I made a request for a 10 kva transformer be installed. This was supposed to be a single phase trans., and by mistake the electrician started to install a 3 phase. He was going to feed it with 3 phase and then only run single phase to a disconect on the secondary side. We rejected the installation. What effect would this have on the transformer? I know it would be out of balance, but would the secondary output still have the same amperage. I know the installation was wrong, but would it still of worked if we didn't catch the mistake?

A 10KVA single phase transformer would provide 83A per line with a 120V 2 wire connection. Or, you could have 42A per line with a 120/240 3 wire connection.

I am not aware of any 10KVA three phase transformer, so I don't know what the contractor was trying to install. You would need a 30KVA unit to get a single voltage secondary of 83A at 120V. It would not be possible to have a 120/240 3 wire secondary connection with this unit. Also, this arrangement would not be very energy effcient and would require a much larger primary feed.

The trans. the contractor was going to use was a 15kva. Sorry I forgot to mention that.

Not sure I complely understand.
First do you have any 240 loads, or are they all 120 loads. If they are all 120 loads you can use the 208Y/120 transformer and keep it balanced. You just distribute the loads evenly with single pole breakers accross the a, b, c.

The advantage of using 3 wire delta input is effiency and reducing harmonic on the line side. So it may have not been a mistake IMO

Thanks for the input. The contractor changed the 15kva 3 phase to a 10kva single phase, which I originally had planned. The loads are single phase, and some single pole breakers.

 
SJT — Is the 1ø transformer secondary configured 120V 2w, 240V 2w or 120/240V 3w?

The secondary is a 120/240v - 3 wire, and 480v single phase for primary. The 10 kva is a 125lb. wall mounted unit. It was set up on a unistrut stand.

Thanks for the followup, SJT. On occasion, with a 2-wire primary, some forget that a 3-wire 1ø secondary cannot rely solely on the primary-overcurrent device for secondary overcurrent protection.

Also, if the proposed 15kVA unit was delta-wye or delta-delta powered only on two primary phases, weird voltages may have been present on the resulting 120/208Y 3-wire or 120/240 3-wire circuit, particularly with increasing load.