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!
I would start by putting a rather large subpanel as close as possible to it, probably whichever Siemens panel ThreePhaseEel recommends, or a CH or QO if space is an issue. 42+ space. Position it with an eye toward rerouting the service wiring there.
Even before that, talk to the power company about upgrade options. If a particular size is possible, get the subpanel with that main breaker, but don't sacrifice spaces to do it.
Short term, if the new subpanel is >=100A it doesn't need any main breaker at all, or even a main-panel breaker. The main breaker in the main panel will protect it. Use at least 100A wire to connect the panels, i.e. At least #3Cu or #1Al. A reason to use cheaper Al on such a short run is the lugs will be aluminum.
Having a breaker in the main panel can be avoided with a lug-breaker, which snaps in like a breaker but has only lugs.
Now, at leisure, move one circuit at a time from the old panel to the new. That's just to get away from those somewhat dodgy Challenger breakers. If wires don't reach, I typically put 4-11/16" square boxes at opportune locations to splice. Ideally metal conduit the rest of the way to the new panel, and THHN wire inside the conduit. Ideally.
With all the circuits moved over, try to straightline the subpanel straight off the bottom (switched side) of the main breaker.
Alternately, if the power company's upgrade path calls for a new meter pan, get one with a main breaker. Gut the old panel, using it only as a gutter, to the new subpanel, using wires rated for main breaker current. If that's the plan, you can use a main lug subpanel instead of a main breaker one, saving some space. 42-space main lug CHs are not that big.
Best Answer
Smurf tube. The name its mom uses to yell at it is "Electric Non-metallic Tubing" or ENT.
You will need to run a separate ground wire since it is not metallic.
You have to watch out for conduit fill, there are fill calculators on the web.
If you plan 4 or more circuits in that conduit, come back and ask us about derating, but you probably won't have space for that.
DIYers must be careful when designing conduit pulls. Too difficult a pull requires an electrician's truck full of pulling tools, and the owner of that truck typically will only agree to do the whole job.
Most people go straight for multiconductor cable because that's what they know. Meet THWN-2: it is an individual wire with a special jacket made for pulling. It is typically stranded and much more flexible than cable. Conduit pulling is much easier. But it lacks the armor of cable so it cannot be used outside conduit.
So you have 3 choices: a) cable all the way including through the conduit, b) extend the conduit (using any conduit type) to your destination and use THWN-2 wires, or c) THWN-2 through conduit to a junction box, there splice to a cable. Any of these are a compromise on difficulty.
I would extend the conduit just because that's my better skill.