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I am try to install an after market usb breakout box on my vehicle.

It has a red wire and a black wire coming out of the back.

The instructions say to connect the red wire directly to a fuse in the fuse box and the black wire to the chassis.

In house wiring connecting the negative to anything metal that a person might touch would be an instant code violation, and exceedingly dangerous, but but it seems to be standard practice with after market vehicle accessories.

Is this how it is normally done, or am mistranslating things

Aaargh Zombies
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2 Answers2

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TLDR;
Yes, this is normal accepted practice and nothing to be concerned about.

In house wiring, the Neutral is connected to Ground in your main panel (and not in or near outlets or appliances) as a safety measure.
If Neutral & Ground were connected together in or near the outlet, and a fault or break in the combined Neutral/Ground wire back to the panel were to occur, then the enclosure of your device could then rise to the Hot/Live 120V potential (or 240V in other parts of the world) - leading to a very dangerous situation.

However in automotive wiring, having the enclosure of the device rise to 12V (or 24V in larger vehicles) during a fault condition poses no danger whatsoever if someone were to touch the 'live' enclosure.
You can grip both terminals of a car battery with both hands all day long and not suffer any ill effects (other than perhaps cramping in your hands ...) because 12V is far too low a voltage to cause any significant current to flow through your body (outside of some exceptional circumstances). Those movies where you see someone being tortured with a car battery are flights of fancy by ignorant script-writers.
The benefits gained by using the chassis as the return current path instead of a negative wire are - reduced vehicle weight due to less wiring, and reduced resistance in the return current path (even though steel has a higher specific resistivity than copper, the cross-section area of the steel chassis is orders of magnitude larger than that of a typical copper wire).

brhans
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Yes, those instructions are fine to follow.

Go look in your engine bay. Identify the starter battery, and trace the fat cable from the negative terminal. It almost certainly goes to the chassis or engine block directly. That shows the chassis is already being used as the negative side of the circuit.

Car wiring is classed as Ultra Low Voltage because it is below 50V and it is Direct Current. Your skin is higher resistance and will not permit a shock - test by touching the positive and negative terminals of the starter battery with your clean dry skin.

Home wiring is 120 or 240V and is Alternating Current and poses a shock hazard to accidental touches.

A lot of techniques transfer between the two, but many number don't. If you're uncomfortable doing anything, run it by us here in a question, or in [chat]

One thing you do need to consider is wire diameter, as it relates to current draw. This USB charger will be labelled to say how much current it provides.

A janky one might offer 1A of current at 5V over all ports combined so it will draw about half an amp at 12V.
By comparison, a laptop charger might offer 3A at 20V or 60W. There, the 12V demand will be 5A sustained at 12V.

So you need to use a wire gauge and fuse that are rated to carry that current at 12V.

A dedicated in-line fuse, mounted near to the battery means that if the converter fails then other accessories aren't affected.

Criggie
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