Electrical – Cable Size for 100 amp service load center

This application has "transformers" written all over it. A big factor in AC vs DC power (Tesla vs Edison) being the ability to haul it long distances. And this right here is where that comes into play.

In every single scenario, your barn needs its own earth grounding system.

Plan 1: Conventional sub-panel (as discussed elsewhere)

Wire cost: $4000. (about a 25% margin over the best prices I can find).

That plan only makes sense if money is no object and the goal is to stay "DIY friendly". But it still won't be, because of the miserable pulls with huge wire (unless you upsize conduit to about 4", not least, because the bends are wider radius.) The only benefit to all this is avoiding transformers. Since this scenario is well covered by other answers, I'll won't elaborate.

Plan 2: Use a transformer to make the barn a main panel.

Wire cost: $2000. Transformer cost: $900

You only need 2 of the huge 350kcmil wires. You feed them with 240V and they go to the primary side of a 25kva transformer. The transformer removes the need to run a "ground" and "neutral" wire (but the primary and secondary sides are different). Your barn is now a main panel. L1, neutral and L2 come off the secondary of the transformer. The transformer and the barn panel are grounded to the barn's grounding system.

You may be able to reduce the size of the wires somewhat, because the transformer has "taps" to compensate for voltage drop.

Plan 3: Also pump up the voltage for the long run.

Wire cost: $300. Really. Transformer cost: $1800

This takes two 4-AWG (maybe 6) AL wires. Instead of one 25KVA transformer, you get two -- with a higher voltage primary, within the rating of the cable you plan to run. So if the cable rating is 600V (most likely), get 480, 575 or 600V transformers (those last two are a thing in Canada). These voltages are serious, but still within the range of common panels and wire. Call an electrician for the final review, megging and hookup of the higher voltage stuff.

You backfeed the secondary of one transformer with 240V from your panel. The primary feeds your long conduit run to the other transformer's primary, which knocks it down to 240V on the secondary. As above, the barn panel is a main panel which gets L1, neutral and L2 from its transformer, and ground from the barn's grounding system.

Plan 4: Tie into the high voltage supply

Wire and transformer cost: ???? depending on voltage rating and delivery.

The power company transports power across farmland at a much higher voltage like 600V or 2400V, and then have a transformer at your house to knock it down to 240/120V. I imagine you've already talked to the power company and asked them the cost of wheeling their high-voltage to your barn and provisioning their transformer and meter, and didn't care for the answer.

I am saying, see if it is feasible to buy their power on the high side of their transformer, and take responsibility for the pole line, underground, transformer etc. It's sorta like "rent the cable modem or buy one". Used transformers can be had. Of course touching their high-voltage is electrician territory, but you can do all the grunt work, so he megs a few things out, connects 2 wires and throws the switch. There's no reason to pay an electrician to trench conduit or punch holes in walls.

The answer is "electrician in trade since 1976". In 1976 a 3-wire groundless connection was legal. Certainly not today.

Even a 1976, a separate neutral and ground wire was an improvement. He made it sound like the AHJ would prohibit it, seems unlikely.

Today, belt and suspenders is mandatory. All sub panels must have ground and neutral brought separately, with separation in the sub panel. And then, an outbuilding or other off site location needs a separate ground rod.

And unless the ground rod tests to 25 ohms, it needs two ground rods. Unless you have the equipment, testing is more expensive than driving a second ground rod.

If you're in conduit, use THWN or XHHW. It's cheaper.

if the conduit run is continuous, don't choke cable down it, use individual THWN or XHHW conductors. Your neutral can be smaller and your ground can be much smaller.

For instance I just priced 2/2/4/6 in XHHW and it was $120 for 100 feet. I don't see any 2/2/2/4 URD or anything out there at that price.

Mind you I got these prices by calling my local electrical supply house: trying to buy wiring on the Internet doesn't really work. Too many prices are sucker prices, or lowball but "gee, we're out of stock but thanks for joining my email list" prices, and then there's shipping to contend with. There is also junk imported from far-east places that use US wire sizes. You don't want that. US big-box stores don't have any selection. Proper electrical supply houses aren't online, these things have a very low price-to-weight ratio, so selling online is a fool's game.

Why URD is not a good fit for feeder

Feeder, service lateral and distribution are three separate and different things. Your location is on your side of the meter, which makes yours feeder.

Power companies follow a completely different and more liberal code. They are allowed to use AA-1350 aluminum, it was designed for power transmission and distribution after all! URD is intended for distribution (pole to pole) or service lateral (pole to demarcation point which is before the meter) and that is abundantly clear:

• from product descriptions (which say that plainly, even if many don't understand the terms and so, disregard them)
• product brochures (note the truly screwball ampacity ratings, and footnote that "by the way" these don't apply to NEC usage which must come from NEC's tables). This is glossed over because product users are expected to know it.

Now if you selected this cable because of these wild ampacity ratings like 120A for #4, that doesn't work at all. You need to use NEC rules for determining ampacity. For instance #4 Al has the same ampacity as #6 Cu, only good for a 60A subpanel.

So why does groundless 2/2/2/4 URD exist? For service laterals for three-phase wye service, hot-hot-hot-neutral, either wye or wild-leg delta. Why does Home Depot sell it? Because it sells. Given how bad their help is, I'm sure a lot of what they sell is misapplied, or wasted when the inspector catches it.

Also, the cable does not have a ground wire. People look at the yellow striped smaller wire and go "that's the ground because yellow is kinda like green". No, it's the neutral. It's legal to remark #4 or larger wires, so it would need to be marked with green tape to make it a ground, and a hot conductor would need to be re-marked with white tape to be neutral. I bet almost nobody does that.

A fair bit of googling reveals many claims that URD is not legal inside any home. The claims are that NEC doesn't mention URD so it's not a proper wiring method. That does make sense.