Does the sub-panel seem over loaded? If so, I could keep the water-heater in the main panel and free up space in the panel another way.
Seems reasonable to me. Most of the equipment won't draw anywhere near the overcurrent rating, at least not during normal operation. Motor loads will draw a higher current on start, but you shouldn't have a problem.
I know I need four-strand wire to run to the sub-panel (2 hot, neutral, ground) but copper or aluminum and what gauge?
You can use either copper or aluminum, though I recommend copper for DIYers. Copper is quite a bit more expensive, but it's easier to work with (in my opinion). If you feel confident working with aluminum conductors, you can save some money using it.
I've covered the topic of feeder sizing here, so I won't go into detail. If you're using copper, you'll want to use 3 AWG conductors. If you choose to use aluminum, you'll need 1 AWG conductors.
If you want to run a single cable, instead of individual conductors in conduit. You can buy what's called 3-3-3-5 SER cable (1-1-1-3 for aluminum), which will contain three 3 AWG conductors (hot,hot,neutral) and a 5 AWG grounding conductor.
When I run the wire along the floor joist, does it need to be secured to the joist or can it just hang there and rest on the drop ceiling? Seems like it should be secured to the joist with wire hanger or something.
You'll have to attach the cable to the joists, using 1 - 1 1/4" staples or other approved means. Check the packaging, to make sure they are rated for the size cable you're using.
What are the things about this project that I don't know that I don't know. :) These are the scary things IMO...i.e. the questions I don't know enough to ask.
The cable you'll be working with is thick and heavy, and it's not going to be fun pulling it. You'll probably want a couple helpers, to help you wrangle it.
Make sure all your connections are tightened to the manufacturer's specified torque.
If you choose aluminum conductors, make doubly sure you tighten the connections. And don't forget the anti-oxidant.
Come back a day or two after the panel has been put into service, and tighten any connections that need it.
Don't forget to remove the bonding jumper between the grounded and grounding bus bars.
You'll need clamps big enough for the cable, to secure it to the panels.
should I put a 100 amp breaker in the sub-panel to act as the "main" for the sub-panel? Or is the 100 amp breaker in the main panel sufficient?
You can usually pick up a main breaker panel, for about the same price as a main lug only (MLO) panel. In my opinion, unless the secondary panel is next to; or within sight of, the main panel. You're better served to install a main breaker panel. It simply offers better protection during maintenance, or other work within the panel.
For example. If you turn off the feeder breaker in the main panel, and start working in the secondary panel. Somebody could easily come along, and flip on the feeder breaker. Since you can't keep an eye on the breaker, you can never be sure the panel will be dead. (unless of course you're using a lockout like you should).
If the secondary panel is in a separate building or structure, then you either need a main breaker, a main disconnect, or the ability to disconnect all ungrounded conductors within 6 or less hand moves.
Best Answer
Watch your stab limits
Sure, you can fit as many 100A breakers as you please. What you cannot do is place them opposite one another so they clip onto the same bus stabs.
Those little bars of metal, which are typically shared by breakers across from each other, have "stab limits" limiting the max current you can put on any one of them. The stab limit may be discussed in the panel's labeling. If not, make sure the breakers opposite them are smallish, like 15 or 20 amp normals (not tandems).
Note that it is common and normal for the individual breakers to total far more than the main breaker. That's because they're not all used to capacity at once.
Don't oversubscribe - but electricity is for using
200A service is more than enough for most homes. Because of that, you have some headroom for things like cottages and shops. And what's more, these days so many people have money that lots are putting in 100A subs on a whim, when in the past they might have fit a 30, 40 or 60A sub. So I'm not particularly worried about these multiple 100A subs. However, it is possible to overdo it, and excessively load your panel.
You haven't given us nearly enough information to know if you are doing that. That is determined by practical load calculations which would evaluate all your potential loads. But it is something to be mindful of.
For instance if I had a lovely wood shop in my garage, and wanted to add a big L3 EV charger, I would simply install the charger so the EV must be pulled into the wood shop area to charge, preventing its use.
Remember though; electricity is there to be used. If you need to, you can always increase your service size.
One last thing: think BIG on the panel
A guy sold us his entire shop for $5000. Suddenly we had eight 240V tools, and I needed 16 spaces in the panel. The moral of the story is: Spaces go fast! Get loads of 'em!
I was certainly glad I installed a 30 space sub panel, however suddenly I really wish I had installed a 40 space. I have backed away from advising 40 space subpanels simply because it seems so absurd, but it's not. Note that 30-40-space subpanels are not made in 100A sizes. You will be getting a sub panel rated for 150-225A and simply feeding it with a 100A feed breaker back in the main. That is fine.
Yes, the far breaker will trip first, but it's gonna do that anyway even if they are of equal trip value, since consumer breakers are too cheap to coordinate. Anyway if you really, really want coordinated breakers, use a shunt breaker as your sub panel main and a little Arduino or Raspberry Pi listening to a current transformer, and write code to have it shunt-trip the breaker before the main would trip.