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
TLDR: 1/0 aluminum with #6 copper ground wire. And yes, you can run straight through with either cable or wires-in-conduit, however, if you splice, you need a really big box.
Hold on. I'm a little unclear on your power requirements.
Provisioning
If you wanted to tandem the 20s to squeeze in to 8 breaker spaces, that's the road to perdition. Breaker spaces are cheap, replacing your panel because you cheaped out on breaker spaces is ex-pen-sive. For a few latte's you can kick up to a 16-space or 20-space (not circuit) panel and never have to worry about spaces. The extra room allows us to put those two 20A circuits on opposite poles. They sit 20A side by side instead of stacking, so we only need to provision 20A for them.
The 15A circuit I assume is for lighting, and in the LED age that'll be negligible, so I'll provision 0A for that. (by the way, if you don't want to have to buy two sizes of wire, you are totally allowed to do lighting in #12 / 20A).
Siemens makes a $30 generator interlock for that panel I linked. It ties two common 2-pole breakers so they can't be on at the same time. If those breakers are for your hot tub and sauna, that means you only have to provision power for one of them :)
So, provisioningwise, with the two 20A circuits on opposite poles (not stacking) I see 50+41.6+10.4+20= 122 amps. Whoa.
Now if we interlock the hot tub+sauna, then 50+10.4+20 = 80.4A, call it 80A.
Wire sizing - distance and metal
Upsizing wires is appropriate for long distances, but that's not a suicide pact. It pays to spend some time with a voltage drop calculator and figure out what happens to voltage drop when you use various size wires. The "rule of thumb" says 3% is an ideal, but Code is more concerned with 8% overall. So let it slip past 3% a good distance if not doing so will waste money.
Speaking of wasting money, at #6 or larger it starts to become time to look at aluminum wire. My answer here really drives that home (they have a 260' run @ 100A, but hold that thought).
If this were a short distance and your load were really 70A, you could use #4 copper or #2 aluminum ($486 or $210). We are using Table 310.15b16, and we're required to work out of the 60C column because we are less than 100A.
But this won't do, for two reasons. First your load is really 80A+++. And second you need to make a wire-size bump because of the distance. So we look at the very next size up, #3Cu or #1Al ($594 or $312), good for 80A @ 60C. But a funny thing happens here. At 100A, we're able to use the 75C column. And these same 3Cu/1Al wires are good for 100A @ 75C. So we get a free bump to 100A.
Mind you, the "free bump" doesn't have any effect on our voltage drop. The voltage drop is still a big deal. So what if we make one more bump, and what does that do to price? So we bump to 1/0Al ($354). (copper is over the moon at this point).
1/0Al is good for 120A @ 75C. They don't make 120A breakers, so we round up to 125A. So we can actually breaker this thing for 125A and power everything, no interlock needed. How bad would voltage drop be at that point? My calc says 3.10% voltage drop, red alert, it's over 3.00% - lol just kidding, 3.1% will be peachy. We can power everything.
But wait. Aren't we paying extra to power everything? Yeah, our previous option was the #1 aluminum for $312. $354 - $312 = $48. We're spending 48 extra dollars to fully provision all the power we'll need. Sounds like a good investment to me!
So you're going to spend about 70 extra bucks listening to me... buying much larger panel and wire. What's this whole project, about $10,000? Don't worry about $70. Besides, if I steered you away from copper, I just saved you about $300.
Routing
Can it be run in one continuous shot? Absolutely. If you want to splice it at the transition, that's allowed, but it needs to be inside a junction box and the junction box needs to be fairly large, practically gutter sized - ask a separate question about that.
In either segment, you can use either cable, or conduit. With conduit you can use either cable or individual wires. Individual wires (THWN-2 or XHHW-2 or USE-2/RHH/RHW-2) must be inside conduit. For practical DIYing, use 1-1/2" conduit. Cable is very stiff, so it requires a bigger 2" conduit to be DIY friendly.
Underground, direct burial cable needs 24" of cover over top of wires. Conduit requires only 18" cover, or merely 6" cover for expensive Rigid and IMC conduit (12" under driveways).
My own preference is conduit the whole way, however theft is a consideration: a lot harder to steal direct burial wire.
Grounding
With two services in such close proximity, you really need to make sure your grounding is absolutely tip-top. There is a genuine likeihood of the ground potentials being quite different between the two services. This is no place to be playing "cheapie" games like forgetting the ground wires in each feeder and thinking the ground rod will suffice.
Imagine there is 120V ground potential difference between the service that supplies the hot tub and the service that supplies the building... they are different transformers, surely. That means between the local ground rod at the hot tub, and the local ground rod at the building, there could be 120V of ground differential 10' apart from each other. That will create a ramped voltage on the ground, meaning different voltage under each foot! Now if that was a product of transformer leakage, the voltage difference could be 2400V, and that will kill you.
This is nothing to toy with, especially around water.
So I would link those two ground rods with a buried copper ground wire. Or simply both tap the same rods.