Cable Size Calculator
Enter the design current, the cable length and the supply type. The calculator picks the smallest copper cable in mm² that carries the current and keeps voltage drop within your limit.
- 1 mm² to 120 mm²
- Single or three-phase
- mV/A/m voltage drop
For copper multicore PVC cable clipped direct at 30°C. Apply correction factors for insulation, grouping and ambient temperature.
Cable size is set by heat and by distance. A cable must carry the current without getting too hot, and it must deliver the voltage your equipment needs at the far end. BS 7671 and IEC 60364 allow 3% drop for lighting and 5% for other circuits from the origin of the installation.
Cable sizes, ratings and voltage drop
The values the calculator uses. Search by mm² or current.
| Size (mm²) | Rating (A) | mV/A/m | Typical use |
|---|---|---|---|
| 1.0 | 16 | 44 | Lighting |
| 1.5 | 20 | 29 | Lighting |
| 2.5 | 27 | 18 | Sockets |
| 4 | 37 | 11 | Immersion, small shower |
| 6 | 47 | 7.3 | Cooker, shower |
| 10 | 64 | 4.4 | Large shower, EV charger |
| 16 | 85 | 2.8 | Submain |
| 25 | 119 | 1.75 | House supply |
| 35 | 147 | 1.25 | Large submain |
| 50 | 179 | 0.93 | Commercial |
| 70 | 229 | 0.63 | Commercial |
| 95 | 278 | 0.46 | Distribution |
| 120 | 322 | 0.36 | Distribution |
Voltage drop figures are in millivolts per amp per metre for a single-phase circuit. Multiply by current and length, then divide by 1,000 to get volts dropped.
Run length and voltage drop
A 32 A load at 230 V on five cable sizes. Slide the length to see where each size crosses the 5% limit.
A 6 mm² cable carries 32 A easily but passes 5% drop at about 49 m. For a garden office or EV charger at the far end of a long plot, the drop limit decides the cable, not the current.
How cable sizing works
Five steps from load to cable. This is the order the wiring regulations follow.
Find the design current
Add up the load. For a 7.2 kW charger, 7,200 ÷ 230 = 31.3 A.
After choosing the cable, check that the protective device trips quickly enough on a fault. Long runs with small cable can fail the earth fault loop impedance limit even when the size passes on current.
Single-phase or three-phase
Three-phase carries more power on smaller cable. Compare the two supply types.
Standard in homes. Current flows out and back on two conductors.
Used in commercial buildings, farms and fast chargers. Load is shared across three conductors.
Three-phase voltage drop per phase is about 0.866 times the single-phase figure for the same cable, and the percentage is measured against 400 V, so long runs are much easier.
Checks before you install
Cable size is one part of the design. Tick each as you confirm it.
Tick each item as you go.
In the UK, new circuits in a home are notifiable under Part P, so they need a registered electrician or a building control notice.
What the result shows
Everything you need to pick a cable. Each tile explains one result.
The smallest mm² that meets both limits.
Capacity for the installation method used.
In volts and as a percentage of supply.
How far this cable can go at your load.
Single or three-phase changes the drop.
The maximum run figure is handy when you are deciding where to put a consumer unit or a subboard, because it tells you how far the chosen cable can go.
Frequently Asked Questions
No question matches that search.
What size cable for a 32 amp circuit?
6 mm² twin and earth for most domestic runs. 4 mm² can carry 32 A clipped direct but voltage drop limits it to short runs.
What size cable for a 9.5 kW shower?
A 9.5 kW shower draws about 41 A at 230 V, so a 45 A breaker and 10 mm² cable are usual. Short runs clipped direct can use 6 mm².
How do I calculate voltage drop?
Multiply the cable's mV/A/m value by the current and the length, then divide by 1,000. 18 mV × 20 A × 30 m ÷ 1,000 = 10.8 V.
What is the maximum voltage drop allowed?
BS 7671 allows 3% for lighting and 5% for other uses, measured from the origin of the installation.
What is the difference between cable and wire size?
Cable size is usually given in mm² of each conductor. Wire size in North America uses AWG gauge numbers.
What size cable for an EV charger?
A 7 kW charger on 32 A normally uses 6 mm² for runs up to about 30 m and 10 mm² for longer runs.
Does cable in insulation need to be bigger?
Yes. Cable fully surrounded by insulation can lose half its rating, so a larger size is often needed.
Size the cable for the whole run
Start from the real load, apply the right installation factors and check both current rating and voltage drop. Then confirm the protective device and have the circuit tested before use.
More size calculators
Each one has its own calculator, worked examples and answers to common questions.