Transformer Sizing Requirement

[bwat]

Is it true that there isn’t a direct explicit NEC requirement for sizing of transformers?



It is my understanding that there isn’t a direct sizing requirement, but you’ll get there indirectly based on ocpd requirements for the load and transformer protection. Load calcs will raise the ocpd to at least be X (a min), and the transformer ocpd requirements based on transformer size will set a cap that the ocpd can only be as high as Y (a max), so therefor the transformer has to be sized to be at least be big enough such that X is not greater than Y.



Am I correct?

 

[augie47]

Is it true that there isn’t a direct explicit NEC requirement for sizing of transformers?

Am I correct?

That's basically a true statement.
As to the 2nd part, the NEC in Art 450, addresses overload protection of transformers and Art 240 addresses protection of conductors.
The transformer should be sized to carry the load taking into account the nature of the load (continuous, non-continuous, motor, etc).
Then the overcurrent selected to carry the load and protecthe transformer and then the conductors sized based on the overcurrent (and load)

 

[electrofelon]

That's basically a true statement.

I agree. I think the most you can undersize a transformer (talking under 1000v here) is going with the load being 125% of nameplate for the 450.3 plus whatever next size up is. Note that where secondary protection consists of multiple OCPD's, unlike a service, their sum can't exceed the transformer rating so there is no loophole there.

 

[bwat]

Thanks for the good responses.

And while I agree with this...

The transformer should be sized to carry the load taking into account the nature of the load (continuous, non-continuous, motor, etc).

... from a "should" perspective regarding design, part of my reason for asking the question is that the requirement to do this isn't actually stated in NEC from what I can tell. There isn't a comparable 215.2 for feeders that applies to transformers, or somewhere that says the 240 requirements for conductor protection applies to transformer windings based on nameplate (please correct if I'm wrong).


As an example that's keeping everything under 1000V. With the next size up rule, I could have a theoretical transformer with a 101A secondary rating feeding a non-continuous load of 150A with the only secondary protection being a 150A breaker and it all is NEC compliant.

101 x 125% = 126.25... next size up is 150A.


The reason I asked is because we had a real world example not too long ago with different numbers but I made the statement that although something like this example could be a poor design and potentially dangerous regarding the transformer, there's nothing in NEC to prohibit it. There's no direct correlation between load and transformer size per NEC requirements. You'll get some type of limiting factor in a roundabout way through the ocpd, but it could end up leaving the transformer rather stressed at times. I was looked at like I had multiple heads when I said this and wanted to confirm.

Thanks again.

 

[electrofelon]

Thanks for the good responses.

And while I agree with this...

... from a "should" perspective regarding design, part of my reason for asking the question is that the requirement to do this isn't actually stated in NEC from what I can tell. There isn't a comparable 215.2 for feeders that applies to transformers, or somewhere that says the 240 requirements for conductor protection applies to transformer windings based on nameplate (please correct if I'm wrong).


As an example that's keeping everything under 1000V. With the next size up rule, I could have a theoretical transformer with a 101A secondary rating feeding a non-continuous load of 150A with the only secondary protection being a 150A breaker and it all is NEC compliant.

101 x 125% = 126.25... next size up is 150A.


The reason I asked is because we had a real world example not too long ago with different numbers but I made the statement that although something like this example could be a poor design and potentially dangerous regarding the transformer, there's nothing in NEC to prohibit it. There's no direct correlation between load and transformer size per NEC requirements. You'll get some type of limiting factor in a roundabout way through the ocpd, but it could end up leaving the transformer rather stressed at times. I was looked at like I had multiple heads when I said this and wanted to confirm.

Thanks again.

There is also ye' old 110.3(B) which would be a reasonable citation. Then the question though is how do you compute the load? From article 220? Nameplate/Actual?