Fuse taps are clever pieces of metal and plastic that can turn 1 fuse position on your fuse box into
2 separate fuse positions. They are a good generic way to separately power an additional circuit.
Obviously I would never recommend these over the proper auxiliary fuse holder which cost about the same.
You remove a fuse, bung it in the fuse tap, fit a 2nd fuse, plug the whole thing in the fuse box and you are done.
These things really can be more complicated than you'd expect and blindly throwing
in fuses and plugging it into your fusebox could end in some serious consequences.
The fuse you remove, it must be placed in the slot nearest to the metal prongs.
This part of the circuit replaces the original position of the fuse.
If you put the fuses in the wrong positions, you may get fuses blowing or electrics getting damaged.
This is because the items will be protected with incorrectly rated fuses.
A fuse can go in either way around because it's just a link in the circuit. However, because you are taking a live feed
from the fuse box to use it elsewhere, it's of upmost importance that you put the fuse tap in the correct way around.
Look again at the photo. You can see that the power goes in on the left terminal and that connects to both fuses.
If you put it in back-to-front then the power is coming in from the right terminal,
through 1 fuse, through a 2nd fuse before it can go out to the additional accessory.
Running 2 fuses in line is bad news. It'll put a strain on the original fuse which may cause it to keep blowing,
(this would also cut power to the new circuit) however, it will also cause the 2nd fuse to blow slower.
This may not blow at the correct amperage or fast enough to protect what ever is attached to that circuit.
Which way around is correct? You'll have to use a multimeter and find out.
1 probe on a car earthing point, multimeter reading DC volts and touch each of the terminals.
1 terminal will show about 12 volts and the other terminal will show no reading.
What fuse do I need for the extra circuit?
This shouldn't be too hard to figure out. Normally the power rating of whatever you are
powering is on the box or in the manual. For example, this car camera runs at 1 Amp.
Ideally, your fuse needs to be as close to the amperage rating as possible without going under it.
However, with some electrics they can peak over that on start up so start off as close as you can.
If the fuse blows immediately or regularly, check the wiring is OK and increase the fuse size slightly.
Can the fuse box handle it?
This is the thing that people won't consider. The fuse box is designed for your car (obviously).
But, the circuit that powers the fuse you removed might only be good enough for the amperage expected.
Using more power through the same circuit could possibly over load the relay, connectors or PCB tracks.
If you kill any of those, you are looking at a new SAM unit (fuse box).
Not even slightly. Use an auxiliary fuse holder where possible.