Voltage drop calculator
cable sizing for 12V, 24V and 48V
Put in your load, your cable run and the size you are thinking of.
You get the volts lost in the cable, and the smallest size that meets your target.
1. Your system
The voltage the battery bank runs at. Most Australian caravans are 12V.
2. The load
What the cable is feeding. Enter amps or watts, whichever your appliance is marked in.
That is 6A at 12V, or 72W.
3. The cable run
Measure one way, battery to appliance. The calculator doubles it, because the current has to come back down the negative as well.
Cable is sometimes sold under a size in millimetres that describes the finished cable with its insulation. Check the conductor cross-section in mm² on the spec, because that is the figure used here.
3% is the tighter target people work to on the circuits that matter, like a fridge or a charging run. 5% is the looser one, used on lighting and short runs. Both are working targets, not a rule.
Your voltage drop
- Volts lost in the cable
- 0.18V
- Volts reaching the appliance
- 11.82V
- Power turned into heat
- 1.1W
- Smallest size that meets 3%
- 4 mm²
6 mm² does the job here, and 4 mm² is the smallest that still would.
The other check a cable needs
A cable also has a current rating: how much it can carry without getting hot. That depends on the cable itself, how it is bundled, how it is run and how warm it gets around it, and it is a separate check from this one.
Have the work done or checked by a qualified auto electrician. A cable can pass the drop target here and still be the wrong cable for the job.
The same 72W on the same cable
Same appliance, same 6 mm² cable, same 5m run. Only the system voltage changes, so the current changes with it.
| System | Current | Volts lost | Drop |
|---|---|---|---|
| 12V | 6A | 0.18V | 1.5% |
| 24V | 3A | 0.09V | 0.4% |
| 48V | 1.5A | 0.04V | 0.1% |
Four times the voltage means a quarter of the current, and a quarter of the volts lost. Measured as a share of the supply it is better again, because that smaller loss is being taken out of a bigger number.
That is the argument for a higher system voltage, and the voltage guide covers what it costs you elsewhere.
How this is worked out
Voltage drop is the resistance of the cable multiplied by the current going through it.
Written out: multiply the run length by 2, by the amps, and by the resistivity of copper, then divide by the cross-section in mm².
The 2 is the round trip. Current leaves on the positive and returns on the negative, so a 5 metre run is 10 metres of copper.
- Copper resistivity, 0.0175 ohm mm² per metre at 20 degrees. The standard engineering figure for copper conductors. It sits a little above the pure annealed-copper value, so answers err toward more cable rather than less.
- Temperature. Copper gains resistance as it warms, by roughly 0.39% per degree above 20. A run through a hot roof cavity drops a little more than the figure above. The calculator leaves that correction out, because we do not know how hot your cable gets.
- The target. 3% and 5% are working targets in common use. You choose which one applies.
- The cable sizes. The mm² figure is the copper cross-section. The B&S sizes are the imperial gauges a lot of Australian automotive cable is sold in, with each one's own cross-section shown beside it.
Use this to check a quote or a plan before you buy the cable. The calculator does not replace an auto electrician.
Cable and voltage drop questions, answered
How do I work out voltage drop in a caravan cable?
Double the one-way run length. Multiply that by the current in amps, then by the resistivity of copper. Divide the result by the cable's cross-section in square millimetres. That gives you the volts lost in the cable. Divide the volts lost by your system voltage to get the percentage most people compare against.
What is an acceptable voltage drop on a 12V system?
Three percent is the tighter target, used on the circuits that matter, like a fridge feed or a charging run. Five percent is the looser one, used on lighting and short runs. Both are working targets rather than a rule. The calculator lets you pick which one applies, and shows the answer against it.
Why does the calculator double my cable run length?
Because the current has to get back. It leaves the battery on the positive cable and returns on the negative. So a five metre run from the battery to an appliance is ten metres of copper. Measure one way with a tape measure, and the calculator does the doubling for you.
Does a 48V system need less cable than a 12V one?
For the same appliance, yes. Power is volts multiplied by amps. So the same watts at 48V draws about a quarter of the current it would at 12V, and voltage drop follows the current. A quarter of the current through the same cable loses a quarter of the volts. That smaller loss also sits against a bigger supply voltage, so as a percentage it is smaller again.
Is voltage drop the only thing that decides cable size?
No. A cable also has a current rating, which is how much it can carry without getting hot. That rating depends on the cable's construction and how it is routed. It also depends on whether the cable runs in a bundle with others, and how warm it gets around it. This page works out voltage drop only. Have the install done or checked by a qualified auto electrician. A cable can meet a drop target and still be the wrong cable for the job.
The decision behind the numbers is in 12V, 24V or 48V, and the power and solar calculator sizes the battery and panels that feed these cables.