Cable conductor resistance and voltage drop table
IEC 60228 maximum conductor resistance for copper and aluminium, with the BS 7671 Appendix 4 voltage drop figure in mV/A/m derived at both conductor operating temperatures.
What these numbers are
R₂₀ is the maximum DC resistance of a Class 2 stranded conductor at 20 °C, as capped by IEC 60228. It is not the resistance you get from dividing resistivity by the nominal cross-section: a 2.5 mm² conductor is capped at 7.41 Ω/km, about 7% more than nominal area predicts, because of stranding, lay length and the manufacturing tolerance the standard permits.
mV/A/m is millivolts of voltage drop per ampere of design current per metre of run — the form BS 7671 Appendix 4 tabulates, because it collapses the conductor resistance, the operating temperature and the go-and-return path into one number you can look up and multiply:
ΔV = (mV/A/m) × I b × L ÷ 1000
The two temperatures are the conductor operating temperatures the regulations tabulate at: 70 °C for thermoplastic (PVC) insulation and 90 °C for thermosetting (XLPE), which is what multicore SWA normally is.
How each figure is derived
mV/A/m = k × R₂₀ × [1 + α(T − 20)]
where k is 2 for DC and single-phase (out and back) or √3 for balanced three-phase, α is the temperature coefficient of resistance (0.00393/K for copper, 0.00403/K for aluminium) and T is the conductor operating temperature. Nothing on this page is transcribed by hand — every value is computed from the same module the voltage drop calculator uses, so the table and the calculator cannot disagree.
Copper
The final column is the difference between the derived 70 °C single-phase figure and the corresponding published BS 7671 Appendix 4 value, as a percentage. It is shown for sizes up to 16 mm² only — see the note below on reactance.
| Size (mm²) | R₂₀ (Ω/km) | 70 °C 1-ph | 70 °C 3-ph | 90 °C 1-ph | 90 °C 3-ph | vs BS 7671 |
|---|---|---|---|---|---|---|
| 0.5 | 36 | 86.1 | 74.6 | 91.8 | 79.5 | — |
| 0.75 | 24.5 | 58.6 | 50.8 | 62.5 | 54.1 | — |
| 1 | 18.1 | 43.3 | 37.5 | 46.2 | 40 | -1.6% |
| 1.5 | 12.1 | 29 | 25.1 | 30.9 | 26.7 | -0.2% |
| 2.5 | 7.41 | 17.7 | 15.4 | 18.9 | 16.4 | -1.5% |
| 4 | 4.61 | 11 | 9.55 | 11.8 | 10.2 | 0.3% |
| 6 | 3.08 | 7.37 | 6.38 | 7.85 | 6.8 | 1.0% |
| 10 | 1.83 | 4.38 | 3.79 | 4.67 | 4.04 | -0.5% |
| 16 | 1.15 | 2.75 | 2.38 | 2.93 | 2.54 | -1.7% |
| 25 | 0.727 | 1.74 | 1.51 | 1.85 | 1.61 | — |
| 35 | 0.524 | 1.25 | 1.09 | 1.34 | 1.16 | — |
| 50 | 0.387 | 0.926 | 0.802 | 0.987 | 0.855 | — |
| 70 | 0.268 | 0.641 | 0.555 | 0.683 | 0.592 | — |
| 95 | 0.193 | 0.462 | 0.4 | 0.492 | 0.426 | — |
| 120 | 0.153 | 0.366 | 0.317 | 0.39 | 0.338 | — |
| 150 | 0.124 | 0.297 | 0.257 | 0.316 | 0.274 | — |
| 185 | 0.0991 | 0.237 | 0.205 | 0.253 | 0.219 | — |
| 240 | 0.0754 | 0.18 | 0.156 | 0.192 | 0.167 | — |
| 300 | 0.0601 | 0.144 | 0.125 | 0.153 | 0.133 | — |
Aluminium
IEC 60228 does not list aluminium conductors below 16 mm², because they are not a standard product. Aluminium has roughly 61% the conductivity of copper, so it needs about 1.6× the cross-section for the same drop.
| Size (mm²) | R₂₀ (Ω/km) | 70 °C 1-ph | 70 °C 3-ph | 90 °C 1-ph | 90 °C 3-ph |
|---|---|---|---|---|---|
| 16 | 1.91 | 4.59 | 3.97 | 4.9 | 4.24 |
| 25 | 1.2 | 2.88 | 2.5 | 3.08 | 2.66 |
| 35 | 0.868 | 2.09 | 1.81 | 2.23 | 1.93 |
| 50 | 0.641 | 1.54 | 1.33 | 1.64 | 1.42 |
| 70 | 0.443 | 1.06 | 0.922 | 1.14 | 0.984 |
| 95 | 0.32 | 0.769 | 0.666 | 0.821 | 0.711 |
| 120 | 0.253 | 0.608 | 0.527 | 0.649 | 0.562 |
| 150 | 0.206 | 0.495 | 0.429 | 0.528 | 0.457 |
| 185 | 0.164 | 0.394 | 0.341 | 0.421 | 0.364 |
| 240 | 0.125 | 0.3 | 0.26 | 0.321 | 0.278 |
| 300 | 0.1 | 0.24 | 0.208 | 0.256 | 0.222 |
The limit of this table: reactance
Above 16 mm², BS 7671 Appendix 4 splits its tabulated figure into a resistive component r and a reactive component x, and the value you must design to is z = √(r² + x²). Reactance depends on the cable’s construction and conductor spacing; it is not derivable from conductor resistance, and the tabulated values are BSI’s.
Every figure in these tables is the resistive component only. Below 25 mm² that is the whole story and the agreement with the published table is within 2%. At and above 25 mm² these values are lower than the figure you must design to, and the gap widens with conductor size. For those sizes take z from Appendix 4 for your specific cable type. This page states that rather than filling the gap with an estimate, because a number that looks authoritative and is not is worse than no number.
Voltage drop limits
| Standard | Lighting | Other loads |
|---|---|---|
| BS 7671 Table 4Ab — public LV supply | 3% | 5% |
| BS 7671 Table 4Ab — private LV supply | 6% | 8% |
| IEC 60364-5-52 Annex G | 3% | 5% |
| NEC 210.19(A) / 215.2(A) | 3% branch, 5% feeder + branch | |
Using this table
It is free to quote and reproduce with attribution to this page. If you want the arithmetic done for you, including the limit check and the smallest conductor that meets it, use the cable voltage drop calculator.
The derivation, the sources and the review process behind these figures are set out in the methodology. The table is reviewed by Salamot Hok, electrician with 10+ years of installation experience. Results are engineering guidance, not a code sign-off — see the disclaimer.
Sources
- IEC 60228 — Conductors of insulated cables — webstore.iec.ch
- BS 7671 — Requirements for Electrical Installations (IET Wiring Regulations) — electrical.theiet.org
- IEC 60364-5-52 — Selection and erection of wiring systems — webstore.iec.ch
ElectroCalculators is not affiliated with BSI, the IET or the IEC. Standard titles and article numbers are cited for identification.