120 mm² copper is rated about 299 A installed per Method C (clipped direct, 70 °C thermoplastic) and drops 0.4 mV/A/m (single-phase figure, matching BS 7671 Appendix 4 within rounding). The chart shows how far it runs before the drop exceeds the limits.
| Load | 230 V @ 3% | 230 V @ 5% | 400 V 3Ø @ 5% |
|---|---|---|---|
| 6 A | 3067 m | 5111 m | 10264 m |
| 10 A | 1840 m | 3067 m | 6158 m |
| 16 A | 1150 m | 1917 m | 3849 m |
| 20 A | 920 m | 1533 m | 3079 m |
| 32 A | 575 m | 958 m | 1925 m |
| 40 A | 460 m | 767 m | 1540 m |
| 63 A | 292 m | 487 m | 978 m |
Computed with ρ ≈ 0.0225 Ω·mm²/m (copper at operating temperature), matching BS 7671 Appendix 4 mV/A/m values within rounding. Grouping, insulation type and installation method change the amp rating — treat 299 A as the Method C reference figure.
The nearest equal-or-larger AWG size is 250 kcmil — see the gauge converter for exact areas.
About 299 A for copper clipped direct (Method C). Enclosed in conduit in insulation it derates substantially — always check the installation method column in BS 7671 Appendix 4.