Okay Bob, here are a few comments that may help: CAUTION I suggest you consult local trained professional electricians or engineers on this one, this is DEFINITELY NOT a question to trust untrained lay Billy Bobs to answer and I'm NOT saying trust me either, as I'm rusty on the latest codes having been retired from electrical engineering wayyyyyyy toooooo longggggg lol That being all said and with NO Warranty here is my "rusty" yet professional OPINION:
PS where to start looking??? Local electrical supply houses, Graingers etc. for a 480 to 240 volt single phase transformer of sufficient wattage.... IFFFFFFF all you need is straight 240 VAC, SEE BELOW for 120/240 or grounded systems!!!
1) If you already have 480 volt three phase, it may be Y or Delta?
a) If its Y its likely configured as 480 Y 277 Volt Three Phase Four Wire (and likely a GrounDED system). In that case you can get 480 volts Single phase from any leg to any other leg (but its only 277 from any leg to Neutral, well DUH). Then if you had a single phase transformer OF SUFFICIENT WATTAGE that can step 480 down to 240, just wire two legs fed from the 480 Y to its Primary input and the Secondary output would be the 240 volts single phase you need.
b) If its straight 480 volt (NO 277) three phase three wire thats a DELTA configuration and likely its a floating NON grounded system (Unless it was corner grounded which I doubt). Similar to the above, its 480 volts across any 2 of the 3 legs. Soooooo take a 480 to 240 step down single phase transformer and wire its Primary input to any 2 of the 3 480 delta legs and on its Secondary output you have 240 volts single phase as needed.
IN SUMAMRY ALL YOU NEED IS A SINGLE PHASE 480 TO 240 VOLT STEP DOWN TRANSFORMER (a 2 to 1 turns ratio transformer) OF SUFFICIENT WATTAGE
NOTES AND MORE CAUTIONS:
1) This gets you straight 240 volts (NO 120) and the output (unless you do otherwise) is a NON grounded floating 240 volts remember. If the load requires 240 only and no 120 and the non grounded floating 240 is okay???? this will work.
2) If the load requires BOTH 120 and 240 this will NOT work. But can still be done, see below.
Your post mentioned 240 ONLY so I wont address what you need to get BOTH 120 and 240 other then it can still be done.
Again, either of the above results in 240 volts non grounded floating, the transformer operates as an isolation transformer and the 240 volts is present ONLY line to line NOT any voltage line to mother earth or line to building steel etc as would be in a grounded system. However, for yearsssssss many industrial plants were wired straight floating NON grounded 480 or 240 and many still are and all is well. When I was a desing engineer our facility and myself preferred a grounded system but floaitng non grounded work fine. There are ways to turn your 480 (Y or Delta) into a 120/240 volt single phase three wire GrounDED system, about as simple as the above, it just requires establishment of a GroundING Electrode to ground the transformers secondary (center tapped)output at the mid point establishing a Neutral.
Sooooooo this is ONLY my OPINION but hopefully it helps you understand why if you dont know what youre doing or whats actually required YOU MAY WANNA CONSULT TRAINED PROFESSIONAL ENGINEERS OR ELECTRICIANS, Your money your risk your choice, be safe.....
ANOTHER PS If theres a need for a few 240 volt circuits and some 120 (single phase), I would probably use a 480 to a 120/240 dry transformer with the center tapped secondary such that I could establish a GrounDED Neutral and use a Grounding Electrode and feed a 120/240 volt single phase three wire sub panel.......
Got it now????????
John T Too dern long retired and rusty grrr EE