- L
- Length: the one-way route from the consumer unit to the load. Don't double it; the tables allow for the return
- I
- Current: the circuit's design current, or the breaker rating to be safe
- V
- Supply voltage: 230 V single-phase, 400 V between phases on three-phase
- ΔV
- Voltage drop: mV/A/m × I × L ÷ 1,000. At most 3% of V for lighting, 5% for other circuits
Voltage drop per amp per metre (mV/A/m)
| Cable | PVC single-phase | PVC three-phase | XLPE single-phase | XLPE three-phase |
|---|---|---|---|---|
| 1.0 mm² | 44 | 38 | 46 | 40 |
| 1.5 mm² | 29 | 25 | 31 | 27 |
| 2.5 mm² | 18 | 15 | 19 | 16 |
| 4 mm² | 11 | 9.5 | 12 | 10 |
| 6 mm² | 7.3 | 6.4 | 7.9 | 6.8 |
| 10 mm² | 4.4 | 3.8 | 4.7 | 4 |
| 16 mm² | 2.8 | 2.4 | 2.9 | 2.5 |
| 25 mm² | 1.75 | 1.5 | 1.85 | 1.6 |
| 35 mm² | 1.25 | 1.1 | 1.35 | 1.15 |
Copper conductors at full operating temperature: 70 °C for PVC (twin and earth, most flex), 90 °C for XLPE (most armoured SWA). A lightly loaded cable runs cooler and drops a little less, so these are on the safe side.
Longest run within 5%: single-phase PVC (230 V)
| Cable | 6 A (m) | 10 A (m) | 16 A (m) | 20 A (m) | 25 A (m) | 32 A (m) | 40 A (m) | 45 A (m) | 50 A (m) | 63 A (m) |
|---|---|---|---|---|---|---|---|---|---|---|
| 1.0 mm² | 43.6 | 26.1 | 16.3 | 13.1 | 10.5 | 8.2 | 6.5 | 5.8 | 5.2 | 4.1 |
| 1.5 mm² | 66.1 | 39.7 | 24.8 | 19.8 | 15.9 | 12.4 | 9.9 | 8.8 | 7.9 | 6.3 |
| 2.5 mm² | 106 | 63.9 | 39.9 | 31.9 | 25.6 | 20 | 16 | 14.2 | 12.8 | 10.1 |
| 4 mm² | 174 | 105 | 65.3 | 52.3 | 41.8 | 32.7 | 26.1 | 23.2 | 20.9 | 16.6 |
| 6 mm² | 263 | 158 | 98.5 | 78.8 | 63 | 49.2 | 39.4 | 35 | 31.5 | 25 |
| 10 mm² | 436 | 261 | 163 | 131 | 105 | 81.7 | 65.3 | 58.1 | 52.3 | 41.5 |
| 16 mm² | 685 | 411 | 257 | 205 | 164 | 128 | 103 | 91.3 | 82.1 | 65.2 |
| 25 mm² | 1095 | 657 | 411 | 329 | 263 | 205 | 164 | 146 | 131 | 104 |
| 35 mm² | 1533 | 920 | 575 | 460 | 368 | 288 | 230 | 204 | 184 | 146 |
Sockets, cookers, showers and other non-lighting circuits: 5% of 230 V is 11.5 V. Voltage drop only: the cable must also be rated for the current where it's installed (clipped, in a wall, in insulation or grouped with others), and the circuit must disconnect fast enough in a fault. A qualified electrician should design and certify new circuits.
Longest run within 3%: lighting, single-phase PVC (230 V)
| Cable | 3 A (m) | 6 A (m) | 10 A (m) | 16 A (m) | 20 A (m) |
|---|---|---|---|---|---|
| 1.0 mm² | 52.3 | 26.1 | 15.7 | 9.8 | 7.8 |
| 1.5 mm² | 79.3 | 39.7 | 23.8 | 14.9 | 11.9 |
| 2.5 mm² | 128 | 63.9 | 38.3 | 24 | 19.2 |
| 4 mm² | 209 | 105 | 62.7 | 39.2 | 31.4 |
| 6 mm² | 315 | 158 | 94.5 | 59.1 | 47.3 |
| 10 mm² | 523 | 261 | 157 | 98 | 78.4 |
| 16 mm² | 821 | 411 | 246 | 154 | 123 |
| 25 mm² | 1314 | 657 | 394 | 246 | 197 |
| 35 mm² | 1840 | 920 | 552 | 345 | 276 |
Lighting circuits: 3% of 230 V is 6.9 V. Voltage drop only: the cable must also be rated for the current where it's installed (clipped, in a wall, in insulation or grouped with others), and the circuit must disconnect fast enough in a fault. A qualified electrician should design and certify new circuits.
Longest run within 5%: single-phase XLPE (230 V)
| Cable | 6 A (m) | 10 A (m) | 16 A (m) | 20 A (m) | 25 A (m) | 32 A (m) | 40 A (m) | 45 A (m) | 50 A (m) | 63 A (m) |
|---|---|---|---|---|---|---|---|---|---|---|
| 1.0 mm² | 41.7 | 25 | 15.6 | 12.5 | 10 | 7.8 | 6.3 | 5.6 | 5 | 4 |
| 1.5 mm² | 61.8 | 37.1 | 23.2 | 18.5 | 14.8 | 11.6 | 9.3 | 8.2 | 7.4 | 5.9 |
| 2.5 mm² | 101 | 60.5 | 37.8 | 30.3 | 24.2 | 18.9 | 15.1 | 13.5 | 12.1 | 9.6 |
| 4 mm² | 160 | 95.8 | 59.9 | 47.9 | 38.3 | 29.9 | 24 | 21.3 | 19.2 | 15.2 |
| 6 mm² | 243 | 146 | 91 | 72.8 | 58.2 | 45.5 | 36.4 | 32.3 | 29.1 | 23.1 |
| 10 mm² | 408 | 245 | 153 | 122 | 97.9 | 76.5 | 61.2 | 54.4 | 48.9 | 38.8 |
| 16 mm² | 661 | 397 | 248 | 198 | 159 | 124 | 99.1 | 88.1 | 79.3 | 62.9 |
| 25 mm² | 1036 | 622 | 389 | 311 | 249 | 194 | 155 | 138 | 124 | 98.7 |
| 35 mm² | 1420 | 852 | 532 | 426 | 341 | 266 | 213 | 189 | 170 | 135 |
Armoured (SWA) cable to a garage, garden office or car charger is usually XLPE. Voltage drop only: the cable must also be rated for the current where it's installed (clipped, in a wall, in insulation or grouped with others), and the circuit must disconnect fast enough in a fault. A qualified electrician should design and certify new circuits.
Longest run within 5%: three-phase PVC (400 V)
| Cable | 6 A (m) | 10 A (m) | 16 A (m) | 20 A (m) | 25 A (m) | 32 A (m) | 40 A (m) | 45 A (m) | 50 A (m) | 63 A (m) |
|---|---|---|---|---|---|---|---|---|---|---|
| 1.0 mm² | 87.7 | 52.6 | 32.9 | 26.3 | 21.1 | 16.4 | 13.2 | 11.7 | 10.5 | 8.4 |
| 1.5 mm² | 133 | 80 | 50 | 40 | 32 | 25 | 20 | 17.8 | 16 | 12.7 |
| 2.5 mm² | 222 | 133 | 83.3 | 66.7 | 53.3 | 41.7 | 33.3 | 29.6 | 26.7 | 21.2 |
| 4 mm² | 351 | 211 | 132 | 105 | 84.2 | 65.8 | 52.6 | 46.8 | 42.1 | 33.4 |
| 6 mm² | 521 | 313 | 195 | 156 | 125 | 97.7 | 78.1 | 69.4 | 62.5 | 49.6 |
| 10 mm² | 877 | 526 | 329 | 263 | 211 | 164 | 132 | 117 | 105 | 83.5 |
| 16 mm² | 1389 | 833 | 521 | 417 | 333 | 260 | 208 | 185 | 167 | 132 |
| 25 mm² | 2222 | 1333 | 833 | 667 | 533 | 417 | 333 | 296 | 267 | 212 |
| 35 mm² | 3030 | 1818 | 1136 | 909 | 727 | 568 | 455 | 404 | 364 | 289 |
5% of 400 V is 20 V. Voltage drop only: the cable must also be rated for the current where it's installed (clipped, in a wall, in insulation or grouped with others), and the circuit must disconnect fast enough in a fault. A qualified electrician should design and certify new circuits.
Longest run within 5%: three-phase XLPE (400 V)
| Cable | 6 A (m) | 10 A (m) | 16 A (m) | 20 A (m) | 25 A (m) | 32 A (m) | 40 A (m) | 45 A (m) | 50 A (m) | 63 A (m) |
|---|---|---|---|---|---|---|---|---|---|---|
| 1.0 mm² | 83.3 | 50 | 31.3 | 25 | 20 | 15.6 | 12.5 | 11.1 | 10 | 7.9 |
| 1.5 mm² | 123 | 74.1 | 46.3 | 37 | 29.6 | 23.1 | 18.5 | 16.5 | 14.8 | 11.8 |
| 2.5 mm² | 208 | 125 | 78.1 | 62.5 | 50 | 39.1 | 31.3 | 27.8 | 25 | 19.8 |
| 4 mm² | 333 | 200 | 125 | 100 | 80 | 62.5 | 50 | 44.4 | 40 | 31.7 |
| 6 mm² | 490 | 294 | 184 | 147 | 118 | 91.9 | 73.5 | 65.4 | 58.8 | 46.7 |
| 10 mm² | 833 | 500 | 313 | 250 | 200 | 156 | 125 | 111 | 100 | 79.4 |
| 16 mm² | 1333 | 800 | 500 | 400 | 320 | 250 | 200 | 178 | 160 | 127 |
| 25 mm² | 2083 | 1250 | 781 | 625 | 500 | 391 | 313 | 278 | 250 | 198 |
| 35 mm² | 2899 | 1739 | 1087 | 870 | 696 | 543 | 435 | 386 | 348 | 276 |
5% of 400 V is 20 V. Voltage drop only: the cable must also be rated for the current where it's installed (clipped, in a wall, in insulation or grouped with others), and the circuit must disconnect fast enough in a fault. A qualified electrician should design and certify new circuits.
About this chart
How much voltage copper cable loses, and the longest run that stays within the UK wiring regulations' limits: 3% for lighting and 5% for everything else (BS 7671 Appendix 4, for an installation fed from the public supply). Choose a table above the chart.
mV/A/m is millivolts lost for every amp and every metre of run. Multiply by the current and the length to get the drop: 2.5 mm² PVC on single-phase is 18 mV/A/m, so 20 A over 20 m drops 18 × 20 × 20 ÷ 1,000 = 7.2 V.
For your own run, or to check a cable that's already in, use the cable voltage drop calculator.
How it's worked out:
- Voltage drop (V) = mV/A/m × current (A) × length (m) ÷ 1,000
- Longest run (m) = limit (V) × 1,000 ÷ (mV/A/m × current), with the limit 3% or 5% of 230 V (6.9 V or 11.5 V), or of 400 V for three-phase (20 V at 5%)
The mV/A/m values are BS 7671 Appendix 4's tables for copper (4D1B, 4D2B and 4D5 for 70 °C PVC; 4E1B and 4E2B for 90 °C XLPE), checked by working them out from the conductor resistances in BS EN 60228. Reactance is ignored, which is accurate up to 35 mm². The limits apply from the origin of the installation, so any drop in a submain counts too. Private supplies allow 6% and 8%.
Last checked 31 October 2026 · Last rev. 31 October 2026 · 53c7