Cable thru Joists Question

Hey all, I've read a lot of posts here and find the information very useful. I thought I would put my question(s) up for grabs and see what you all have to say. I'm looking to get an air compressor in the next month or so and update my detached garage accordingly. It currently has a 30amp circuit split to 2 15amps breakers. The compressors I'm looking at range from 220V/15amp to 220V/30amp. I'd prefer the 30amp more powerful ones if I can afford it. Anyways, the current wiring "should" be in conduit since its under my paved driveway(was done 15 years ago). How probable is it to pull additional cable to the garage to get this 220V service? Ideally, I'd like to add a 60-100Amp panel in the garage and have room for a possible wall A/C unit and small 220V welder. I'd have to dig a trench around my driveway to run the wiring if I can't use the existing run, which could be a pain. Any ideas??? Oh yeah, sheesh, back up resistance emergency heating sure does use a large breaker!! In trying to see if I have room for this garage upgrade, I was surprised to see such a large breaker, especially for something that has never come on since I've lived here over the last 4 years. Thanks for your responses - Drew

Hi, When a 15 amp breaker goes bad can it possibly trip while an appliance drawing 7amps is being used or does a bad breaker supply less voltage to an outlet? Thanks, Angelo