240.4(F) Question

[augie47]

240.4(F) provides that a delta/delta transformer need no have secondary conductor protection provided (among other things) that the secondary conductors are protected by the primary overcurrent with the voltage ration applied. Does this apply to the total current on parallel conductors ?
In particular, given a 14.4 kv/480 transformer with (11) 750 kcmil per phase (5225 total ampacity). Would primary overcurrent of 175 amps be considered as adequate protection. (30:1 ratio) ?

 

[skeshesh]

I'm not sure what the ratio has to do with protection. Take a look at 30.4/Exception No. 2 (c). Although this exception is talking about conductors in sizes smaller than 1/0AWG, it might not be a bad consideration for overcurrent protection anyway. Other than that I can't think of anything else that would be a problem.

 

[SEO]

Gus it looks like your math is correct and you comply with section 240.4 (F). It would have to apply to parallel conductors as well as single conductors. Parallel conductors electrically joined at both ends per 310.4 IMO are one conductor.

 

[augie47]

Thanks, Steve
I guess this is a good place for Cable Limiters

 

[pfalcon]

I'm not sure what the ratio has to do with protection. Take a look at 30.4/Exception No. 2 (c). Although this exception is talking about conductors in sizes smaller than 1/0AWG, it might not be a bad consideration for overcurrent protection anyway. Other than that I can't think of anything else that would be a problem.

Per 240.4(F) last sentence.
(Primary Protection) >= (Secondary Conductor Ampacity) / (Turns Ratio)
175A >= (5225A) / (30 turns) = 174.2A

 

[skeshesh]

Per 240.4(F) last sentence.
(Primary Protection) >= (Secondary Conductor Ampacity) / (Turns Ratio)
175A >= (5225A) / (30 turns) = 174.2A

I stand corrected.