Well, I would not say you're doing anything stupid. You have some very good questions.
First,
- If you did decide to direct bury the wire the minimum depth for direct burial is 24", not 18".
- At least three #6 ( black, red, white) and one #10 insulated ground ( green coating).
- Anything <= 60amps just requires a #10 insulated ground with green coating.
Second, Consider voltage drop:
- Load: 30 Amps @ 240V Single Phase.
- Length of run: 80 feet
- Wire Size: #6 Copper
- Voltage: 240V
- Voltage Drop: .81 %
- Voltage At End of Circuit: 238.05
That is less than 3% which the NEC recommends for a feeder. Very Good !
Note: I would hesitate to install the ground rod if you have a in-ground pool in line of the transformer.
Correction: This being a feeder would require a grounding electrode!
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
Fat conduit and fat panels are your friends here
Given that you've already gone through all that work to dig a 140' trench, why would you want to have to do that again if you ever need more than 60A at your greenhouse, as you would if you had to replace a direct buried cable? PVC conduit is not much more expensive, allows you to use a slightly shallower trench depth (21-22" instead of 25-26"), and provides you with the ability to have much more expansion room in the future, as a 1.5" PVC conduit can easily accommodate a 125A feeder.
Inside that conduit, 3 6AWG stranded copper THHNs (hot, hot, neutral) with an 8AWG or 6AWG bare copper grounding wire (to return wayward utility electricity to the utility) are the simplest way to ship 60A out to your greenhouse. You'll of course need the correct 60A two pole breaker for your pole's panel, as well as two 8' grounding rods driven 8' or more apart at the greenhouse and connected to the subpanel at the greenhouse with 6AWG bare copper to form a grounding electrode system so that wayward natural electricity has a path back to nature.
Furthermore, when you're going subpanel shopping, there's absolutely no reason to skimp on the subpanel, as again, saving a few bucks now is not worth the hassle a full panel causes down the road. You'll want to get a 24-space or 30-space, 100A or 125A, main breaker panel for the greenhouse and fit a matching accessory ground bar or two to it as well as removing the neutral-to-ground bonding screw or strap from the panel. This way, neutral and ground are separated at the subpanel, as they should be, so that normally flowing power doesn't try to return on the grounding system and cause problems in the process.