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12 V vs 24 V vehicle wiring

Cars run on 12 V; most trucks, buses and military vehicles on 24 V. The circuits look the same, but the numbers change — and mixing parts from the two systems burns them. Here is what is different.

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What changes on 24 V

Two 12 V batteries in series give 24 V; the alternator charges at about 28 V.
The same power needs half the current: a 70 W lamp draws 5.8 A on 12 V but 2.9 A on 24 V.
Half the current means thinner wires and smaller fuses for the same load, and a quarter of the power lost in the wiring.
Lamps, relays, motors and ECUs must be rated for 24 V.

Mixing 12 V and 24 V parts

A 12 V lamp on 24 V gets four times its power and burns out at once.
A 24 V lamp on 12 V gets a quarter of its power and glows dim.
A 12 V relay coil on 24 V overheats; a 24 V coil on 12 V may not pull in.
For 12 V accessories on a truck use a 24-to-12 V converter, not a tap from one of the two batteries — that drains them unevenly.

Try it in the simulator

Draw a battery, a fuse, a switch and a 12 V lamp; simulate — the lamp lights normally.
Set the battery to 24 V: the lamp burns out and the simulator says why.
Set the lamp to 24 V and compare the current with the 12 V circuit.

Questions

Can I use 12 V bulbs on a 24 V truck?

No. A 12 V bulb on 24 V gets four times its rated power and burns out immediately.

Why do trucks use 24 V?

Large starters and long harnesses: at 24 V the same power needs half the current, so cables are thinner and voltage drop smaller.

Can I take 12 V from one truck battery?

It works, but it discharges one battery more than the other and shortens both. A DC-DC converter is the right way.

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