Transformer to transformer bonding 208Y to 480Delta

[jim dungar]

would assume there isn't an XO on the secondary side.

The primary and secondary side of transformers are not designated using H and X.
"X" is the designation for the Low Voltage side of a transformer. "H" is the designation for the High Voltage side.
"0" is the terminal number for the neutral point of transformer wye connected windings.

There is always a problem with connecting the X0 point of the primary side of a transformer, unless you are a utility.

Some of the engineering is explained by @winnie in post #19, which usually leads to a severally overloaded neutral.
The other reason not to do it is the NEC requires reverse fed transformers to be installed per manufacturer's instructions. In 50 years I have never seen a major manufacturer that approves any connection to an X0 terminal on the primary side of a general purpose dry type transformer.

 

[winnie]

But the bad situation (what happened to the OP) is you have a small wye:delta transformer trying to control the neutral voltage on a larger system. You get circulating currents that damage the transformer or the neutral connection.

I just read @Strathead 's post, and went back to the OP, and realized that the connection to X0 _didn't_ cause a problem.

As I explained, when X0 is connected to the supply neutral, the transformer acts to 'balance' the neutral. This is just circulating currents and an additional load on the transformer. If that load is small enough, then it isn't a problem, and acts to 'support' the service neutral.

As an example, imagine you have a 208/120V service with a long service drop. If you then connect a single unbalanced load (say a large space heater on one leg only) then you get voltage 'drop' on the long neutral. With a wye:delta transformer and X0 connected, the transformer will supply some of the neutral current to try to prevent the voltage drop. If _intentionally done_, this can be a useful tool, and as I noted the utility company sometimes does this on their distribution system.

The _problem_ is that this is often an unexpected load on the transformer, and if you have for example a small transformer intended to feed a small load connected to a large service you maybe don't what that small transformer trying to maintain the balance of the rest of the system.

 

[tortuga]

@8sparky8 you could check a few things like age and condition of 3-wire feeder, is it in good condition? is it OH or UG? Was the TN-C feeder installed prior to 2005 and thus is grandfathered? Is the load around 30kVA or less?
If those things are all good and I'd personally look into a delta-delta (Δ-gΔ) transformer for the step up side, corner ground it, and keep the step down side as is.