But then the building is supplied with two 13.8 kV feeders. I think we do our
own on-site transformations from there on down.


Most likely.

While at my former employer we NEVER allowed one of our customers to connect to
our transmission systems (115 kV ac through 500 kW ac, and 800/1,000 kV dc), we
DID allow our large industrial customers/municipal partners to connect to our
subtransmission systems (34.5 kV, most, and 69 kV, a few).

Note that MOST (but not all) system voltages are multiples of 115 ...

13.8 kV = 120 X

34.5 kV = 300 X

69 kV = 600 X

115 kV = 1,000 X

138 kV = 1,200 X

230 kV = 2,000 X

287.5 kV = 2,500 X (Hoover to L.A., Circuits 1 and 2, e.g., in L.A.'s system)

345 kV = 3,000 X (not used in L.A.'s system, but used within the "Western
System", APS, e.g.)

But ...

500 kV / 115 = 4347.8261 (Hoover to L.A., Circuit 3, e.g. and the "Western
System" ac Intertie, e.g.), and

765 kV / 115 = 6652.1739 ("Eastern System" ac Intertie, e.g.).

Oh, well.