If you're spending the money to add a panel anyway, you might as well reduce the changes of having to expand it later. Unless you're installing the second panel really far from the main, you're likely not going to spend much more to put in the 100 amp panel over a 60 amp panel. Oversizing the panel will not hurt anything but your wallet, so why not do it?
The installation will be exactly the same either way, the only differences will be the size of the breaker in the main panel, the main breaker in the new panel, and the size of the conductors between the panels.
You can read this answer for more detail, but likely you'll need four 3 AWG copper conductors to feed a 100 ampere panel. Of course the size will change based on the length of the run, and other factors. So make sure you verify the size once you know exactly how long the wires will be, and how they'll be run.
Did a quick search for wire prices, and it looks like it's about $0.20 per foot difference between a 6 AWG stranded copper wire and a 4 AWG stranded copper wire. Then another $0.20 per foot difference between 4 AWG stranded copper wire and 3 AWG stranded copper wire. So you'd be looking at an $0.80 per foot difference between 6 and 4 AWG feeders, and an $0.80 per foot difference between 4 and 3 AWG feeders.
Correct. An Outbuilding needs a main shutoff switch even if it has one breaker. It does not need a main breaker, but it absolutely needs a shutoff switch.
It is unclear whether a "connected by roof" building is an outbuilding or not. Code plainly says it's not an outbuilding and does not need a main switch. Your local inspector is the final word on the subject.
Spend some time in the Eaton price book, and you find out that if you optimize for "cheap" or "compact", the best way to get a main shutoff switch is to get a panel with a big switch that by wild happenstance is a circuit breaker too. We don't care about that, we just need a shutoff switch. We only care that its "circuit breaker" trip value does not unnecessarily limit us. If your supply is 100A, then any main breaker 100A or larger will suffice.
This thing is not a breaker for us, and coordinating the breaker trip is hopeless - expecting this local breaker to trip first for our convenience violates Murphy's Law.
The one special characteristic this main "switch" might have is GFCI -- using an oversize hot tub panel is one way to provide necessary GFCI protection to every garage circuit at once, at the cheapest cost. The problem is, hot tub panels are woefully small, though this is helped by our ability to use double-stuff breakers if AFCI is not needed.
And "small panel" is death to a project like this. A person who brings 100A of service to their garage means to run some 240V loads. Those go through breaker spaces like congressmen through taxpayer dollars. Most loads in a garage need to be GFCI with some AFCI, so these will be full-space breakers.
The upshot is, don't even think of a panel less than 24-space...
Best Answer
Wow, very mysterious.
First of all, if the AC unit trips a breaker, it may be the compressor is intermittantly having a hard start. Mine was doing this in cooler weather, or at night. Adding a hard-start capacitor to the compressor fixed it.
But your event looks more troublesom than that, as you have unrelated circuits tripping together. What I would first guess is that you are having brown outs. The brown outs could be caused by the AC compressor having a hard-start. Or the brown-out could be caused by the grid voltage being low.
When a AC compressor starts, it acts like a dead-short for a brief moment, until the motor is spinning. The unit starts in the "locked rotor" condition, and the locked-rotor current is usually many times that of the breaker, so a unit that has a 40 amp breaker might have an locked rotor amp (LRA) rating of over 100amps. Most breakers trip on heat, so they can take a load 3 or 4 times higher than they are rated, for a few miliseconds.
So if your AC is slow to start, it might hit your house with a 100+ amp load for a second, causing your house voltage to drop. If the voltage drops, then many things you have plugged in with reactive loads will increase in current as the voltage drops out. The breakers run off current only, and don't care that the current is caused by a brown out, so if the brown-out causes current to exceed ratings, breakers trip.
So the first thing I would do is tell your AC repairman you are popping the AC breaker from time to time, and ask him to investigate putting a hard-start kit on your compressor. What the hard-start kit does it ads a little extra boost of start-up power to the compressor, so a tired old compressor can start easier, and avoid tripping the breaker.
Replacing the AC bereaker may be called for, but don't put one in that is larger than the name plate rating on the compressor. The compressor will say on the nameplate how large a breaker you should use. Don't put a larger one in, a larger breaker will not make a hard-start compressor better, it will make it burn-up quicker. The only fix is a new compressor or a hard-start kit, never a larger breaker. Of course that assumes it is your compressor. It could be something else entirely, like a power grid brownout.
In my case, the compressor hard-start was only when it was cool outside, but that is just one odd case, most of the time a hard start compressor will hard start all the time, or only when hot, or only when the unit is short cycles, or only when it's been off for a while. Your AC guy should know to look for hard-start when you mention the breaker tripping intermittently. You should hear him talk about trying a hard-start kit even if you don't mention it to him, but that doesn't mean you can't suggest it to him anyway... if he thinks you know what is going on, he is less inclined to give you B.S... If he is unscrupulous, he may suggest a compressor replacement first, but really a hard start kit is a better first move if a hard-start condition is suspected.