step up transformer calc.

[Dustin M Morris]

i have a 120/208 3 phase service... in need to add a 460v 3phase rtu that needs a 50 amp ocp. i have a 30kva 3phase transformer that i could wire in reverse to get my step up voltage.. not sure what breaker size i need for primary side to the transformer or if 30 kva is enough?? could someone please help me out with this one?

 

[augie47]

The 50 amp breaker really doesn't tell you what you need to know.
One could assume the FLA would be around 40 amps in which case a 30 kva would be a bit small.
IMO, the only way to assure you have an adequate transformer is to get the FLA and RLA of all motors plus any other loads and run that by a transformer rep. As a rough guess I'd say you would need closer to a 45 kva.

 

[fmtjfw]

You also might want to check if the manufacturer recommends that the transformer can be run backwards. Sometimes there are inrush current problems when you use the secondary as the primary.

 

[Dustin M Morris]

Just wanted to say thank you for ur input and ur time.. it was very helpful...

 

[kingpb]

Make sure the RTU can take the 480V unless the transformer has taps that you can adjust, since you say it needs 460V.

Also, if the RTU needs a neutral input then it won't work either, because you will have the Delta side as the 480V, and no neutral available.

If the transformer windings are fully rated, i.e. full capacity at all tap positions, and the RTU truly needs 460V, then changing the taps would allow you to get about 38A @ 457V. check transformer nameplate to see ratings at various taps.

 

[kwired]

Another thing to remember and has been hinted at is the delta high voltage side of your transformer will not have a neutral so you will have to ground a phase or use ground detection. Grounding a phase is simplest and least cost.

I will not get into details but you can search for other threads on backfeeding a transformer as there are several there, but you do not want to connect the 120 volt neutral to the low voltage side of your transformer either, leave the X0 terminal "floating".