Thanks Jon.
Just the wire leads. I don't care too much about the internals of the "black box" at this time.
My concern was more with the secondary side of the xfmr due to the fact that the primary side had two pairs of wires (Your explanation of the 21 amp thing makes sense on the primary side.) The concern is can the transformer handle unbalanced loads between phases up to 10 KVA on a single phase (idk why you would do that on purpose)? Specifically, I think the design would have to account for up to an 84 amp current draw on one phase (plus a factor of safety). In a perfect world, you would put 42 amps on both phases and be done with it. From that 84 amp number, 12 gauge wiring appears to be small.
But that is critical to what you can do. When you connect the secondary as 120 volts you put two 5 kVA coils in parallel to one another and can supply the entire 10 KVA rating of the unit. If you wire it as 120/240 you put those same two 5 kVA coils in series with one another. If you put all 120 volt loads on just one side - the max the design can safely handle is only 5 kVA, with the other 5 kVA coil sitting there idle.
But at same time if you wired it for 120 volt output you would have twice the overcurrent protection level you would have if you wired it for 120/240, if you correctly protected it for max rating.
Remember use of 12 AWG for most general wiring - 90 C conductor is rated 30 amps. 240.4(D) often forces us to limit it to 20 amp protection, but ampacity adjustments still can start with the 90C value of 30. Then remember the transformer leads are often higher temp rated. The ampacity is based on what the insulation can take, the conductor itself is generally fine at much higher temperature than the insulation can handle.
General ampacity of NEC conductors is somewhat conservative on the side of safety as well, they can often withstand higher temps, and often are designed to operate at higher temps within listed equipment and/or may also incorporate methods of cooling to gain higher ampacity in listed equipment.
Max rated current on one of those 5 kVA @ 120 volt secondary coils is 42 amps. (full 10 kVA secondary rating of 84 amps includes two of those coils in parallel to one another) Really not all that high if you consider THHN @ 90C has 30 amps ampacity as a general rule.