240.21(C)and 3 Phase Delta-Wye Transformers

[Alan Fraser]

240.21(C)(1) specifically states that "...multiphase (other than delta-delta, 3-wire) transformer secondary conductors are not considered to be protected by the primary overcurrent protective device." I believe this also applies to C(2) through C(6), if not, what am I missing?

 

[kwired]

240.21(C)(1) specifically states that "...multiphase (other than delta-delta, 3-wire) transformer secondary conductors are not considered to be protected by the primary overcurrent protective device." I believe this also applies to C(2) through C(6), if not, what am I missing?

Part 1 is for situations where the conductors are considered protected by the transformer primary side overcurrent protection device. If you have 20 amp primary @ 480 volts protection, and a single two wire or a delta to delta three wire secondary @ 240 volts then the 20 amp primary protection is effective as 40 amps protection for the secondary because of primary to secondary ratio.

For the other parts we are talking about tap conductors - the primary overcurrent device does not necessarily limit the secondary current to the ampacity of the conductors or even to the potential current that could be taken from only a portion of the transformer windings.

Say we have a 480 single phase input with 20 amp primary overcurrent protection and 120/240 secondary output. You could load the secondary to 80 amps on just one 120 volt half of the secondary winding - which is double the (assumed from the 20 amp primary OCPD)) rated current of this portion of the windings, yet the primary overcurrent device is still only seeing 20 amps and does not operate.

 

[raider1]

Alan you are correct.

240.21(C)(2) through (6) permits transformer secondary conductors to be protected by an overcurrent device located at the load end of the conductors when installed in accordance with those sections.

Chris