What cross section in mm² should the lift retrofit kit's supply wire be?
Use at least 2.5 mm² over a run of about 3 m one way; the cross section is set by distance, not current. Go up one size for longer or hotter routes.
On a typical installation where the kit is connected roughly 3 m one way from the battery, use a cross section of at least 2.5 mm². What really sets the cross section is not the current but the distance: on a 12V system the 3% voltage drop limit leaves only 0.36 V of margin, and the outgoing and returning conductors spend that margin between them. If the distance grows, or if the wire stays in the heat of the engine bay, go up one size.
Four headings that set the cross section
The factors to be taken into account when choosing a cross section are these:
- Total length: The calculation is made on the sum of the positive and the negative line. 3 m one way means a circuit length of 6 m; the reason is under counting the round trip.
- Temperature: Temperatures reaching 90-105 °C in the engine bay raise the resistance of copper by roughly 27-33%. On wires that pass through a hot area, moving up one size is sensible.
- Quality of the connections: A loose lug or an oxidised chassis point adds enough resistance to make the cross section calculation meaningless. Crimp the lugs with the proper tool and clean the paint off the chassis surface.
- Matching the fuse: The fuse rating has to stay below the carrying capacity of the wire; a 2.5 mm² cross section suits a 15A fuse.
When to go up one size
| Situation | What to do |
|---|---|
| Short route inside the cab (of the order of 3 m one way) | 2.5 mm² is enough |
| Battery at the far end of the vehicle, clearly a long route | One size up |
| Wire passes through the engine bay or close to the exhaust | One size up and a heat-resistant route |
| An extension joint comes in between | The cross section is not reduced; the joint is lugged or soldered and insulated |
"One size up" means moving to the next value in the standard range of cross sections; do not try to close the gap with intermediate sizes. When you increase the cross section, check that the lugs, terminals and cable glands suit the new size too — a lug the wire will not fit into gives back at the connection the cross section you have just gained.
Verify it with a single measurement after installation
The check after installation is simple: with the kit running, measure the voltage at the supply terminals of the kit and at the battery terminal at the same moment. Since the 3% voltage drop limit at 12V leaves about 0.36 V of margin, the aim is for the difference between them not to exceed that order noticeably; if it does, the cross section or the connections are inadequate. This threshold is an engineering derivation, not a manufacturer's declaration. Always take the measurement while the kit is drawing load — a reading taken with nothing running does not show the problem. The steps of the method are under the voltage drop test.
Problems that a bigger cross section does not solve
- A poor ground. If the negative side is weak, increasing the cross section on the positive side achieves nothing; the chassis connection has to be made with the same care.
- A loose or oxidised lug. One bad lug can add more resistance than the whole length of wire.
- A tired battery. If the voltage collapses under load the trouble is not in the wiring; repeat the measurement at the battery terminal.
- The wrong fuse. If the cross section is increased and the fuse left as it was, the protection no longer matches the wire; fuse selection is reviewed together with the cross section.
- A roundabout route. Taking the wire the long way round lengthens the line in fact. The cross section calculation is made on the distance the wire really covers, not on the distance you drew.
The practical order: measure the route with a tape before running anything, work out the round-trip total, choose the cross section from that and set the fuse to the cross section. When the installation is finished, run the kit under load and take the two-point voltage measurement; if the difference is small the line is right, and if it is large, check the connections before changing the wire. If your vehicle is 24V the voltage drop margin is more comfortable for the same job, but the cross section is still chosen on the distance.
The full topic: Converting a tractor hydraulic lever to electronic control