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.
I'm fine with the idea of a main panel with only 2 things in it feeding a sub-panel with everything in it.
Question #1 - I'd rethink putting a service panel outdoors at all. Weather is rough on panels, even if they claim to be outdoor rated. I'm a little nervous about a 100A breaker supplied from the normal bus bars, but if the manufacturer stands behind it, okay. The 100A wires are going to be a mother to wrestle onto that 100A breaker. Are you quite sure the power company has provisioned you 125A service? 100A is more common.
Question #1 (the second): You're gonna want more slots than 24, since this box powers pretty much your whole house. Nobody ever installed an addition and went "Gosh, that job was sure made harder and more expensive by having too many slots in the panel". It's a false economy, especially since bigger boxes are often bundled with more breakers. Your house may be ok now, but do a kitchen remodel and lookout!
Question #2 (the second): Don't bond your grounds to random plumbing that happens to be going by. It's not code, and someday you might have a plumbing problem and the plumber replaces a downstream chunk of it with PVC. Whoops. Also, they've been upgrading customers to PLASTIC water meters. Double whoops. Bond properly and to code.
Question #3 (the third): Bond ground and neutral only in the (singular) main panel. As such, you need 4 wires between main and subpanel.
Just for your edification, it's only a sub-panel if it's fed from a main panel. If it's fed directly from a transformer, it's a main panel.
Best Answer
You start with an 8AWG copper wire...
You are correct that for a 100A feeder, the associated ground wire needs to be an 8AWG copper or 6AWG aluminum wire. If you are using a four-wire direct burial cable, then just use the ground wire in the cable as it will be quite amply sized for the job it is doing; if you are using PVC conduit, then bare 8AWG copper would be my choice to keep conduit fill down.
...and then upsize from there based on how much upsizing of the other wires you do
When you start upsizing wires for voltage drop control, the size of the ground wire is governed by NEC 250.122(B):
For instance, using 1/0 aluminum wire for the hots and neutral in your example vs. the 1AWG that is required to meet Code ampacity rules gives us a size ratio of 53.49mm2/42.41mm2 based on the NEC Chapter 9, Table 8 conductor sizes (we can do this math in mm2 since the conversion factors cancel out here) or about 1.26. Multiplying that by the 8.367mm2 area of an 8AWG copper conductor gives us about 10.55mm2 for the resulting wire. This isn't a standard conductor size though, so we should use a 6AWG copper conductor (which has 13.30mm2 of area) or a 4AWG aluminum conductor for this ground wire.