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.
Hooboy. This may be prohibitive to bring to full code, so let me discuss some options to get the most safety increase for the least buck.
Romex NM-B is not legal for direct-burial or for running in conduit. But what really has me spooked is bootlegging a neutral off that ground wire. If anything breaks in that ground wire, it will energize every ground in your electrical system at 120VAC - touching a conduit, the panel, light switch screws, all the things that are supposed to be safe will be dangerous! Far away from the house you could be dead before they find you! Do not leave this situation to fester merely because bringing it all the way to current code would be hard: Don't let "perfect" be the enemy of the "good". Every option here restores the proper function of the safety ground and separates neutral from it.
Ed Beal discusses an issue with the ground rod.
I am willing to assume some defects may be grandfathered or permitted via local exemption.
Option 1: Go 120V-only
If you can bear having only 120V, then remove all markings from the white wire. In the main panel, black goes to a 120V breaker (or one side of a 240V breaker), white to neutral bar, and bare to ground bar (they may be the same in the main panel). In the sub-panel, black wire goes to one phase, white to the neutral bar, bare to the ground bar which are isolated. Every other row of breakers will not work - just move your needed breakers to rows that do work.
Cost: $30ish per building, for a separate neutral bar for the sub-panel
Option 2: Go 240V-only - temporary transformers for 120V
If your primary loads are 240V-only and/or multi-voltage loads (lighting, some gadgets), and your 120V loads can be hooked up temporarily as needed, easy - just reconfigure for 240V-only. Stop using neutral; the bare wire is ground-only.
Mark the white wire with tape on both ends to designate it a "hot". In the main panel, black and remarked-white to a 240V breaker. Bare to the ground bus. In the sub-panel, black and remarked-white to opposite "hots", bare to the ground bar. No neutral bar - no neutrals will exist in this panel! No exceptions! Every white wire in wiring will be marked with tape as it's not a neutral.
To power 240V loads, done. Many machines will run on 120-240V. Lighting particularly - check your fluorescent lights, the newest ballasts are 120-277V automatic switching, if yours isn't, just change ballast, they're $10-20. Good time to upgrade to T8 bulbs while you're at it. Many LED bulbs are 120-277V, I've been replacing 175W barn lights with 15W LED lights that are $10 and multivoltage. Old mercury, sodium and halide lights have transformers, and they are often jumpable for 240V. For plug-connected devices that can accept 240V, change your wired receptacles and plugs to NEMA 6-15 or 6-20.
To temporarily power loads that are 120V, use a common step-up/step-down transformer.
Cost: est. $250 per building for new bulbs, ballasts, outlets, plugs, power supplies and a step-up/down transformer.
Option 3: Go 240V, install a 120V transformer permanently
This is like Option 2, except with a permanent 120V transformer feeding a third panel.
I often find 5KVA transformers around "used" for about $100. These have 240V on the primary, and 20 amps of 120/240V split-phase on the secondary (jumpable for 40 amps of 120V only). Do the sub-panel per option 2, and add a 20A breaker to feed this transformer. The third panel gets its hots and neutral from the secondary of the transformer. It gets its ground from the ground rods feeding the sub-panel. Because the third panel is fed from a transformer, it's actually not a sub-panel at all - it's a main panel, and you do bond neutral to ground here. The transformer makes this safe.
You only have 20A per leg (or 40A at 120V-only) - so keep the loads here to a minimum.
Cost: est. $300 per building for used transformer, enclosure, additional mini-panel, breakers and wiring.
Option 4: Use a transformer for all loads.
Here, we use a larger transformer (12 KVA or larger, single phase) to feed the entire sub-panel. Like above, this makes it a main panel.
In the house's main panel, we mark the white wire with tape, and punch it down into a 240V breaker. Ground goes to the ground bar. At the transformer, we jumper the transformer's primary for 240V and attach the black and marked-white wire. Ground goes to the transformer chassis or enclosure.
We jumper the transformer's secondary for 120/240 split-phase. Its hots and neutral go to the 2 hots and neutral bar. Being fed by a transformer makes it a main panel, so ground and neutral are bonded together and the ground must go to the rods and also to the transformer chassis. The now-main panel is hooked up in the normal way. This gives you best of all worlds at higher cost.
Cost: est. $500 per building for a used transformer and wiring; $1000 for a new one.
Option 5: Pull 4 wires (in existing conduit??)
ThreePhaseEel addresses this well in his answer. You're pushing the limits of what that conduit can handle, out and in, and it's going to be one miserable pull. I prefer to install conduit much larger than the wires need, mainly to make the pull easier (and provide room for expansion). I would bring in an electrician for this - simply because they'll have all the right pulling tools on the truck, and the skill to do the pull without tearing up the wire. The right tools make all the difference in the world.
Keep in mind if there's any damage to this conduit, the pull will fail and you will end up digging up the yard. Water in the conduit is no big deal; that's why you use THWN wire.
And don't presume it's in conduit. If it's direct-burial cable, Code requires the cable exit the ground in conduit. That would explain using barely-big-enough conduit that he couldn't possibly pull 200 feet through. If it's direct-buried, you'll have to re-trench it.
Cost: est. $700-1200 per building: $400 for wire; $300 for tools or $800 to hire an electrician for the pull. If the conduit is not pullable, more.
Best Answer
You need to use wiring methods compatible with the wire. For instance XHHW wires aren't allowed outside conduit, so you either extend the conduit the full run, or transition to a type of cable that is rated for direct use. They do make individual wires rated for direct use, check with your supplier (who I hope isn't a big-box store).
You cannot splice in an LB. * Put that right out of your head. You'd need an appropriately sized junction box, and since we're dealing with large size wire, you need to follow the wire-radius rules, not just the cubic-inch rules. So you'd want a gutter, specifically.
1/0 may be larger than you need. 1 AWG Al should be legal for 100A (you are allowed to work out of the 75C column for >=100A feeders) and your distance isn't enough to warrant a voltage bump (put your distance, 40A for the charger, 240V and 6% into the voltage drop calc and see what it says).
The feeder run must be protected. The main breaker in the subpanel is in the wrong place to do that, so you need a 100A breaker in the main panel A normal breaker is fine.
The subpanel doesn't need a main breaker. However since it is an outbuilding, it needs a shutoff switch. Using a panel with a main breaker is an overkill, but cheap, way to do that.
When used as a shutoff switch, as in here, the size of the subpanel's main breaker does not matter. If you're getting a small-ish panel to obtain a panel with a 100A main breaker, don't do that! Spaces are rather valuable (super cheap to add now, super expensive to add later) and you don't want to have to have a future project compromised because you refused to spend a few latte's on a bigger panel today.
Also you want to avoid the necessity to go to "double-stuff" (twin, duplex) breakers simply to achieve needed density; most breakers today need AFCI,GFCI or both and that is they don't make those in double-stuff.
You need 1 ground rod if proper testing proves it to be <25 ohm to earth (this is specialized testing, not a pocket VOM) - otherwise you need two.
* Ever buy a product with 10 warning labels because some people need disclaimers to not eat things they shouldn't? Well, here's one. If the LB is excessively large for the wires that are in it, e.g. if you used a 6" LB, then you get to treat it under the junction-box/gutter rules, and count the dimensions to determine whether a splice here is legal. But normal people don't do that, because it's an expensive way to get a junction box.