Sizing a transformer

[Btwixt]

I’m trying to price a job that has 2 existing 600 amp 120/208 panels. I plan to use one of those panels to step up to 480 on the secondary side. The transformer is going to need to be 225 kva and at least 200 amp. My question is how do I figure out what size ocpd I need on both the primary and secondary side? ( I intend on changing the panels to newer cutler hammer panels)

 

[augie47]

Over-current devices for transformers is covered by Art 450. Commonly, the primary is protected at 125% but you need to review that Section.
Once you select your over-current device, go to Art 240 for conductor sizing and 240.21(C) fro secondary conductor sizing.

 

[Btwixt]

Thanks for that. I did that but I wanted to make sure I wasn’t doing something wrong because it would be a costly mistake if so

 

[roger]

Thanks for that. I did that but I wanted to make sure I wasn’t doing something wrong because it would be a costly mistake if so

So what did you come up with?

Roger

 

[augie47]

I believe once you do the math you wil find a glitch in the plan.

 

[Btwixt]

So the way it seems to me it would be a 250 amp breaker running 250’s on the primary side and 300’s on the secondary side

 

[augie47]

One of us has a major misunderstanding If you are looking at a 225 kva transformer with a 208 primary, the primary overcurrent device is likely to be larger than your 600 amp existing panel.

 

[Btwixt]

Yeah it was me sorry. So I’ll need at 700 amp on the primary side and a 300 on the secondary

 

[augie47]

With the primary protected at 125%, your secondary protection is only limited by the conductor sizes per 240.21(C) but your numbers will likely work,.

 

[Btwixt]

Thanks a lot.