What Size Cable Do I Need? (And Why Voltage Drop Decides)
Every electrical installation is the same three things: a source that supplies the voltage, a cable that carries it, and a load that uses it. Cable sizing is the question of how much copper (or aluminium) that middle link needs so the load still receives a usable voltage.
The cable is not a perfect conductor. Its resistance is R = 2ρL / A: it grows with the length of the run, falls as the cross-sectional area grows, and depends on the metal — copper at 0.0172 Ω·mm²/m, aluminium at 0.0282 Ω·mm²/m at 20 °C. Resistance also climbs with heat, by roughly 0.4% per °C, so the calculator corrects it to the conductor working temperature — 70 °C for PVC under load. Current flowing through that resistance loses voltage as heat, so the far end of a long, thin cable sees less than the source delivers.
Worked example: a 3 kW heater at 230 V draws 13.0 A. Over 25 m of copper at its 70 °C working temperature, 1.5 mm² loses 8.9 V (3.89%) — outside a 3% budget — while 2.5 mm² loses 5.4 V (2.33%). So 2.5 mm² is the answer here. Stretch the same run to 40 m and 2.5 mm² reaches 3.73%: the cable has not changed, the answer has, and 4 mm² becomes the smallest size that passes.
That is exactly what the scene above is for. Change the load and the demand changes; change the length, the material or the limit and the verdict moves with it; pick a different size in the comparison strip and the cable in the drawing visibly thickens or thins while every number follows. The tool recommends the smallest candidate that keeps the calculated drop inside your selected limit — nothing more, nothing less.
What it does not consider: current-carrying capacity, ambient-temperature and grouping derating, protective device selection, fault-loop impedance and disconnection times are all out of scope. A real design has to clear those gates separately — this calculator tells you what the physics of the run does to the voltage.
The Cable Sizing Equations Behind the Scene
Resistance of the run
R = 2 × ρ × L / AResistance grows with length (L) and falls with cross-sectional area (A). ρ is the conductor resistivity — 0.0172 Ω·mm²/m for copper, 0.0282 Ω·mm²/m for aluminium at 20 °C. The factor 2 is the round trip: out on the line conductor, back on the neutral.
Voltage drop
ΔV = I × R [AC: ΔV = 2 × I × L × (r cos φ + x sin φ)]Ohm’s law applied to the whole loop. Every candidate cable size is scored with this by the shared voltage-drop engine, so the ladder and the answer can never disagree.
Design current
I = P / (V × cos φ) [AC] · I = P / V [DC]The demand the load places on the run. A 3 kW heater at 230 V draws 13.0 A; a 100 W lighting load draws 0.43 A.
Voltage actually delivered
V_load = V_source − ΔVWhat arrives at the far end. This is the number that matters to the lamp, the motor or the heater — not the nominal voltage at the origin.
Drop as a percentage
ΔV% = ΔV / V_source × 100The figure the limit is compared against: 3% of 230 V is 6.9 V, 5% is 11.5 V.
How to Size a Cable by Voltage Drop: Step-by-Step
- 1
Set the source
Choose AC or DC and the nominal voltage (230 V AC by default). Three-phase is out of scope in this version.
- 2
Choose the load
Pick Lighting, Fan, Motor, Heater or Appliance — or Custom, where you enter the power in watts and the power factor yourself. The scene swaps to that load immediately.
- 3
Describe the cable run
Select copper or aluminium and enter the one-way cable length from source to load.
- 4
Choose a voltage-drop limit
Pick 3%, 5% or your own figure. Treat it as a design choice: 3% / 5% are the values most standards quote for a public LV supply, but local rules differ.
- 5
Read the recommendation, then experiment
The tool recommends the smallest candidate that passes. Click any size in the comparison strip to inspect it: the cable in the scene thickens or thins and every number follows.
BS 7671 & IEC 60364 Permissible Voltage Drop Limits
Under the UK Wiring Regulations (BS 7671:2018+A4:2026, Regulation 525.1 with the limits tabulated in Appendix 4 Table 4Ab) and the international IEC 60364-5-52 Annex G guidance (Table G.52.1), the voltage drop between the origin of the installation and any fixed equipment must not exceed:
| Installation Type & Standard | Lighting Circuits (Max %) | Other Uses: Sockets, Power, Heating (Max %) |
|---|---|---|
| Public LV Supply — BS 7671 (UK) | 3.0% (6.9 V at 230 V) | 5.0% (11.5 V at 230 V) |
| Public LV Supply — IEC 60364-5-52 Annex G, Table G.52.1 | 3.0% (6.9 V at 230 V) | 5.0% (11.5 V at 230 V) |
| Private LV Supply (generator, on-site transformer, solar PV) | 6.0% (13.8 V at 230 V) | 8.0% (18.4 V at 230 V) |
| Three-phase 400 V final circuits (both standards) | 3.0% (12 V) | 5.0% (20 V) |
| Long runs — IEC Annex G note | Beyond 100 m of main wiring the ceilings may be raised by 0.005% per extra metre, capped at +0.5% | |
These are the installation allowances only. The distributor's own drop sits outside them: EN 50160 lets a 230 V public supply run between −6% and +10% of nominal at the meter, so a circuit calculated at exactly 5% can still arrive at the equipment hungrier than expected. Where a board is fed by a submain the budget is shared — a common split is about 0.5% for the consumer mains, 1.5–2% for the submain, and the remainder for the final circuit.
Ways to Bring an Oversized Cable Back Down
- Shorten the run: drop is directly proportional to length, so a better route is cheaper than thicker copper — halving the distance halves the drop.
- Upsize one step at a time: each step up the ladder cuts resistance roughly in proportion to area, so the first step buys the most (1.5 → 2.5 mm² takes the heater example from 3.89% to 2.33%).
- Raise the supply voltage: at twice the voltage the same load draws half the current, and the same volt drop is half the percentage — a 4× improvement in margin.
- Choose copper where the run is long: aluminium is lighter and cheaper but drops about 1.6× more voltage for the same size; on the 25 m heater example aluminium needs 4 mm² where copper is comfortable on 2.5 mm².
- Split the load: two shorter circuits each carrying half the current keep both inside the limit where one long radial would not.
- Reconsider the limit: 3% and 5% are the values most standards quote for a public LV supply, but where the limit is measured from and which rules apply are design decisions — make them deliberately, and check them against your local regulations.
Frequently Asked Questions
How does this calculator choose a cable size?
It evaluates every candidate cross-section with the shared voltage-drop engine and recommends the smallest one whose calculated drop stays inside the limit you selected. A 3 kW heater at 230 V draws 13.0 A; over 25 m of copper that is 7.48 V (3.25%) on 1.5 mm² — over a 3% limit — and 4.49 V (1.95%) on 2.5 mm², so 2.5 mm² is the recommendation. Push the same run to 40 m and 2.5 mm² reaches 3.12%, so the answer becomes 4 mm².
Why does a bigger cable reduce voltage drop?
Because resistance is ρL/A: doubling the cross-sectional area halves the resistance of the run, and the drop is I × R. On the 25 m heater example, going from 1.5 mm² to 2.5 mm² takes the drop from 7.48 V to 4.49 V, and 10 mm² brings it down to 1.12 V.
Is the 3% or 5% voltage-drop limit a legal requirement?
It is a design parameter here, not a compliance certificate. 3% and 5% are the figures most commonly quoted for a public low-voltage supply (BS 7671 Reg 525.1 and IEC 60364-5-52 Annex G both band it that way), but the permitted drop depends on your supply, your local regulations and where the origin of the installation is taken to be. This calculator tells you what the physics does — it does not certify a design.
Does this tool check current-carrying capacity or protective devices?
No. It sizes the cable by voltage drop only. Current-carrying capacity, ambient-temperature and grouping derating, MCB/RCD selection, fault-loop impedance and disconnection times are all out of scope, and a real design has to clear those gates separately.
Copper or aluminium — how much difference does it make?
Aluminium’s resistivity is about 64% higher than copper (0.0282 vs 0.0172 Ω·mm²/m at 20 °C), so the same size drops about 1.6× more voltage. On the 3 kW heater over 25 m at a 3% limit, copper is comfortable on 2.5 mm² (1.95%) while aluminium needs 4 mm² (2.5 mm² aluminium drops 3.20% and fails).
Why does my answer differ from a cable manufacturer’s table?
Two assumptions differ. (1) Temperature — this calculator uses the resistivity at 20 °C unless you reason about the hot cable, while tabulated mV/A/m figures are quoted at the conductor’s maximum operating temperature (70 °C for thermoplastic), roughly 20% higher resistance. (2) Power factor and reactance — the tables fold inductance in above 16 mm² and assume cos φ ≈ 0.8, where this tool models resistance with the power factor you set. Use the Voltage Drop calculator to explore the hot-cable case directly.
What cable size do I need for a 3 kW heater?
At 230 V it draws 13.0 A. With copper and a 3% limit: 2.5 mm² up to about 38 m, 4 mm² up to about 61 m, and 6 mm² beyond that — rounding down, because a longer run spends the same 6.9 V budget faster. Switch to aluminium and each of those distances shrinks by roughly a third. Always confirm the size against current-carrying capacity and the protective device for the installation method you are actually using.
Does length really matter that much?
Yes — drop is directly proportional to length. Double the run and you double the drop, so a cable that is comfortable at 20 m can fail at 40 m. That is why the length slider is one of the first things worth dragging in the scene: the cable does not change, but the verdict does.