Underground Conduit Rules

[Bill Dufour]

While the tench is open...back fill 6" or so then throw in two or more 3/4 conduit runs, another 6" of back fill then the marker tape or old iron pipe etc, then fill to the top. The extra conduit can just be capped at both ends underground and left unconnected for future use.
Bill D

[Bruce King]

You need an underground rated cable even though it’s in conduit.
3 conductor plus ground. Be sure to run large conduit, looks like 1.5 inch will work if you are going with 100 amp. This cable is about one inch diameter.
https://www.wireandcableyourway.com/...BoCtTgQAvD_BwE

[Bill Dufour]

I do not think the cable has to be rated for underground just for water?
Bill D.

[Jim Becker]

I do not think the cable has to be rated for underground just for water?
Bill D.

The cable through the underground conduit must be rated for outdoor/underground/direct-burial. The conduit provides a level of physical protection but is considered a "wet" environment. I could be wrong, but I don't believe there's a multiple cable types for this application.

[Charlie Velasquez]

The cable through the underground conduit must be rated for outdoor/underground/direct-burial. The conduit provides a level of physical protection but is considered a "wet" environment. I could be wrong, but I don't believe there's a multiple cable types for this application.

Mobile Home Feeder is considered an assembly. I used mhf 2-2-2-4 aluminum to run my panel. If used as a service to run your dwelling it is rated at 100 amps. If used as a feeder it is rated at only 90 amps. Check with your local AHJ ahead of time. Many will sign off on its use as a 100amp feeder, mine did.

Clarification: #2 aluminum is always rated at 90 amps. It is allowed for 100 amp service if it is a dwelling service.
If you decide to go this route instead of copper thwn, get the AHJ to sign off, or ask via email and print the approval, keeping it with your permit.
If not, in a residential shop, probably not much difference between 90 and 100 amps. You may need to special order the 90 amp breaker.

[Doug Dawson]

While the tench is open...back fill 6" or so then throw in two or more 3/4 conduit runs, another 6" of back fill then the marker tape or old iron pipe etc, then fill to the top. The extra conduit can just be capped at both ends underground and left unconnected for future use.
Bill D

Or to give the trench digger a good harmless scare. :^)

[Charlie Velasquez]

While the tench is open...back fill 6" or so then throw in two or more 3/4 conduit runs, another 6" of back fill then the marker tape or old iron pipe etc, then fill to the top. The extra conduit can just be capped at both ends underground and left unconnected for future use.
Bill D

I would suggest a 3/4” spare and a 1” or larger.
The most likely upgrades are a/v, internet which can be handled by the 3/4” conduit; and 4 #6awg for a generator inlet to feed your main panel which can be handled by the 1” (assuming a 9600 watt to 12000 watt portable generator). The communication conduit does not need to be at the 18” depth, but the possible generator inlet does.

While 1 1/4” conduit meets code for a 100 amp pull, I would upsize it. Not sure how many turns you have, but it’s a lot easier with oversized conduit.
Even if you go with 1 1/4” conduit, if you have a number of turns consider upsizing your LB or pulling elbows. NEC says you cannot use a conduit body smaller than the size of your conduit, but you can use one bigger to help with the pull.
Plan your pull. Make sure all the bells are going in the direction of the pull.

[james manutes]

Years ago , I was in a condo . Soon as you added that , alarms went off in my mind . Do you have an HOA , or a "board" to answer to ? It can be worse than local code and building hoops to jump thru . Condo's suck !

[Bill Dufour]

I believe in conduit you can have only four turns before you have to add a box for a pull point. That would be one at each underground end and maybe one at each end into the wall or side of a external panel.

[Charlie Velasquez]

I believe in conduit you can have only four urns before you ahve to add a box for a pull point. That would be one at each underground end and maybe one at each end into the wall or side of a external panel.

Kinda, no more than 360° of bends.
So LB’s at the entry points, short stub, expansion fitting, stub to trench depth, then no more than 360° of bends underground. You need 90° at each end to make the vertical/horizontal change, so you’re limited to 180° in your run before you would have to add another conduit body.

The cable Bruce linked to is commonly called URD. It is rated for direct burial. If you have a number of underground turns you could forego the underground conduit and just lay it in the trench after stubbing it to your trench. (edit: you may have to go deeper than 18” if direct burial. If you’re paying someone to do the trenching, probably not an issue.). But URD is not rated for indoors and must transition to indoor rated conductors immediately, usually to SER (some ahj’s interpret that to mean “immediately” others allow a few feet; check with your local ahj). transitioning to SER allows you to run the cable following the rules for regular nm-b (romex)

I used MHF. Like URD, it is also rated for direct burial, but it is allowed indoors if enclosed in conduit. It saves on large junction boxes and connectors, probably split-bolt or morris; but you have to run conduit to your panels.