Step-up transformer sizing guide

[owlit]

Hello,
I am looking for NEC or other generally accepted code that outlines the process of sizing step-up transformer capacity for a VFD chiller.
One electrical engineer has told me that I need to use MCA of the equipment, but I can't find any codes that state this.
Is anyone aware of such a code or any reliable source in writing that either states that MCA should/shouldn't be used to size step-up transformers?

I apologize in advance if this question has been asked previously or that it's common sense...
Any help is appreciated.

Thank you.

 

[Jraef]

Hello,
I am looking for NEC or other generally accepted code that outlines the process of sizing step-up transformer capacity for a VFD chiller.
One electrical engineer has told me that I need to use MCA of the equipment, but I can't find any codes that state this.
Is anyone aware of such a code or any reliable source in writing that either states that MCA should/shouldn't be used to size step-up transformers?

I apologize in advance if this question has been asked previously or that it's common sense...
Any help is appreciated.

Thank you.

It seems as though you are looking to the NEC for design guidance, which is not what it is for. The NEC is a set of minimum and maximum allowances, based on the concept (originally) of not starting fires. Remember, WE call it the NEC, but officially it is called NFPA 70, National Fire Prevention Assoc standard #70 for Electrical equipment installations.

So the NEC is not going to tall you how to size a transformer, that's the job of the engineer or master electrician. The NEC is going to tell you how to connect the transformer, size the conductors, fusing etc., once YOU have sized it for whatever you had in mind. The closest the NEC will come to telling you how big it must be is in Article 100.3, stating that the equipment must be suitable for the intended use. In other words if you have a machine needing 100A and you chose to use a transformer capable of only supplying 10A, that transformer is not suitable. But if you were to put in a transformer capable of 1,000A, that's fine per the NEC, as long as you properly select the cables, protective devices, clearance distances, etc. etc.

So back to your problem, I think your EE is giving you good advice. MCA = Minimum Circuit Ampacity. Why would you want to size it for less than that? Now, there are reasons to NOT over size it by too much as well, but technically, you could. If it were me, I would look carefully at the application. Constant Torque rated drives are going to be able to deliver 150% of rated output current for at least 1 minute, which from a transformer sizing standpoint might as well be forever. So if your machine is going to need that capability, and some types of chillers do, then you should size that transformer accordingly.

 

[Julius Right]

See:
http://forums.mikeholt.com/showthread.php?t=161109

 

[owlit]

Thank you so much for your response!
Makes sense now.