You can always start at 15 have them on for awhile then if it blows go to 20. I doubt it will blow that. The highbeams on a Ford Crownvic(Po-Po car) don't pull more than 20. I wire those up all day. I know on my fogs My98 style I have a 15A fuse hooked up to them and have no problems. What wattage bulb goes in the newer style? Mine are 55w if I recall correctly.
I've got upgraded bulbs in my fogs, 110W I think. Any reason to switch from the factory 15A fuse to a 20A fuse, even thouth the 15A hasn't blown? Will the 20A somethow let more current through than the 15A, or am I just messing with something that's not broken?
You're basically playing with fire. If you put a bigger fuse in, more current will be able to go through the wires, but they were set up for 15A instead of 20A. Worst case scenario, the wires would catch on fire! I would just leave it alone and replace it with a 15A fuse if it blows. Just my 2 cents!
My theory is "If it doesnt blow the fuse, there is no need to put a higher amp one in its place"
Darrell has a valid point though, the actual wiring is not capable of handling 20A. So if something was to short you run a higher risk of the wire heating up and melting the insulation. The first time you might be fine but after the 3rd or fourth time you might end up with a dead short(more than likely somewhere you cant get to easily) Then you would have to run new wires out to your fogs.
By the way I'm not BSing......this is from trial and error on a daily basis working with electronics in cars. In my earlier post I hadn't thought about the size of the actual wire.