Xfmr primary conductor size

[msteiner]

When sizing transformer primary conductors for low voltage xfmrs, are the cables required to be sized based upon the primary OCPD, or can they be based on the xfmr primary FLA?

 

[don_resqcapt19]

The cables must be protected at or below their ampacity.
Don

 

[augie47]

size

size

expanding slightly on don's answer, you may find that transformer "inrush" current will require you to use an overcurrent device larger than the transformer FLA in which case the OC device would be the prime determing factor.

 

[mayanees]

...except for...

...except for...

NEC learning is a never-ending experience.
We talked about this one at work today, and I don't have my code book with me, but there is a tap rule exception that allows transformer primary conductors to be sized lower than the ampacity of the primary OCPD. It's the 25 feet total length of primary and secondary conductors, maybe 240.21(b)(2).
There are a number of stipulations that must be met, but it allows inrush through the breaker without having to upsize the cable.
JM

 

[don_resqcapt19]

JM,
That would be 240.21(B)(3).

(3) Taps Supplying a Transformer [Primary Plus Secondary Not Over 7.5 m (25 ft) Long] Where the tap conductors supply a transformer and comply with all the following conditions:
(1) The conductors supplying the primary of a transformer have an ampacity at least one-third the rating of the overcurrent device protecting the feeder conductors.
(2) The conductors supplied by the secondary of the transformer shall have an ampacity that is not less than the value of the primary-to-secondary voltage ratio multiplied by one-third of the rating of the overcurrent device protecting the feeder conductors.
(3) The total length of one primary plus one secondary conductor, excluding any portion of the primary conductor that is protected at its ampacity, is not over 7.5 m (25 ft).
(4) The primary and secondary conductors are protected from physical damage by being enclosed in an approved raceway or by other approved means.
(5) The secondary conductors terminate in a single circuit breaker or set of fuses that limit the load current to not more than the conductor ampacity that is permitted by 310.15.