Verify earth fault loop impedance to BS 7671:
Zs = Ze + (R1+R2), corrected to operating temperature.
Compares the hot design Zs against the tabulated maximum (Table 41.3).
For design guidance only. Always verify with a qualified engineer.
About this earth fault loop impedance calculator
This free earth fault loop impedance calculator works out the design Zs of a final circuit and checks it against the maximum value permitted by BS 7671:2018+A2:2022 for your protective device. It is built for electricians, designers and inspectors who need a quick fault-loop check during design or verification, before confirming the result with a measured Zs on site. All calculation runs in your browser. Nothing is uploaded.
How earth fault loop impedance is calculated
The earth fault loop impedance is the sum of the external loop impedance and the circuit conductors: Zs = Ze + (R1+R2). The conductor contribution is taken from the IET On-Site Guide Table I1 (mΩ/m at 20 °C) and corrected to the conductor's operating temperature: (R1+R2) = mΩ/m × length × temperature factor ÷ 1000, using 1.20 for 70 °C thermoplastic and 1.28 for 90 °C thermosetting cable. The temperature-corrected (hot) design Zs is then compared directly against the tabulated maximum in BS 7671 Table 41.3, which already incorporates the Cmin = 0.95 voltage factor (Reg 411.4.5). The calculator also surfaces the informational 0.8 (80 %) rule-of-thumb limit used for a cold site-measured Zs.
Frequently asked questions
What is Ze and what value should I use?
Ze is the external earth fault loop impedance at the origin of the installation. Use the measured value where available; otherwise the BS 7671 maxima are 0.35 Ω for TN-C-S (PME) and 0.8 Ω for TN-S supplies.
Why is the design Zs compared to the full Table 41.3 limit?
The Table 41.3 maximum Zs values already include the Cmin = 0.95 voltage factor, so the temperature-corrected design Zs is compared directly against the full tabulated limit with no further de-rating.
What is the 0.8 rule of thumb for?
The 0.8 (80 %) rule gives the limit a cold site-measured Zs should meet, allowing for the rise in conductor resistance when warm. Because this calculator's design Zs is already temperature-corrected, the 0.8 figure is shown for reference only and is not the pass/fail threshold.