Choosing a transformer size

[alixenos]

Hello,

I'm designing an electrical network and I want to connect a high voltage (277/480) three phase panel with a low voltage one(120/208).

Anyone can help me to do the correct calculations to choose the correct size of a step-down transformer?

What if I have a single phase stepping-down?

Thank you all.

 

[tom baker]

Start with the load on the secondary side. Convert amps to KW. A single phase panel will be 120/240. You will want to determine what equipment you will have operating phase to phase. 240 volt loads can’t be mixed with 208 volt loads.

 

[Jpflex]

240 volt loads can’t be mixed with 208 volt loads.

I wrote a post regarding this in the past. actually although not designed for this it will work and we actually had to do this since our company boss owner demands we use what ever equipment he had available

We were able to step down a 3 phase 480 volt delta source feeder to a single phase 30kva 120/208 transformer to feed 120 volt loads loads and a 220 volt load from this lower voltage rated single phase transformer by adjusting the primary transformer taps.

By placing the taps for a setting for what should be for a lower incoming voltage into the primary, the secondary output voltage were “boosted up.” So we were able to obtain 220 - 240 volts output from what was designed for 208 volts.

This may shorten the lifespan of the transformer but if you have a stubborn or ignorant boss it works indefinitely

 

[Jpflex]

We are not mixing 208 volt loads with 220-240 loads however

 

[david luchini]

We were able to step down a 3 phase 480 volt delta source feeder to a single phase 30kva 120/208 transformer to feed 120 volt loads loads and a 220 volt load from this lower voltage rated single phase transformer by adjusting the primary transformer taps.

That's a three phase transformer