Cable Size & Voltage Drop Calculator
Find the right cable size for a power run, or check the cable you have. Length, load, voltage, temperature and how the cable is installed all count, and you'll see the voltage drop rated from excellent to unacceptable, how hot the cable runs, the power it wastes and the fuse to use.
- AC, DC and 3-phase
- Voltage drop rated
- Capacity, heat and fuse
Result
Recommended sizes
Every size for this run
How much voltage drop is acceptable?
| Voltage drop | Rating | Good for |
|---|---|---|
| Up to 1% | Excellent | Solar panels to the controller, battery to inverter, sensitive electronics, long-term efficiency |
| 1% to 3% | Good | Lighting, electronics, chargers, most DC circuits. The usual target, and the UK limit for lighting circuits. |
| 3% to 5% | Acceptable | Heaters, pumps, fans and other non-sensitive loads. The UK limit for other mains circuits. |
| 5% to 10% | Poor | Only short-term or non-critical loads. Motors may struggle to start; 12V devices may cut out. |
| Over 10% | Unacceptable | Wastes power as heat and can be a fire risk at high current. Use a thicker cable. |
Why length matters so much
- Many charts give only a cable's current rating, which is about heat. Over a long run the voltage drop, not heat, usually decides the size: 100 A over 50 cm and over 5 m need very different cables.
- Low voltages suffer most. The same 1 V lost is under half a percent at 230 V but over 8% at 12 V, which is why 12V and 24V systems need such thick cables.
- Hot cable has more resistance. We estimate the conductor's real temperature from the load and correct the resistance, so a cable near its limit shows a bigger drop.
Safety first
- Avoid CCA (copper-clad aluminium). It has about 55% more resistance, carries less current and its connections overheat. If you must use it, go up about 1.5 sizes.
- Fuse to protect the cable: the fuse or breaker must be at least the load current and no more than what the cable can carry. We suggest a rating with 25% headroom for continuous DC loads.
- Use AC-rated breakers and switches for AC and DC-rated ones for DC: DC arcs don't go out by themselves.
- In the UK, new mains circuits must be designed and certified to BS 7671 by a competent electrician. This calculator is guidance, based on IEC 60364-5-52 style ratings, and doesn't replace that.