The problem here is that some people, including you, seem to think that their car fusebox works the same way as their house fusebox. Unfortunately it doesn't, the house fusebox is actually a
distribution panel, the car fusebox isn't.
In the house fusebox, each fuse is supplied by a busbar capable of supplying as much power as the main fuse protecting it, thus it is not possible to overload it. In a car fusebox the fuses are supplied by individual wires from various sources, the 5 amp fuses are likely to by supplied by 5 amp cable, quite likely from a 5 amp relay or 5 amp transistor, put a 10 amp fuse in (or a wrong way around fuse tap with 2x5 amp fuses) and you can draw double the current that can safely be supplied and either damage things or set fire to the car. Even worse some of these circuits are safety critical; causing a failure in the ABS, stability control, electric steering systems etc. could kill someone. Car manufacturers always fit the minimum rated cable required for each circuit in order to save weight and reduce costs. Saving weight improves performance and fuel consumption, it's important, reducing costs improves profits.
- You should not bypass the circuit's main fuse by installing the fuse tap the wrong way around.
- The fuse tap should tap into the circuit after the existing fuse so that the entire circuit (both before and after the fuse) is still correctly protected by the original fuse as it was designed to be.
Hi if you had read my postings in full you will note that I was an electrical officer in the merchant navy. I worked my way up to chief electrician. This means I have a full working knowledge of electrical systems from 12 volt batteries emergency power generation right through to massive turbine generators my first ship that I went to sea on could produce enough power to supply a town of 75000 people not just housing factories offices the lot.
During my 50 years at sea I have worked on every kind of system that it is possible to from a single high capacity 2 volt battery cell to multi megawatt generation. This included both AC and DC systems in varying voltage levels up to and including three phase electricity. What I am trying to impart is my knowledge for people to do a job correctly without endangering themselves I have tried to do this in a simple manner as not everyone is electrically minded. You implied I was getting confused between household Systems and vehicle systems WRONG I have worked on both many many times you're post is not giving anyone the confidence to carry out a very very simple task safely and is causing confusion to those with limited knowledge of the subject. You are correct in that you need to be wary of cable size and that some are related to a vehicle safety system it is for that very reason that I suggested that a spare fuse location be used and to check that it is live when the ignition is operated to the position you need depending on what use you are putting it to. As for the piggyback method see my earlier advice on the subject plus note this apart from the spotlights for reasons that I mentioned before most accessories that you fit in a car nowadays have a very low current draw. The main exception to the rule is an additional 12 volt socket now if you look at the red wire on the piggyback it is fairly thick and should handle the promised
20 Amps my concerns are with the ability of the piggyback itself to withstand 20 Amps the point being that the fuse has to be the weakest link. Generally the cable that most accessories have is fairly light therefore you now have a double safety feature if the fuse is a bit on the slow side to blow you have a small degree of protection from the piggyback cable being that little bit more heavy duty. I apologise for the misspelling in my last post I was using my wife's phone and it was not until later after posting my reply I realised that her spell checker has dyslexia.