3 phase transformer sizing

[TaylorElectrical]

Need some input on transformer applications.
Changing out a farm service from 120/240 single phase to a 480/277 3 phase service 500kva. Once we bring in the 3 phase service, we can use a 3 phase transformer to step voltage down to 120/208 to feed lights, receptacles etc but we cannot use the existing 120/240V single phase panels. We will have to install 3 phase panels for these loads but can existing motors wired single phase 240V be wired up 208V? I assume they are multi-tap but most small ones are single phase. Im concerned about the existing single phase loads. Or can we use a transformer to get from 480/3ph to 120/240 1phase? Thanks for the input.....

 

[templdl]

Need some input on transformer applications.
Changing out a farm service from 120/240 single phase to a 480/277 3 phase service 500kva. Once we bring in the 3 phase service, we can use a 3 phase transformer to step voltage down to 120/208 to feed lights, receptacles etc but we cannot use the existing 120/240V single phase panels. We will have to install 3 phase panels for these loads but can existing motors wired single phase 240V be wired up 208V? I assume they are multi-tap but most small ones are single phase. Im concerned about the existing single phase loads. Or can we use a transformer to get from 480/3ph to 120/240 1phase? Thanks for the input.....

How did you come up with 120/208 which I would 'assume' would be be 1ph3w which would be feed from a 3ph 480d - 208y/120 step down transformer? Now this is only a guess as you weren't clear on this.
But you would be going in the right direction should you consider a 1ph 480-120/240 transformer which to me would make more sense.

 

[kwired]

Most general duty motors will be dual voltage for 120 and 240, no 208 volt tap.

That doesn't mean they will not work on 208. Some may even be marked as suitable for use at 208 volts - FLA will typically be about 10% higher then it is for 240(is actually probably rated 230).

What is best to do is likely dependent on how much single phase load you actually have, and maybe considerations of future expansion, and what kind of equipment you are dealing with. General purpose motors may be easier to change to three phase motors, specialty motors are more expensive and maybe worth leaving as is. When converting grain storage areas to three phase 480 volts from single phase 120/240 I generally recommend replacing all motors over 5 hp with 3 phase motors - lessens the size of 240 volt transformer needed, but each motor needs its own evaluation, sometimes there is good reason to leave it as 240 single phase.

 

[TaylorElectrical]

Transformer

Transformer

Probably didnt explain it very well.......
We have current single phase service to grain bins, 120/240V. Putting in a new 480/277V 3 phase to help the huge motor loads. Trying to figure out transformer set-up to keep existing 240 motors running until they are changed out at a later date. So I need to set a transformer off of 480V panel to re-feed current 120/240 panel that carries single phase motors, lighting, receptacles etc. We are going to re-use the existing single phase 800amp distribution panel to re-feed single phase panels. So do we size transformer off of 240V, 800Amp load size and is it a single phase panel since load is or is it a 3 phase since line side is 3 phase supply?

 

[kwired]

Probably didnt explain it very well.......
We have current single phase service to grain bins, 120/240V. Putting in a new 480/277V 3 phase to help the huge motor loads. Trying to figure out transformer set-up to keep existing 240 motors running until they are changed out at a later date. So I need to set a transformer off of 480V panel to re-feed current 120/240 panel that carries single phase motors, lighting, receptacles etc. We are going to re-use the existing single phase 800amp distribution panel to re-feed single phase panels. So do we size transformer off of 240V, 800Amp load size and is it a single phase panel since load is or is it a 3 phase since line side is 3 phase supply?

Figure out what you have for loads and do a load calculation. Just because you have an 800 amp panel doesn't mean it is loaded to 800 amps. If you can get demand data from POCO you may be surprised at the actual demand.

Still may be worth converting some larger motors and/or more often used motors to three phase to lessen the size of transformer needed.

Grain storage - fans are something that runs continuously. Augers, conveyors, often are limited time and seldom do you run all at same time either. A conveyor coming from multiple bins to a load out usually only unloads one bin at a time, so only figure one bin auger in the calculation and not all of them to run at one time. You generally are not using loading and unloading equipment at same time - at least not typically for farm storage bins, maybe at a plant that is processing the grain you could load and unload a bin at the same time.