Transformer OCPD Design

steve66

Senior Member
Location
Illinois
Occupation
Engineer
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

Member
Location
USA
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

Moderator
Staff member
Location
Wisconsin
Occupation
Retired Electrical Engineer - Power Systems
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.
 
Last edited:

kwired

Electron manager
Location
NE Nebraska
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.
 
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