Correct way to install fuse tap?

[92saturnsl2]

What's the correct way to install a fuse tap?

If you plug it in one way, the two circuits (existing and auxiliary) are in parallel at the source, both draw from the same source and are fused independently.

If you rotate it 180 degrees, the auxiliary circuit (and fuse) is wired in series with the existing fuse. So the current from the auxiliary circuit first has to travel through the main (existing) fuse, then branching off to its own fuse. In this configuration the auxiliary fuse would have to have the same or smaller rating as the main fuse or the main fuse blows. (the load of both circuits can not exceed the main fuse).

My thinking is that the latter configuration is the correct way, as it protects the original circuit from being overloaded. For example, if you have a 20 amp circuit for say a cigarette lighter, then you use a fuse tap to add some LED spotlights using a 10 amp fuse. If you orient the fuse tap using the first method, you could then draw 30 amps from that circuit (presumably designed for 20 amps) with no fail-safe. Using the second method you are still limited to a 20 amp current draw which protects the wiring, relays, etc that are part of that circuit.

I've seen some listings on Amazon stating to install these using the first method, and there's endless threads on various forums saying the same. I think this is wrong. Am I the only one?

 

[Oildudeny]

Can the wiring handle 30 amps?

 

[92saturnsl2]

Can the wiring handle 30 amps?

Well if we know the wiring can handle 30 amps, then it's a moot point.

But given that many circuits in a fuse box are provided by a relay, in some cases the ECU itself, there's more than just the wiring to consider.

 

[ctechbob]

The best way is, don't.

There's so many other options for supplying power to auxiliary items that fuse taps just shouldn't be used, especially for something drawing more than milliamps of current.

If you're specifically talking about powering external LED's, the best way to do it (If you're using a standalone interior switch) is with a fused relay in the engine compartment and the switch inside the car switching to ground to trigger that relay. This does several things. You provide power directly from the battery, and you avoid running a switched 12v through the firewall. If your switch wire accidentally goes to ground, the worst it will do is activate the relay and turn the light on as opposed to blowing a fused circuit.

If you want them to turn on and off with the ignition, then there are other options, but none of them should involve powering the lights directly from that fusebox circuit.

 

[92saturnsl2]

The best way is, don't.

There's so many other options for supplying power to auxiliary items that fuse taps just shouldn't be used, especially for something drawing more than milliamps of current.

If you're specifically talking about powering external LED's, the best way to do it (If you're using a standalone interior switch) is with a fused relay in the engine compartment and the switch inside the car switching to ground to trigger that relay. This does several things. You provide power directly from the battery, and you avoid running a switched 12v through the firewall. If your switch wire accidentally goes to ground, the worst it will do is activate the relay and turn the light on as opposed to blowing a fused circuit.

If you want them to turn on and off with the ignition, then there are other options, but none of them should involve powering the lights directly from that fusebox circuit.

I can think of lots of things a fuse tap would be handy for, but I agree, your example of wiring switched headlamps, light bars, etc. to the exterior of the vehicle would make more sense to use a relay.

If I were installing a dash cam (or something of reasonable current draw) and I had a fuse panel on the interior of the car nearby giving me easy access to a switched circuit (either ACC or igniton on), no way would I be running separate power and switching for that purpose. I'll use the fuse tap every time.

My question is more of principle than it is directed at a specific application.

 

[ctechbob]

In that case, for low draw items, I'd be inclined to pull the power from the switched radio circuit if it were reasonably easy to get to. You're usually talking no more than an amp of draw for things like that, so just adding it into that circuit is usually no big deal.

I still usually go out of the way not to use a fuse tap. The switched ignition wires are usually right there, those can be tapped fairly easily in most cars.

If push comes to shove, tap off the load side and be done with it. If it's over an amp or two of current, do something else.