12 V socket · USB · fuse

How to wire an extra 12 V socket or USB charger

An extra socket in the boot for a cool box, or USB chargers for the rear seats — both are small circuits, but they need their own fuse and a decision: should they stay live with the car off?

Open the editor · ▶ Run the live demo

Permanent or with the ignition?

Permanent + (straight from the battery through a fuse): a cool box keeps running at a stop — and can flatten the battery overnight.
Ignition-switched (ACC): chargers turn off with the car. Best for USB.
For a cool box on a permanent feed, a low-voltage cut-off or a second battery protects the starting battery.

Fuse and wire

Load	Current	Fuse	Wire (2–3 m)
USB charger 2 × 15 W	≈ 2.8 A	5 A	0.75 mm²
Phone + laptop chargers	≈ 6 A	7.5 A	0.75 mm²
Cool box 120 W	≈ 10 A	15 A	1.5 mm²

A 12 V socket is usually rated 10–15 A; its fuse is chosen for the wire, and never larger than the socket rating.

Try it in the simulator

Run the live demo: a socket for a 120 W cool box on a permanent 15 A fuse, and a USB charger on ACC.
With the ignition off the socket works and the USB charger is off; switch ACC on and it comes on.
Click the socket wire: about 10 A.

Questions

Can I take power from the existing cigarette lighter?

For a USB charger, yes — it adds little. A second high-power socket on the same fuse will blow it; give it its own circuit.

Do USB chargers drain the battery when idle?

Cheap ones take a few milliamps even with nothing plugged in. On ACC that does not matter.

Which wire is +?

In a 12 V socket the centre contact is + and the sleeve is −.

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