12V Cable Size & Voltage Drop Calculator
The right cable for a circuit has to pass two tests. It must carry the current without overheating ‐ its current rating ‐ and it must deliver the power without losing too much voltage along the run ‐ the voltage drop. Too much drop and winches run slowly, lights dim, and fridges or chargers cut out. Enter your circuit details below and the calculator checks both tests at once.
Nominal system voltage ‐ double it for a 24 V system.
The cable run from the power source to the accessory, along the route the cable actually takes. The return run is counted automatically.
Technical details
Cable resistance per metre: ‐ Ω
Total circuit resistance (incl. return path): ‐ Ω
Continuous current rating of selected cable: ‐ A
Based on a copper cable resistance of 0.0185 Ω·mm²/m. Current ratings are for a single thin-wall cable (flexible PVC battery cable in the larger sizes) in free air ‐ derate for bundled looms, trunking or hot engine bays. Heating effects on resistance are not accounted for.
A maximum voltage drop of around 3‐4% is generally acceptable. This calculator doubles the one-way length to allow for the return circuit, giving a worst-case figure to work with.
Cable Size & Voltage Drop FAQ
How do I use the cable size calculator?
Pick a cable size, choose your battery type, then enter the current the accessory draws and the one-way length of the cable run. The calculator shows the voltage drop, the percentage drop and the voltage left at the accessory. It also checks the cable's current rating, so you can see straight away if the cable is too small.
What voltage drop is acceptable on a 12V system?
Around 3–4% is generally acceptable for most circuits. If the maker of your equipment gives a maximum drop or a recommended cable size in the fitting instructions, go with theirs.
Why does the calculator double the cable length?
Current has to flow out to the accessory and back again. The calculator adds the return run for you, so enter the one-way length only, measured along the route the cable will actually take. That gives a worst-case figure to work with.
Should I choose AGM (12V) or lithium (12.8V)?
Choose the battery that powers the circuit. A lithium battery sits at a slightly higher voltage, so the same drop in volts is a slightly smaller percentage.
Can I use it for a 24V system?
Yes. Enter double the voltage. For the same current, the drop in volts is the same but the percentage is half. For example, 20 A over 3 m of 2.5 mm² cable is 7.4% on 12V but 3.7% on 24V.
My cable is rated for the current. Why does it still fail?
Current rating and voltage drop are separate tests. A cable can carry the current without overheating and still lose too much voltage over a long run. On a 12V AGM system, 20 A over a 3 m run on 2.5 mm² cable passes the current rating (29 A) but drops 0.89 V, which is 7.4%. Going to 6 mm² brings it to about 3.1%.
Worked example: a 12V fridge
A fridge drawing 5 A, 6 m from a lithium battery (12.8V): 2.5 mm² drops 0.44 V (3.5%), 4 mm² drops 0.28 V (2.2%). Fridges with low-voltage cut-outs are better on the bigger cable.
Worked example: a 25 A DC-DC charger
A charger drawing 25 A, 4 m from the starter battery (12.8V lithium shown): 6 mm² drops 0.62 V (4.8%), 10 mm² drops 0.37 V (2.9%), 16 mm² drops 0.23 V (1.8%). Always check the charger's fitting instructions, as most give a recommended cable size for the run length.
What's the difference between mm² and AWG?
UK and European cable is sized by the conductor's cross section in mm². AWG (American Wire Gauge) is the US system you'll see on imported kit. The conversion table below shows the nearest AWG for each metric size. The match is never exact, so for critical circuits size up.
Does it allow for looms, trunking or hot engine bays?
No. The current ratings are for a single cable in free air. Derate for bundled looms, trunking or hot engine bays, and the calculator doesn't account for cable heating. If in doubt, go up a size.
Does the calculator tell me what fuse to use?
No, it only sizes the cable. A fuse protects the cable, so it should be rated above the accessory's normal current but no higher than the cable's rating, and fitted as close to the battery as practical.
Cable Sizing Conversion Table
Metric cable is sized by conductor cross section in mm²; American Wire Gauge (AWG) is the US equivalent you'll see on imported kit. This table shows the nearest AWG size for each metric size.
Nearest AWG size by conductor area ‐ the match is never exact, so for critical circuits size up.