Doug is right (as usual:thumbsup

, but I disagree just a little in the real world applications.
First, my experience is that carmakers are conservative in the wiring and amp ratings. There is a little bit of leeway.
I'll agree so far - especially with the part where I am right!
Second, most electrical components have an initial surge when turned on, then their power requirements drop to maintain the appliance.
So, when you hit your seat heater, there is a big initial draw for the first second or two, and then it goes down some. If you go up 5 amps on your fuse rating, you will probably be safe.
You's guys is killing me :dunno: Actually, what you really are doing is making me go deep into electrical theory, and I really don't want to...
So... you asked for it!
Wire is rated for a certain amount of continuous current at
a maximum voltage. Fuses are rated using the amperage
at which they will blow if it is exceeded for a certain amount of time.
For example, a "slow blow" 25A fuse will allow about 35A or so for a short period of time (the startup you refer to). Automobiles typically use slow-blow fuses to avoid nuisance problems during minor surges.
The issue here is that you may have a fault to ground. A fault to ground (meaning a live wire touching the chassis) does 2 things: First, it causes a huge spike in voltage. Second, it causes a huge spike in amperage.
What happens with a huge spike in voltage and amperage? Something melts. It can be the fuse (as intended), or, if you use to large a fuse, it can be the wire.
THAT is why you don't use a larger fuse. And as a side note, there is NO need for a larger fuse when testing. Use a test light (self powered with a small lightbulb) to check for shorts to ground.
But, you still should check out the cause of the original blown fuse, as it could be something as simple as the heating element getting old and inefficient, causing that higher intial draw which is OK, to something serious like a short developing, which is not OK.
Just to clarify, if the heater is getting old and inefficient, it needs to be replaced, not run with more power. If you have a short you need to fix the wire...
So, think up uprated amp as a SHORT TERM solution until the problem is properly checked out.
Regards,
Jerry
I think my opinion on this one is probably pretty clear now
d-