Single phase or three phase
Single phase gives you 120/240 volts and feeds houses and small services. Three phase gives you 208Y/120 or 480Y/277 and feeds motors, chillers, lifts and anything with real horsepower. A three phase machine on a single phase supply will not run, and no transformer fixes that, because a transformer changes voltage and not the number of phases. Going from one to three requires a phase converter or a rotary converter, which is a different product.
Why the amperage is not what people expect
Three phase current is kVA times 1000 divided by the voltage times the square root of three. That square root of three, about 1.732, is what trips people up. A 75 kVA three phase transformer at 480 volts gives about 90 amps. The same 75 kVA single phase at 240 volts gives about 313 amps. Same kVA, wildly different current, because the power is spread across three conductors instead of one.
208 volt three phase is real
A frequent confusion is whether 208 is single phase or three phase. It is both. In a 208Y/120 system you get 120 volts from any phase to neutral and 208 volts between any two phases. So a 208 volt single phase load is connected across two phases of a three phase system. There is no 208 volt single phase service as such, it is derived from a three phase one.
Getting 120 volts out of a 480 volt service
Two ways, and they are not equivalent. A 480 to 208Y/120 transformer gives you a full three phase panel with 120 volts line to neutral on every phase, which is what a building needs. A small 480 to 120 control transformer gives you one small single phase circuit, which is what a control panel needs. Ordering the second when you wanted the first is a common and expensive mistake.
Common questions
How many amps is a 75 kVA three phase transformer
About 90 amps at 480 volts and about 208 amps at 208 volts. Three phase current is kVA times 1000 divided by the voltage times the square root of three.
Is there such a thing as 208 volt three phase
Yes. 208Y/120 is one of the two standard commercial three phase voltages in North America. You get 208 volts between any two phases and 120 volts from any phase to neutral.
How do you get 120 volts from 480 three phase
With a 480 to 208Y/120 transformer if you need a full 120 volt panel, because that gives 120 volts line to neutral on all three phases. With a small 480 to 120 volt control transformer if you only need a single circuit for controls. They are different products for different jobs.
How many kVA is 400 amps three phase
At 480 volts, about 332 kVA. At 208 volts, about 144 kVA. Multiply amps by volts by the square root of three and divide by 1000. Because the answer depends entirely on voltage, a service described only in amps is not enough information to size a transformer.
Free to cite with a link to this page. Work out current at any rating with the kVA to amps calculator.