Zs ≤ 0.8 × Zs(max) (for UK BS 7671 rule of thumb) Draw a table like this:
(Add conversion for higher temps: multiply by 1.2 for operating temp) Draw this table (at rated tripping time, typically 0.4s for final circuits up to 32A): fault loop impedance calculator
| Cable size (mm²) | R1+R2 (mΩ/m) at 20°C | |------------------|----------------------| | 1.0 / 1.0 | 36.2 | | 1.5 / 1.0 | 24.2 | | 1.5 / 1.5 | 30.2 | | 2.5 / 1.5 | 19.5 | | 2.5 / 2.5 | 18.1 | | 4.0 / 1.5 | 13.9 | | 4.0 / 2.5 | 12.9 | | 6.0 / 2.5 | 9.5 | | 6.0 / 4.0 | 8.4 | | 10.0 / 4.0 | 5.8 | | 10.0 / 6.0 | 5.2 | Zs ≤ 0