET On-Site Guide Appendix A diversity conventions. EV charging is evaluated at 100% continuous demand (zero diversity) unless automated Dynamic Load Management is declared per BS 7671 §722.311. The contractor retains full design responsibility under the Electricity at Work Regulations 1989. Not valid as a statutory Electrical Installation Certificate (EIC).