How much can the fuse box take?

[jabba2020]

I want to run some extra power to the back of my passat. The 2 12v outlets in the boot are not powerful enough. Is there any maximum rating fuse that I can put in the fuse box. I have found a spare space with permanent power but need to know what extra I can put in? Will it cause any ecu problems if I put another 30a or higher in? Please help??

[gazza57]

The size of the fuse you can put in there is ultimately determined by the size of the cable which runs from the battery to the fuse box, which is called the feed cable and the size of the cable which you intend to run to your 12V outlets.
Firstly in order to protect the feed cable to the fuse box the main fuse is probably 50 or 60 Amps, the maximum rated current for the feed cable will be above that probably 70A. This means that if too much current is drawn say 62A the fuse will blow before the current can reach 70A and begin to affect the feed cable.
Increasing the current rating of your 12v outlets will place extra load on that circuit and extra load on the feed cable.
If you put a 30A in there you have to be sure that the extra current drawn by the power outlets plus what is already on there will not exceed the feed cable fuse rating and that the extra current in that particular circuit does not exceed 30A If either do your circuit fuse or the main feed fuse will blow.
A 30A fuse will give you approx 350 Watts of power at 12 volts but just how much extra capacity you have in the fuse is a difficult thing to quantify.
It pays to leave well alone, most manufacturers pare their fuse ratings to the bone, ie very close to the cable max current to save on cable and hence costs.
Don't be tempted to increase the fuse size until it stops blowing, if you do and the maximum current rating of your circuit cable or feed cable is exceeded the insulation will simply melt, you will most probably get a massive short and the wiring loom will just catch fire.
Gazza 57