May be a dumb question, but I can't answer it....  If a baitcaster reel says the line capacity is 12LB mono or 17lb mono does that mean it's the largest diameter line you can put on that reel?  If not, how do you know (aside from trial and error) how far above that rating you can go before you screw yourself up?

The given capacity does not dictate the maximum line test you can use on a reel, it simply gives a reference point for comparison to other reels as well as helping determine what techniques the reel is best suited for. Most manufacturers use 12lb or 14lb mono as a basepoint line simply because they are very commonly used line sizes in bass fishing. As far as maximum test on baitcasters, it depends on what you want to use the reel for, but I've run lines up to 0.46mm (20lb. Berkley Big Game) on low profile baitcasting reels with no issues. 

The given capacity does not dictate the maximum line test you can use on a reel, it simply gives a reference point for comparison to other reels as well as helping determine what techniques the reel is best suited for. Most manufacturers use 12lb or 14lb mono as a basepoint line simply because they are very commonly used line sizes in bass fishing. As far as maximum test on baitcasters, it depends on what you want to use the reel for, but I've run lines up to 0.46mm (20lb. Berkley Big Game) on low profile baitcasting reels with no issues. 

 

Thanks BassJunkie, looking to spool like 20-30lb mono on one of my baitcasters to throw some smaller/medium sized swim baits....  Wondering how that will work, but feel a little better after your reply....

The line will work fine on your regular baitcaster.  The main issue with heavier line is line capacity.  As long as your reel doesn't have a shallow spool, you should be ok.  If you're throwing smaller swimbaits, less than 2 ounces or so, your reel should be fine, assuming it's a decent reel.

Required casting distance and species dictate how much line you need.

Thanks BassJunkie, looking to spool like 20-30lb mono on one of my baitcasters to throw some smaller/medium sized swim baits....  Wondering how that will work, but feel a little better after your reply....

I was actually throwing swimbaits up to 9" with the 20lb. Big Game and experienced no issues