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
Option A is necessary if your present panel is Zinsco or FPE Stablok, but we can do that in a sneaky way that isn't so impactful. Otherwise, option C.
Option A-1 is to install a replacement panel right next to the defective panel, fit double-lugs in the original panel so you can daisy-chain the new panel off the old -- then cut over one circuit at a time at your leisure. Once all the circuits are gone from the old panel, bypass and remove it.
Now I'll discuss why option B is out of the question.
This referred to DC power; the Code edition was 1897. Current code says the same but splits it up, all in Article 230. Nonetheless, its terse language implies several things.
Only one set of service conductors per building.
And those exceptions call out fire pumps, generators, apartments, too-large services, and different voltages/phases. There's some blather about 230.40, which is full of very confusing terms and you will need to keep referring back to Article 100 to clarify them - but there's nothing there that helps you.
One shut-off switch per service, practically
This is no good at all. It means even if you can snake two sets of service conductors by the inspector by snowing him with 230.40, the panels need to be right next to each other. The service conductors would be uselessly doubled, lay right next to each other.
This "Rule of Six" is a liberalization of the 1897 rules, often seen in obsolete split-bus/"Rule of Six" panels. Note that if your service panel is a split-bus/Rule-of-Six, a second panel would be a seventh throw, violating the Rule of Six. The exception "6 per service" doesn't work because you have only a single service.
Option C
Being out of space is an intolerable situation. We constantly advise people to get the largest panel they can bear to get, and then some. I aim to finish projects with 50% of spaces unused. Where we see full panels, we often see twin/duplex/"double-stuff" breakers, which have hazards of their own, not least, they actively impede upgrading circuits to AFCI/GFCI.
So right off the bat, I recommend a subpanel right next to your "main" subpanel, and cutting over circuits from the main to the new sub, as convenient. This will free up spaces at your leisure and at a scale that is DIY viable.
As we will always recommend, go really huge on the panel. Space is cheap, regrets are expensive.
You can move circuits at your leisure from the main-sub-panel to the sub-sub-panel. Right away move enough circuits to liberate two spaces for the sub-sub-panel's feed breaker.
My recommendation for the addition is a second subpanel, so liberate 2 more spaces.
Option C part 2: Service to the addition
I gather from your question that you'd rather have a panel in the addition serving loads there. Just another sub-sub-panel.
If you can accomplish this with only the addition sub-sub-panel and move enough house circuits to it, that will suffice.
Again, the addition sub-sub has its own feed breaker in the main-sub. The wire between the two decides the size of the feed breaker (and this is picked out of the tables for branch circuits, 310.16, not for service drops). If you're looking at #4 or larger wire, seriously consider aluminum, it works extremely well for feeders like this, and the lugs are aluminum anyway. Again, the sub-sub-panel's rating must be larger than the feed breaker. (or equal).
I recommend two subpanels to make it simple to describe, and because being out of spaces is a major annoyance. If you can reroute enough circuits from the main-sub to the addition sub-sub, then perhaps that is the only one you will need. Regardless, don't scrimp on spaces. A 42-space subpanel is not excessive at all. Doing all the work to install a 12-space, and then running out again, is excessively wasteful of your time.
** However, if your main subpanel is old, and maybe worthy of replacement, it would be clever to choose a sub-sub-panel which is capable of performing all the functions of the main-sub. Then you can slowly migrate every circuit over to the sub-sub, and one day, switch the service conductors over and eliminate the old main-sub entirely.