?
Friday, November 24, 2023
Question:
Greetings fellow CQD travelers. It is fairly common practice to see the "round up" rule for OCPD's rated 800A in Section 240.4. However, this rule is not allowed to be used for transformer secondary conductors, according to 240.21(C). No one in our office was aware of this, and this has been in the NEC since 2002!
My question is: Why are transformer secondary conductors (and feeder tap conductors) exempted from the "round up" rule? Why is this exemption in 240.21 (which deals with location) and not even mentioned in 240.4, which is the section specifically for conductor overcurrent protection? Cheers! Ciaran Scott
A
Answer:
Hello Ciaran, thanks for the great questions. When transformer secondary conductor OCPD is required per Section 450.3 (it is not always required), the provisions found in Section 240.4(B) cannot be applied, to ensure that adequate overcurrent protection is provided for those secondary conductors.
The NEC® requires protection of both conductors and transformers. Article 240 provides requirements for protection of conductors, whereas Article 450 provides the requirements for protecting the transformer. There are circumstances where the transformer and conductors one OCPD can protect the transformer and conductors can be protected by the same protective device, if that device can satisfy the requirements of both Articles.