Transformer primary & secondary OCPD

[cppoly]

If you decide to use both, there’s nothing wrong with putting 125% on both right? Not sure what the purpose would be to put 250% on the primary and 125% on the secondary, just seems more intuitive to put 125% on both.

 

[david luchini]

If you decide to use both, there’s nothing wrong with putting 125% on both right? Not sure what the purpose would be to put 250% on the primary and 125% on the secondary, just seems more intuitive to put 125% on both.

No problem with that.

 

[electrofelon]

If you decide to use both, there’s nothing wrong with putting 125% on both right? Not sure what the purpose would be to put 250% on the primary and 125% on the secondary, just seems more intuitive to put 125% on both.

some people like the higher OCPD setting on the primary to account for the inrush current when energizing the transformer. The disadvantage is you then need larger conductors. I dont recall ever having an issue with 125%.

 

[cppoly]

Thanks, are you required to size secondary conductors and OCPD to the LOAD or the full rating of the transformer?

 

[david luchini]

Thanks, are you required to size secondary conductors and OCPD to the LOAD or the full rating of the transformer?

The load.

 

[electrofelon]

Thanks, are you required to size secondary conductors and OCPD to the LOAD or the full rating of the transformer?

See 240.21(C). You have to coordinate the 450.3 requirements with the 240.21(C) requirements. Also dont forget about 408.36. Typically 1 OCPD meets all three requirements.

 

[ron]

If you decide to use both, there’s nothing wrong with putting 125% on both right? Not sure what the purpose would be to put 250% on the primary and 125% on the secondary, just seems more intuitive to put 125% on both.

In a critical application, I've found it too risky with newer DOE2016 efficient transformer inrush to be as low as 125% on the primary. I usually go to 150% minimum, unless the transformer and breakers come from the same source they they pick it. Schneider tests their combinations, so those are fine.