Transformer Secondary Conductors

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bnewport

Member
Location
Indiana
I am working on a customer designed installation. The proposed installation is as follows: A 700A breaker is feeding a 500 kVA transformer - 480 primary - 208/120 secondary. The secondary will feed (2) 1200A main lug only panels which are installed right beside the transformer (less than 10' conductor length) Are the secondary conductors required to terminate in an OCPD and what code section(s) apply?

Is the primary/secondary ratio the proper way to size the secondary feeders? (480/208=2.31) (700A primary X 2.31 = 1617A secondary feeder)
 

kingpb

Senior Member
Location
SE USA as far as you can go
Occupation
Engineer, Registered
500KA/(208V x 1.732) = 1,388A

The cable will need to carry no less than 1.25 x 1388A = 1,735A but shall be capable of the same ampacity as the protection device. Refer to Article 240.4.

IMO: I would install sceondary protection regardless of what minimum may be allowed by NFPA 70.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
As long as the calculated load for each of the panels is 1200 amps or less, you are premitted to run cables rated at a minimum of 1200 amps to each of the MLO panels and be in compliance with the NEC. 240.21(C)(2)
Don
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
The 700 amp primary potection is less than 125% of the transformer primary rated current so the transformer itself does not require overcurrent protection. That brings us to 240.21(C) for the protection of the transformer secondary conductors.
Don
 
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