Voltage drop · wire length · cross-section

Voltage drop calculator for car wiring

Every metre of copper has resistance. On a long or thin wire the load gets less than battery voltage: lamps go dim, motors run slow, relays fail to pull in. Enter the load and the length to see the drop and the wire that fixes it.

Open the editor · ▶ Run the live demo

How voltage drop is calculated

Resistance of a copper wire = 0.0178 Ω·mm²/m × length ÷ cross-section.
Voltage drop = current × resistance. 10 A over 4 m of 1 mm² loses 0.71 V.
With the return through the car body only the feed wire counts; with a separate ground wire, count both lengths.
A common target is under 3 % of the supply: 0.36 V on 12 V, 0.72 V on 24 V.

Typical loads on 12 V

Load	Power	Wire length	Current	Fuse	Wire
Parking lights	10 W	4 m	0.83 A	2 A	0.35 mm² (22 AWG)
Headlight bulb H7	55 W	3 m	4.58 A	7.5 A	0.75 mm² (18 AWG)
Pair of headlights	110 W	3 m	9.17 A	15 A	1.5 mm² (16 AWG)
Horn	60 W	2 m	5 A	7.5 A	0.5 mm² (20 AWG)
Wiper motor	120 W	2 m	10 A	15 A	1 mm² (17 AWG)
Radiator fan	250 W	2 m	20.83 A	30 A	4 mm² (12 AWG)
LED light bar	300 W	4 m	25 A	35 A	6 mm² (10 AWG)
Audio amplifier	1000 W	5 m	83.33 A	125 A	35 mm² (2 AWG)

See the drop in a simulation

Draw a battery, a fuse, a lamp and ground in the editor.
Give the lamp wire a length and a cross-section.
Simulate: the lamp’s brightness follows the voltage it really gets; above 0.5 V of drop the simulator warns.
Click the wire to read the drop, then make it thicker.

Questions

How much voltage drop is acceptable?

Around 3 % for most loads and under 0.5 V for lighting; starter and alternator cables are often allowed a little more while cranking.

Does a thin ground cause voltage drop?

Yes. The current flows through the ground the same way; a bad ground is often the hidden half of the drop.

Why do my headlights get brighter when I rev?

Because the alternator raises the voltage to about 14 V; a big difference also hints at voltage drop in the lamp wiring.

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