Glad to hear it - that was a drastic idea and not really a solution. Since you're putting a new circuit in (here in BC I don't think you're allowed to use a 15a circuit for power, it's only allowed for lighting) I'd suggest you put the printer on the 20a and keep the computer on the 15. I too wonder about the accuracy of the printer's draw but at the same time suggest you do consider replacing it, if you can verify that excessive draw. The 80% you were mentioning is more a thing of good housekeeping - for safety's sake, a circuit should always be de-rated as it were - just as with electronic components like capacitors, they're usually rated far above the operating voltage you should actually use. For example, I got a collection of capacitors from a surplus store very cheap because they were odd values - one is rated at 100,000 microfarads which is huge but rated only at 15 volts dc - I couldn't use it say, in a car as a noise suppressor in my stereo, as car alternators can put out as much as 13.8 to 14 volts, which would be too close to its peak rated voltage....its absolute limit would probably be 12 volts, but in practice, it'd be better for 6 or 8 volts. The other thing to note in the life of circuit breakers is they do not have an indefinite life, I've heard talk they should be replaced as soon as they've been tripped a few times, since they can start to act up like yours is, and not trip when they should....also, if you're running them at or above their peak, the likelihood of them failing to trip is increased. A really safety-conscious person probably ought to replace ALL their breakers every few years, although I doubt many do.

If there's a reasonable probability that you'll need 30 amps sometime in the future, then you can install the 10Gauge wire. Even if it doesn't fit the receptacle or the breaker (which it probably will), you can pig-tail it in. As long as you're installing single receptacles on each circuit, I don't see any problem. If planning for the future, I'd run 10/3 so that the neutral would be availble if the device you're connecting in the future needs both 240 and 120V power.

If you decide on a three-wire service, then I believe you'll need to run 2-2-2, not 2-2-4. A true equipment ground--not a neutral--can be a size lower (#4), but I don't believe that's the case when neutral and ground are combined. [Since you'll have more than six breakers, I'm guessing that a good number of the circuits will be 120V. As Brick noted, if you had only 240V loads, no neutral would be needed at all, but that doesn't seem to be the case here, and usually isn't with multiple-circuit outbuildings.] I know for certain that the three-wire service you're proposing would not fly in my county, but that is because local restrictions here (not necessarily the NEC) won't allow it. (My jursidiction doesn't buy into the "no other connections" exception for 240/120V service.) Perhaps they figure that sooner or later someone's going to run a phone line or intercom or water pipe, and anything like that--any other connection!--instantly puts the wiring out of code. I can't speak for your area, of course, but that begs the question of whether you plan to pull a permit and have this installation inspected. In any event, it might be a good idea to consult with your AHJ to verify that your plan meets local code. For a variety of reasons--not least the flexibility of adding things in the future--you really might be better off with four-wire service (2-2-2-4). As noted above, you'll probably need 2 ground rods driven at least 6' apart. While it is true that one will suffice if it can be demonstrated to have a resistance of 25 ohms or less, don't bank on that. I've put in three ground rods in the past six months in three different states (Virginia, Maine and California) and none of the three passed the 25 ohm test, but that depends upon local soil conditions, moisture etc. at any given time. Use #6 wire for the ground connection and avoid any splices! (That's one of the problems with adding a second rod "after the fact".) I've also run into the problem of not being able to find subpanel "main" breakers under 100amps with the code-required lockdown feature. Keep in mind that you can use a larger breaker there (i.e., 100amps) because it is serving simply as a disconnect rather than primary overcurrent protection for the wiring and your box is rated at 125A. With #2 wire coming in, it'll probably fit easier into a 100A breaker as well. The one slight disadvantage is that if the "main" does trip, it'll be the one in the house not the outbuilding. But this is not something that should be happening often. (We hope!) Just out of curiosity, how long will the service wiring run be? I ask that because you might want to price #4 (or possibly even #6) copper instead of #2/AL for the sake of comparison, if you haven't already. There's no question that #2/AL will do the job (unless it is an inordinately long run), but #4/CU (with #6/CU ground) would be a good bit easier to pull and work with. Still, I know that copper's gotten very expensive in recent years, so YMMV.

OK, to answer the question, you can't tell about the loads without doing a proper analysis. What used to be connected to the 30A breaker. Seaborn: The way it works is that the panel is appropriately sized for the service and there are limitations on what you can put in the "top" section. That's where we run into a problem. The code that allowed these panels in the first place requires that all the lighting and appliance e branch circuits (the ones it sound like he wants to add) CAN NOT be connected to the top section (mostly the understanding is you don't know what someone might connect to them as opposed to the things in the top section which are fixed items: furnace, stove, dryer, hot water heater. This requirement persists in the modern code. 408.36 covers it. It was formerly 384-16. There were some exceptions but they don't seem to apply here. So the answer is NO, YOU CAN'T DO THIS by code. ProSource is correct. You could connect a subpanel to that 30A breaker and put your new branch circuits on that.