Transformer OCPD Design

[Le7316]

The secondary conductors still need to be protected per article 240, so you can't completely eliminate the OCP on the secondary, if that is what you are asking.

Isn't that the Table 450.3(B) allows us to do that " Primary only protection" ?

 

[steve66]

Isn't that the Table 450.3(B) allows us to do that " Primary only protection" ?

450.3(B) covers protection for the transformer, but you still need to protect the transformer secondary conductors per 240.21(C).

Assuming you are still talking about a 3 phase transformer (like the original post), you still need secondary protection for the conductors.

 

[Le7316]

450.3(B) covers protection for the transformer, but you still need to protect the transformer secondary conductors per 240.21(C).

Assuming you are still talking about a 3 phase transformer (like the original post), you still need secondary protection for the conductors.

Yes i am and to satisfy the 240.21(C) and 450.3(B) , shouldn't the primary OCP be 225A ( 250%) and secondary OCP be 250A (125%) ??

 

[jim dungar]

Yes i am and to satisfy the 240.21(C) and 450.3(B) , shouldn't the primary OCP be 225A ( 250%) and secondary OCP be 250A (125%) ??

Secondary protection is optional per 450.3(B). There is no requirement to upsize the primary based on a secondary protective device.

 

[kwired]

Isn't that the Table 450.3(B) allows us to do that " Primary only protection" ?

take a look at the primary protection requirements in the cases where no secondary protection is required.

A typical general power distribution transformer will be in the 9 amps or more category and will require 125% OCPD on the primary. This will be sufficient to also provide protection to the secondary in 2 wire:2 wire transformers or for three wire delta:three wire delta transformers, as in those situations you can't really overload the secondary without also overloading the primary and the primary protection device will catch that.