Max Zs and expected Zs calculator
The calculator needs JavaScript. The worked example, formula and figures below show exactly how it works, so you can do the sum yourself. A guide, not advice. Check your supplier's or maker's figures before ordering.
Short answer
Worked example
| You enter | Value |
|---|---|
| External earth loop impedance | 0.35 Ω |
| Cable | 2.5/1.5 mm² twin and earth |
| Cable length | 30 m |
| R1+R2 per metre at 20 °C | 19.51 mΩ/m |
| Temperature multiplier | 1.2 × |
| Protective device | Type B MCB or RCBO |
| Device rating | 32 A |
| Maximum Zs for the device | 1.37 Ω |
| Rule of thumb for measured values | 0.8 × |
Expected Zs: 1.05 Ω (indicative)
- Within the guidance Expected Zs within 80% of the maximum: 1.052 Ω UK-wide: BS 7671:2018+A4:2026 Tables 41.2–41.4. Check with the source
| Result | Value |
|---|---|
| Expected Zs (indicative) | 1.05 Ω |
| Expected R1+R2 at operating temperature | 0.702 Ω |
| Maximum Zs for the device | 1.37 Ω |
| 80% figure (for measured values) | 1.1 Ω |
| Longest run within the 80% figure | 31.86 m |
| R1+R2 per metre used | 19.51 mΩ/m |
Show the working
- R1+R2 per metre: 19.51 mΩ/m (your figure)
- Expected R1+R2: 19.51 × 30 m × 1.2 ÷ 1000 = 0.702 Ω
- Expected Zs: 0.35 Ω + 0.702 Ω = 1.05 Ω
- Maximum Zs: 1.37 Ω (your figure); × 0.8 = 1.1 Ω
- Longest run: (1.1 − 0.35) × 1000 ÷ (19.51 × 1.2) = 31.9 m
How it works
- Expected R1+R2 = (R1+R2 per metre in mΩ/m) × length (m) × temperature multiplier ÷ 1000
- Expected Zs = Ze + expected R1+R2
- 80% figure = maximum Zs × 0.8: a rule of thumb for comparing a measured (cold) value with the tabulated maximum
- Longest run = (80% figure − Ze) × 1000 ÷ (R1+R2 per metre × multiplier)
Maximum Zs values come from BS 7671 Tables 41.2–41.4 for the device and disconnection time; ours are still being checked against A4:2026.
Indicative only: an electrician measures Zs and checks it against BS 7671.
Our default (adjustable)
These are typical planning figures and our own allowances, not facts about your product. Change any of them in the calculator to match your bag, pack or tin.
| Figure | Our default (adjustable) | Kind | Source |
|---|---|---|---|
| Temperature multiplier | 1.2 × | Our assumption Not yet checked against the source | Our choice: change it to suit your job brief v2 E5 (ZS1): multiplier taking R1+R2 at 20 °C to conductor operating temperature. Researcher to cite IET Guidance Note 3 / On-Site Guide. |
| Rule of thumb for measured values | 0.8 × | Our assumption Not yet checked against the source | Our choice: change it to suit your job brief v2 E5: the 80% rule of thumb for comparing a measured Zs with the tabulated maximum. Researcher to cite. |
Regulations and standards
- BS 7671:2018+A4:2026 Tables 41.2–41.4 (maximum earth fault loop impedance)
- BS 7671:2018+A4:2026 Reg 411.4 (automatic disconnection, TN systems)
- IET Guidance Note 3 / On-Site Guide (R1+R2 per metre, temperature multiplier)
Explained: BS 7671 (the IET Wiring Regulations), The IET On-Site Guide.
BS 7671 is used across the UK.
| Table | Clause | Nations | Status |
|---|---|---|---|
| R1+R2 per metre at 20 °C for flat twin and earth (line/CPC) | IET On-Site Guide / Guidance Note 3 (R1+R2 per metre); prefer the cable maker's conductor resistance data | UK-wide | 0/7 values entered Not yet checked against the source |
| Maximum earth fault loop impedance (Zs) by protective device and rating | BS 7671:2018+A4:2026 Tables 41.2–41.4 (0.4 s and 5 s disconnection; Researcher to confirm the A4:2026 values and which table applies) | UK-wide | 0/46 values entered Not yet checked against the source |