Feeding a sub-panel

[Flint]

You may not be able to use the existing wires between barn and house in reverse to feed the house subpanel. It depends on whether the existing feeder has an EGC which is compliant with current code. It used to be allowable to run only three wires (two hots and a grounded conductor) without either a wire or raceway EGC and originate the building EGC from the feeder neutral and a local ground system.

It's an existing conduit (2.5"). It's empty, so I can choose what to run through it.

Southwire's calculator says that 100A through 4/0 for 250 feet will have 2.66% drop. If I went to 300MCM, I could get it down to 1.97%. Not sure if that's worth the extra cost - I'll have to get a quote on 300. Also says that I should use a #6 aluminum ground wire - that seems small, to me?

[Flint]

Feeding from the barn to the house won't work due to a variety of factors (primarily, the power company being difficult, but a few others with layout).

So, here's the plan (unless someone tells me I missed something critical):

Existing house panel will be left alone. Above it, I'll use insulation-piercing connectors to lead off a branch which will immediately connect to a 60A fusible disconnect, which will feed the barn.

4/0 wire will run through the conduit to the barn.

At the barn end, the conduit will connect to the REC meter housing. Millbank has a dual-tap kit for this meter housing, so the 4/0 will connect to one set of taps, and the other set of taps will feed the barn sub-panel (through 2ga THHN in conduit).

The inverters will connect to the 4-space 125A main lug panel I already have sitting around - one through a 40A and one through a 50A breaker. The lugs will be connected to the line side of the REC meter (through 2ga THHN in conduit).

In the future, before the 10kW inverter is installed, I will have to install a 100A disconnect by the utility meter and move the 4/0 barn feed to that disconnect, since an external disconnect is apparently required on any system 10kW and over, but not on ones below that. I'd love to do it all now, but it will be a good bit of work to install that external disconnect.

I believe that meets all of the code requirements, right? Thanks in advance for any confirmations (or problems noted).