Why does voltage drop matter on a vehicle feed?
A long thin cable wastes part of the battery voltage as heat before it reaches the device. A device that gets less than its minimum input voltage can restart, drop its recording or show wrong readings. The fault appears when the engine starts or the battery is low, so it is hard to catch in the workshop.
The tool answers three questions in order: how much current each part of the feed carries, how much voltage each cable loses, and which fuse protects each cable.
How does the tool calculate the drop?
It uses Ohm's law on the round trip of the cable, with the resistance of copper at the temperature you enter.
P is the device power in watts, V the supply voltage, L the one-way length in meters, A the wire cross-section in mm² and T the conductor temperature in °C.
- Resistivity of copper at 20 °C is 0.017241 Ω·mm²/m, the annealed-copper standard (1/58).
- The temperature coefficient is 0.00393 per °C. Both values are in NIST Handbook 100, Copper Wire Tables. Source and date checked: 3 November 2026.
- The tool counts the return conductor, so the length is doubled.
- Branch devices share the main cable. The main cable carries the sum of all device currents.
Check against the AWG table: AWG 18 (0.823 mm²) has 20.95 Ω per km. NIST Handbook 100 gives 6.39 Ω per 1,000 ft for AWG 18, which is 20.96 Ω per km. The resistances in the table below come from the same formula.
How do you choose the wire?
Start from the allowed drop, then take the smallest wire that stays inside it for the current and the length. The next size up costs little and leaves room for heat and for the next device you add.
| Wire size | Resistance (Ω per km) | On 12 V (m) | On 24 V (m) |
|---|---|---|---|
| 0.5 mm² | 34.48 | 1.7 | 3.5 |
| 0.75 mm² | 22.99 | 2.6 | 5.2 |
| 1 mm² | 17.24 | 3.5 | 7.0 |
| 1.5 mm² | 11.49 | 5.2 | 10.4 |
| 2.5 mm² | 6.90 | 8.7 | 17.4 |
| 4 mm² | 4.31 | 13.9 | 27.8 |
| 6 mm² | 2.87 | 20.9 | 41.8 |
The table is for one 3 A load on one run. The Wire sizes tab does the same for your own current and length.
| AWG | Cross-section (mm²) | Resistance at 20 °C (Ω per km) |
|---|---|---|
| 22 | 0.326 | 52.9 |
| 20 | 0.518 | 33.3 |
| 18 | 0.823 | 20.95 |
| 16 | 1.31 | 13.2 |
| 14 | 2.08 | 8.29 |
| 12 | 3.31 | 5.21 |
| 10 | 5.26 | 3.28 |
Sizes in AWG are given by their diameter formula. Sizes in mm² are the metric series. Source for the AWG values: NIST Handbook 100, Copper Wire Tables, Table 5. The AWG sizes themselves are set in ASTM B258. Checked 3 November 2026.
How do you choose the fuse?
A fuse protects the cable from a short circuit or an overload. It does not protect the device. Think of it as the weakest part of the cable.
- Find the current. Add the device currents for the main cable. Use the device current alone for a branch.
- Add a margin. Multiply by the margin you chose, 125 % as a starting value, so normal peaks do not blow the fuse.
- Take the next standard rating. The tool picks the smallest rating in its list that is at or above the result. The list is editable.
- Check it against the cable. The fuse rating must not be above the current rating of the cable. Enter the cable rating from its datasheet and the tool checks it.
Put the fuse as close to the battery or power source as you can. A cable that is not fused between the source and the fuse has no protection. Fuse type, fuse ratings and the vehicle standard that applies to you are for you to verify against the current official text.
Is 24 V better than 12 V?
For the same power, 24 V draws half the current. Half the current loses half the voltage in a wire, and that is measured against a supply twice as big, so the drop in percent is four times smaller. A long run that fails on 12 V often passes on 24 V with the same wire.
Many devices accept both. A vehicle has the supply it has: use the 12 V and 24 V tab to see what the same load does on each, and check the input range of every device.
How do you power an MDVR and its cameras?
An MDVR draws more than a tracker, and cameras add to it. Give each camera cable its own length and wire size, and let the main cable carry the whole sum. Cabin cameras are usually short. Rear and side cameras can mean long runs, where the wire size matters most.
The example rows (15 W MDVR, 3 W and 4 W cameras, 3 W tracker) are typical values for illustration. They are not Tracom specifications.
Take the real power and the supply range of every device from its datasheet. A device that starts up, heats or records to a disk can draw more than its running value for a short time.
Assumptions and limits
- The tool sizes wire and fuse from the numbers you enter. The device powers, the allowed drop and the fuse margin are your inputs. The values shown first are typical examples.
- It treats the load as constant power at the supply voltage. Devices that start up, motors and surges are not modeled.
- The supply is taken as exactly 12 V or 24 V. A real vehicle varies, from a low battery to the charging voltage.
- The smallest main-cable wire shown assumes the main cable alone may use the whole allowed drop, so it is a lower limit and the branch wires then have no allowance left.
- The cable current rating is an optional input. The tool does not know the rating of your cable, its insulation or how it is routed.
- Connector and joint resistance, and fuse holder drop, are not included. Allow extra margin for them.
- Verify the allowed drop, the fuse and the wire against the current official text of the standard or the manufacturer instructions you work to. The 3 % and 10 % values are common guidance for important and other circuits, from common wiring practice.
Vehicle wiring has safety rules. This tool is a design aid and not an engineering approval. Have a qualified person check the final design.