First off, credit where it's due; thanks to Brittan for clearing up any confusion on this topic.
For anyone who's installing a Dashcam, GPS, media adapter for a reverse camera or other powered accessory, it's a good idea add a fuse in the circuit to protect your wiring and the device in the event of a fault. The advantage of using a piggy-back fuse tap over cutting and splicing wires is that the change can be easily undone in order to return to the original setup.
For the Audi TT Mk2, the fuse box you'll most like want to use is located behind the dash panel end cover on the driver's side. Check the fuse panel label or owner's manual to identify the location the fuse slot from which you want to tap power. Generally, the best choice is the cigarette lighter since it's only "hot" when the ignition is on; e.g. #30 @ 20A Cigarette lighter. However if that's already used for something else, you can tap off another fuse slot. Note that the fuse arrangements will differ depending on the OEM equipment and options of the vehicle.
Deciding which Fuse Slot to Tap
• Select a fuse slot that is currently empty (unused) over one that is occupied.
• Go for a utility function (e.g. cigarette lighter, heater, radio, etc.) rather than a critical system function (e.g. engine, ABS, lights, etc.)
• Choose a fuse slot in the fuse panel that is located such that the piggy back will fit properly when installed and the cover replaced.
Testing the Fuse Slot
To test that the circuit is only "hot" with the ignition on, it's a good idea to use either a test light or digital multi-meter to verify the power at the fuse slot. Remove the fuse from the slot you want to tap and switch on the ignition. Connect the negative (black or alligator clip) end of your test tool to a place on the chassis of the car (an unpainted metal body part). With the positive (red or pointy tip) end of your test tool, touch each of the two fuse connectors separately to determine which side has power; the "battery side" or (+) side vs. the "load side" or (-) side. Note which is which and put the fuse back into the slot for now.
How the Piggy-back Works
As shown in the illustration below, in a piggy back fuse tap both fuses receive power from "A" (+). Power flows in from the battery (+) at "A", through the primary 10-Amp fuse, to the OEM device from "B" and then ground (-). The piggy-back also receives power from the battery (+) at "A", through the 5-Amp fuse and out through the "Accessory" line to the device and then to "B" ground (-).
Installing the Piggy-Back Backwards
While not immediately obvious, there is a right way and a wrong way to install a Piggy-back fuse tap. If installed incorrectly, the accessory will receive voltage from both fuses. To determine if the piggy back is installed incorrectly, remove only the primary fuse (10-Amp in the diagram). Then, with ignition ON, if the accessory power is lost, the piggy back is the wrong way round. Simply remove the piggy back adapter from the fuse box, rotate it 180° and reinstall it the other way around.
Accessory Fuse Current Rating
While there is some debate about the recommended fuse rating for the accessory, the best shop practice is to take the primary fuse rating (in this case 10-Amps) and go with half that value for the accessory, in this case 5-Amps. This will ensure the circuit isn't pulling too much current which could result in over loading the wires in the circuit. Generally, for small power electronics like Dash Cams, a 5-Amp fuse will suffice.
Tapping Power off the Battery
In some cases, such as when installing a reverse camera or a 12V-power outlet in the trunk, it's easier and more convenient to pull power directly off the battery. When this is necessary, you should always use an in-line fuse of the appropriate rating. There are several "accessory" screw points at the battery positive terminal which you can tap off. Be sure to remove the fuse from the fuse holder during installation to protect the circuit until you've completed the installation and are ready to apply power. For grounding, there's a chassis ground lug conveniently located next to the battery.
As an added bonus, here's an excerpt from the workshop manual on the fuse panels and fuse assignments.
View attachment A005TT200801 - Fuse Panels and Assignments.pdf
![piggy back 1.JPG]( "piggy back 1.JPG")
![piggy back 2.JPG]( "piggy back 2.JPG")
![PiggyBackFuseTap01.JPG]( "PiggyBackFuseTap01.JPG")
![Fuses.JPG]( "Fuses.JPG")
![SSP382.jpg]( "SSP382.jpg")
For anyone who's installing a Dashcam, GPS, media adapter for a reverse camera or other powered accessory, it's a good idea add a fuse in the circuit to protect your wiring and the device in the event of a fault. The advantage of using a piggy-back fuse tap over cutting and splicing wires is that the change can be easily undone in order to return to the original setup.
For the Audi TT Mk2, the fuse box you'll most like want to use is located behind the dash panel end cover on the driver's side. Check the fuse panel label or owner's manual to identify the location the fuse slot from which you want to tap power. Generally, the best choice is the cigarette lighter since it's only "hot" when the ignition is on; e.g. #30 @ 20A Cigarette lighter. However if that's already used for something else, you can tap off another fuse slot. Note that the fuse arrangements will differ depending on the OEM equipment and options of the vehicle.
Deciding which Fuse Slot to Tap
• Select a fuse slot that is currently empty (unused) over one that is occupied.
• Go for a utility function (e.g. cigarette lighter, heater, radio, etc.) rather than a critical system function (e.g. engine, ABS, lights, etc.)
• Choose a fuse slot in the fuse panel that is located such that the piggy back will fit properly when installed and the cover replaced.
Testing the Fuse Slot
To test that the circuit is only "hot" with the ignition on, it's a good idea to use either a test light or digital multi-meter to verify the power at the fuse slot. Remove the fuse from the slot you want to tap and switch on the ignition. Connect the negative (black or alligator clip) end of your test tool to a place on the chassis of the car (an unpainted metal body part). With the positive (red or pointy tip) end of your test tool, touch each of the two fuse connectors separately to determine which side has power; the "battery side" or (+) side vs. the "load side" or (-) side. Note which is which and put the fuse back into the slot for now.
How the Piggy-back Works
As shown in the illustration below, in a piggy back fuse tap both fuses receive power from "A" (+). Power flows in from the battery (+) at "A", through the primary 10-Amp fuse, to the OEM device from "B" and then ground (-). The piggy-back also receives power from the battery (+) at "A", through the 5-Amp fuse and out through the "Accessory" line to the device and then to "B" ground (-).
Installing the Piggy-Back Backwards
While not immediately obvious, there is a right way and a wrong way to install a Piggy-back fuse tap. If installed incorrectly, the accessory will receive voltage from both fuses. To determine if the piggy back is installed incorrectly, remove only the primary fuse (10-Amp in the diagram). Then, with ignition ON, if the accessory power is lost, the piggy back is the wrong way round. Simply remove the piggy back adapter from the fuse box, rotate it 180° and reinstall it the other way around.
Accessory Fuse Current Rating
While there is some debate about the recommended fuse rating for the accessory, the best shop practice is to take the primary fuse rating (in this case 10-Amps) and go with half that value for the accessory, in this case 5-Amps. This will ensure the circuit isn't pulling too much current which could result in over loading the wires in the circuit. Generally, for small power electronics like Dash Cams, a 5-Amp fuse will suffice.
Tapping Power off the Battery
In some cases, such as when installing a reverse camera or a 12V-power outlet in the trunk, it's easier and more convenient to pull power directly off the battery. When this is necessary, you should always use an in-line fuse of the appropriate rating. There are several "accessory" screw points at the battery positive terminal which you can tap off. Be sure to remove the fuse from the fuse holder during installation to protect the circuit until you've completed the installation and are ready to apply power. For grounding, there's a chassis ground lug conveniently located next to the battery.
As an added bonus, here's an excerpt from the workshop manual on the fuse panels and fuse assignments.
View attachment A005TT200801 - Fuse Panels and Assignments.pdf
