What size cable do I need?
A 12V, 24V and 48V cable size and volt drop calculator that checks current rating and volt drop together, and shows its working
Tell us the load, the run length and where the cable goes. We will tell you the size, the fuse and the holder, which check decided it, and exactly how we worked it out. Then we will make the lead up if you want us to.
Common inverter runs, worked through
Worked at 12V and 24V, lead acid, a 2 metre run one way, cable on its own in a cabin, on the 10% general-circuit limit. Change any of those and the answer changes, which is what the calculator above is for.
| Load | Current | Cable | Volt drop | Fuse |
|---|---|---|---|---|
| 1000W at 12V | 93A | 25mm² | 2.60% | MEGA 150A |
| 2000W at 12V | 185A | 50mm² | 2.57% | Class T 250A |
| 3000W at 12V | 278A | 70mm² | 2.72% | Class T 400A |
| 2000W at 24V | 93A | 25mm² | 1.30% | MEGA 150A |
| 3000W at 24V | 139A | 35mm² | 1.38% | MEGA 200A |
Notice that the cable is sized to carry the fuse, not just the load. A 2000W inverter draws 185A, which needs a 250A fuse to avoid nuisance blowing, and a 250A fuse has to sit on a cable that can carry 250A. Size to the bare load and you end up with a lead nothing in any range will protect.
Why this gives a bigger answer than most calculators
Three reasons. We use the real published resistance of tinned Class 5 flexible copper, which is higher than the plain solid figure most tools assume. We correct that resistance for the temperature the cable actually runs at, which nobody else in this market does. And we size the cable to carry the fuse that protects it, not just the load it feeds.
The result is a larger cable than a free tool will suggest, and we would rather show you why than quietly agree with everyone else.
Read why our calculator recommends a bigger cable than everyone else's.
The standards this is worked to
- BS EN ISO 13297:2021 requires that calculated volt drop does not exceed 10% of nominal voltage. For equipment vital to safety it requires that the equipment be supplied with the proper voltage to achieve its rated performance, and offers 3% as an acceptable figure for such equipment. The 3% is a note against a functional requirement, not a limit the standard sets.
- ABYC E-11 is the US practice and makes both numbers mandatory, 3% and 10%. Its tables are based on a constant that corresponds to a conductor at about 29°C, so they are a room temperature basis and do not allow for a warm engine bay.
- BS EN 60228 classifies the conductor. Our cable is Class 5 flexible, and the tinned range is declared to it by the manufacturer.
- Conductor resistance is corrected for temperature by (234.5 + T) / 254.5. A 60°C conductor carries 15.7% more resistance than the same cable at the 20°C its datasheet figure is quoted at.
This is a design aid and not a design. The installer remains responsible for the installation.
Not sure, or want a second opinion? Ring us on 023 8045 5129 and we will work it through with you.
When you know the size, we will make the lead up for you: cut, crimped with matched dies, heat shrunk and labelled, with a certificate of conformity.