Underground conduit question

[Bluedog225]

Now that I have my conex for the power shack, things are falling into place. The next step will be to run underground cable from the conex to the cabin. The cabin is out in the country and completely off-grid. No code. No inspections.

I’ve got a bunch of 2” schedule 80 PVC and a fair amount of 2” rigid metal conduit. I’ll have a dedicated trench at 12” deep (no frost line) and run the schedule 80 as conduit with the part under the road in the rigid metal conduit.

I’m thinking I’ll run split phase AC to the cabin from some victron product/transformer in the power house. I’ll use copper. It’s about 75’.

I’ve got a couple of questions.

1. If I’m running split phase on an underground feeder line (3 conductors and one ground wire), I think I perform all my wire size calculations at 120 volt. Giving me 8awg. Does this sound right? There is no “there and back” as in DC, correct?

2. Is it a big mistake not to run 240 to the cabin with a transformer at the cabin? I am trying to avoid electrical hum in the small cabin. The 8 awg copper looks like it’s about $3/foot. Not terrible.

3. I’m thinking of a single grounding rod at the conex. The cabin will be a sub panel and grounded back to the conex ground. Am I on the right track?

4. Do I also ground the array to this ground rod?

I’m pretty new to most of this and any general thoughts also welcome.

Thanks I’m advance.

 

[LydMekk]

4. Yes, ground the panel frames/rails to the same ground.

 

[v_green57]

Tape the metallic with pipe wrap. It will corrode.

Or, better yet, bury it deeper in the road and use the PVC.

Metallic conduit buried can cause problems down the road (no pun intended).

 

[Bluedog225]

Good to know. I can simply run the pvc inside a larger pvc as is done with sewer pipe. Thanks.

 

[timselectric]

I’ve got a bunch of 2” schedule 80 PVC and a fair amount of 2” rigid metal conduit. I’ll have a dedicated trench at 12” deep (no frost line) and run the schedule 80 as conduit with the part under the road in the rigid metal conduit.

You can stick with PVC all of the way. 2ft deep under the road. Should be 18" deep everywhere else.

1. If I’m running split phase on an underground feeder line (3 conductors and one ground wire), I think I perform all my wire size calculations at 120 volt. Giving me 8awg. Does this sound right?

How many amps? This determines the conductor size.

2. Is it a big mistake not to run 240 to the cabin with a transformer at the cabin? I am trying to avoid electrical hum in the small cabin. The 8 awg copper looks like it’s about $3/foot. Not terrible.

I would keep the transformer at the connex.
You will probably want 120v there. And may need to run it somewhere else, later.

3. I’m thinking of a single grounding rod at the conex. The cabin will be a sub panel and grounded back to the conex ground. Am I on the right track?

Yes

4. Do I also ground the array to this ground rod?

Yes

 

[Bluedog225]

Good point. I don’t think I’ll ever draw more than 40 amps at 120 volts at the cabin.

I didn’t think to check the assumptions behind the conductor size table I glanced at. Thanks.

 

[timselectric]

Voltage doesn't affect conductor size, only amperage.
Voltage only affects insulation type and thickness.
#8 is fine for 40a at 75ft.