I was hoping to install and extra 12v usb port

June 12, 2022

Hey guys,

Basically I was hoping to install an extra usb 12v port to the left of the stereo on the passenger side so about the opposite place to the standard port that's on the driver's side of the stereo.

All I was really wondering was can I take power from the back of the stereo or piggyback from the other 12v socket and have an inline fuse? Or would it be best to run to the fuse board or Battery?

Has anyone done this at all please? Car is a 2007 2.2 SR

August 13, 2022

On some older cars you can add a fuse and new output line into a blank space on a fusebox, but on the Toyota, it's far more effort than benefit.

The 12v port is on it's own fuse (Fuse #9 in the body module behind glove box). That's probably a 15A fuse* and this is an ideal choice to run another low power accessory. (*e.g. if the spec says it should be 15A.. and someone previous bodger has put a bigger fuse in, swap it back!)

Sorry if I'm not sure on your electrical skill level. Generally fuses protect the wiring and not much more. This is very important as anything can happen that causes more current than the wire can carry safely (equipment fault, wire gets rubbed or cut and contacts something else etc etc.).

If you run double the allowed current that the wire will handle (less on a very hot day..) it will gradually get hot and burn through the insulation. It'll keep burning and may burn into other wires or trim plastic and eventually your'e likely to need the fire brigade. Therefore if you've got 20A rated cable and fit a 15A fuse, the fuse should blow before the cable melts.

If you want to patch in to a circuit with 15A fuse by adding a 3A rated cable (e.g. if you're adding a <20W device) then add an inline 2A fuse.

If you don' know the current (amps) rating of the cable then you can google it from the cable core size and use a conservative value unless you know it's automotive cable (typical automotive cable allows 30+ ambient temp and low heat dissipation to air in case it's bundled or under trim.

So - the answer is a choice.

For low power stuff like a USB port, you can piggyback on other circuits (the 12v point is a good one for this) but you should either

use cable that's rated higher than the fuse value unless the manufacturer of the device states a fuse rating (then see second choice anyway)
or add an additional inline fuse to protect the new cable (or the smaller fuse size recommended by the equipment spec)

Issues with piggy back.. If you want to run the 15A from the 12v port AND you run more than 20W power to other consumers you're likely to stress the 15A fuse.

(not much takes 15A on a cig lighter socket but maybe a tyre pump can be 15A. For >15A there are some very dodgy mains inverters about! (e.g. don't run a 300W+ inverter from a standard vehicle cigarette lighter socket.. some people try and then they swap the 15a fuse for a 30A one. They're asking for smoke and probably worse)