2014 NEC handbook commentary 240.21(C)(2)

[robeward]

The blue text commentary for this section on page 156 contains an example for calculating the minimum secondary conductor size. In the first step they calculate 1/10th of the primary OCPD rating. Why do they do this? I am missing something here.

i can type out more the example if i need too. Thanks.

 

[iwire]

The blue text commentary for this section on page 156 contains an example for calculating the minimum secondary conductor size. In the first step they calculate 1/10th of the primary OCPD rating. Why do they do this? I am missing something here.

i can type out more the example if i need too. Thanks.

Check out 240.21(C)(2)(4)

(4) For field installations where the secondary conductors
leave the enclosure or vault in which the supply connection
is made, the rating of the overcurrent device
protecting the primary of the transformer, multiplied by
the primary to secondary transformer voltage ratio,
shall not exceed 10 times the ampacity of the secondary
conductor.

 

[robeward]

dang it. thanks. i glossed over that like 5 times because it doesn't apply to my situation. just to make sure i understand part (4), can you provide an example of when an conductor would leave the place that it terminates? Would this be if a main was double tapped in a panel to feed another panel?