25 mm² copper is rated about 112 A installed per Method C (clipped direct, 70 °C thermoplastic) and drops 1.8 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 | 639 m | 1065 m | 2138 m |
| 10 A | 383 m | 639 m | 1283 m |
| 16 A | 240 m | 399 m | 802 m |
| 20 A | 192 m | 319 m | 642 m |
| 32 A | 120 m | 200 m | 401 m |
| 40 A | 96 m | 160 m | 321 m |
| 63 A | 61 m | 101 m | 204 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 112 A as the Method C reference figure.
The nearest equal-or-larger AWG size is 3 AWG — see the gauge converter for exact areas.
About 112 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.