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Elecwork Design Calculator Suite
BS 7671:2018+A2:2022 — PFC • Adiabatic • Voltage Drop • Design Calcs • EFLI • Max Zs Lookup
Quick Reference Tool Only — Not a Substitute for Full Engineering Design
This calculator suite is provided by Elecwork Contractors Ltd as a quick estimation and reference aid only. Results are based on simplified formulas and standard tabulated values from BS 7671:2018+A2:2022 and the IET On-Site Guide. They do not account for all installation-specific variables, site conditions, or manufacturer data. All calculations must be verified by a qualified electrician or electrical engineer before use in any design, installation, inspection, or certification document. Elecwork Contractors Ltd accepts no liability for any loss, damage, or non-compliance arising from the use of these tools. Always refer to the current edition of BS 7671, the IET On-Site Guide, and relevant Guidance Notes for authoritative design guidance.
Prospective Fault Current Reg 411.4
Calculate PFC from supply impedance Ze and combined cable resistance R1+R2
Ipf = U0 / Zs | Zs = Ze + (R1+R2)
U0 = nominal supply voltage • Ze = external loop impedance • R1+R2 = combined phase & CPC resistance (Ω)
Loop Impedance Zs
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PFC at Board
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Assessment
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Adiabatic Equation — Minimum CPC / Conductor CSA Reg 543.1.3
Verify the protective conductor withstands fault energy without thermal damage
S = √(I² × t) / k
S = minimum CSA (mm²) • I = fault current (A) • t = disconnection time (s) • k = material constant (BS 7671 Tables 54.2–54.6)
Min CSA Required
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Proposed CSA
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Result
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Voltage Drop Appendix 4
Standard voltage drop check using BS 7671 Appendix 4 tabulated mV/A/m values
ΔV = (mV/A/m × Ib × L) / 1000
Limit: 3% for lighting • 5% for power / other circuits • mV/A/m values from BS 7671 Appendix 4, Table 4D2B
Voltage Drop
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% of Supply
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Permitted Limit
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Result
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General Design Calculations BS 7671 Ch.43 / App.4
Three-condition cable sizing check — select cable from BS 7671 tables, apply correction factors, verify compliance
Condition 1: Ib ≤ In | Condition 2: In ≤ Iz | Condition 3: Iz = It × Ca × Cg × Cs × Cc ≥ In
Ib = design current • In = device rating • It = BS 7671 tabulated current • Iz = derated current-carrying capacity. All three must be met simultaneously.
Circuit Currents
Cable Selection — BS 7671 Tabulated Current (It)
Auto-filled from BS 7671 Table 4D2A (Method A). You can override this value manually.
Correction Factors
Combined Factor
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Derated Iz (It × factors)
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Tabulated It
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Overall Result
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Cond 1: Ib ≤ In
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Cond 2: In ≤ Iz
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Cond 3: Iz ≥ In
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Headroom Iz − In
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EFLI & Max Cable Length Reg 411.4 / 643.7
Enter your circuit data once — instantly get Zs verification and the maximum permissible cable length for your device
Zs = Ze + [(r1+r2) × L × Ct] / 1000 | Lmax = (Zs(max) − Ze) × 1000 / [(r1+r2) × Ct]
Ct = 1.00 (ambient) • 1.20 (70°C PVC) • 1.28 (90°C XLPE) • 80% limit used for measured site readings (OSG Table 7.2.7ii)
Circuit & Supply Data
Protective Device
Earth Fault Loop Impedance
Calculated Zs
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Max Zs 100% (design)
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Max Zs 80% (measured)
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Design Result
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Fault Current If
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Margin to 100% limit
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Measured compliance (80%)
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Maximum Cable Length
Max length (100% design)
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Max length (80% measured)
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Available Z for cable
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Length headroom
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100% vs 80% explained: The 100% value (BS 7671 Table 41.3) is the design limit — use for cable selection and recording on test certificates. The 80% value (OSG Table 7.2.7ii) is for comparing your measured on-site loop reading at ambient temperature — if your meter reading exceeds the 80% value, the circuit may not disconnect in time under load conditions and further investigation is required.
Maximum Zs Reference Table OSG Table 7.2.7(ii)
Maximum permissible measured earth fault loop impedance values at ambient temperature (80% of BS 7671 Table 41.3 values, incorporating Cmin = 0.95), 230V TN system
When to use 100% vs 80% values
100%
BS 7671 Table 41.3 — Design & Certification
Use these values when designing a circuit (calculating Zs from Ze + R1+R2) and when recording the maximum permitted Zs on an Electrical Installation Certificate or EICR schedule of circuit details. These are the figures for conductors at their maximum operating temperature.
Use these values when designing a circuit (calculating Zs from Ze + R1+R2) and when recording the maximum permitted Zs on an Electrical Installation Certificate or EICR schedule of circuit details. These are the figures for conductors at their maximum operating temperature.
80%
OSG Table 7.2.7(ii) — On-site Testing
Use these values when comparing your measured loop impedance reading from a site tester. Because testing is done at ambient temperature (conductors are cold), the measured Zs must not exceed 80% of the BS 7671 limit. If your reading exceeds the 80% value shown in the table below, further investigation is required per BS 7671 Appendix 14 before the circuit can be certified.
Use these values when comparing your measured loop impedance reading from a site tester. Because testing is done at ambient temperature (conductors are cold), the measured Zs must not exceed 80% of the BS 7671 limit. If your reading exceeds the 80% value shown in the table below, further investigation is required per BS 7671 Appendix 14 before the circuit can be certified.
The table below shows the 80% (measured) values. The quick-check panel above shows both 100% and 80% when a rating and Zs reading are entered.
| Device / Rating (A) | MCB/RCBO Type B | MCB/RCBO Type C | MCB/RCBO Type D | BS 88-2 HRC | BS 3036 Rewireable | |||||
|---|---|---|---|---|---|---|---|---|---|---|
| 0.4s | 5s | 0.4s | 5s | 0.4s | 5s | 0.4s | 5s | 0.4s | 5s | |
Values shown are maximum permitted measured Zs at ambient temperature (approx. 80% of BS 7671 Table 41.3 values, Cmin = 0.95 applied, 230V). N/A = device not available at this rating or not appropriate for fault protection. Compare your measured site loop impedance against these values. If your reading exceeds the relevant value, further investigation is required per BS 7671 Appendix 14.
R1+R2 & Cable Length Calculator OSG Table 9A
Fill in what you know — get the unknown calculated instantly
Cable Details & Temperature Correction
1
I have a measured R1+R2 — find the cable length
On-site continuity / loop test result
L = R1+R2 × 1000 / (r1+r2) • length (m) = measured resistance (Ω) × 1000 ÷ tabulated mΩ/m
Estimated one-way cable length
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2
I have a cable length — find the expected R1+R2
Pre-test estimate / design check
R1+R2 = (r1+r2) × L × Ct / 1000 • estimated resistance (Ω) = tabulated mΩ/m × length (m) × Ct ÷ 1000
Estimated R1+R2
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Ct correction: For cold on-site testing (standard continuity / loop test) leave Ct = 1.00. Apply Ct = 1.20 (PVC) or 1.28 (XLPE) only if the cable is at its full operating temperature — typically used when back-calculating from a live circuit reading or when designing to worst-case conductor resistance.