Yet another transformer secondary OCPD question

[sparky 134]

The installation is as follows: We have a 480v 3phase 3wire 75Kva transformer feeding a 240v 3phase 3wire panel. The transformer will be located with 25' of the panel. The primary OCPD is 100amps. The panel has a 200amp main breaker. This is also in an industrial setting.

Does the OCPD on the primary side satisfy the requirements for protection of the transformer secondary's ? If not, does the Main Breaker in the panel satisfy the protection requirements for the secondary conductors ?

I reviewed the EC&M article (Feb. 08) in which Mike Holt explained when secondary OCPD's are required but this sentence from the article puzzles me:

"You can install secondary conductors up to 25 feet long, without overcurrent protection at the point they receive their supply, if they (Fig. 3):
Have an ampacity that is at least the value of the primary-to-secondary voltage ratio multiplied by one-third of the rating of the OCPD that protects the primary of the transformer.

I'm not understanding "Having an ampacity that is at least the value of the primary-to-second voltage ratio multiplied the one-third of the rating of the OCPD that protects the primary of the transformer."

In this case the voltage ratio would be 2:1 (480 to 240) and one-third of the rating of the OCPD that protects the primary of the transformer would be 33.3amps.

We also have another installation coming up which is similar expect the transformer will be a 480v 3phase primary to 120/240v 3phase 4wire secondary. The primary OCPD will be 100amps and the panel will have a 200amp main breaker. The secondary conductors will not measure more than 25'. Are we safe with this scenario or do we need a fusible disconnect located immediately after the secondary conductors leave the transformer ?

Thanks for the help.

 

[don_resqcapt19]

First, assuming a delta to delta transformer, and a secondary conductor with a minimum ampacity of 200 amps, there is no need for any type of OCPD on the secondary side of the transformer. The transformer secondary, the secondary conductors and the panel are all protected by the primay OCPD in this application. See 240.21(C)(1).
In the second application the secondary conductors cannot be protected by the primary because you are not using a delta to delta transformer. You will have to apply the "tap" rule in 240.21(C)(6), assuming that you don't qualify to use 240.21(C)(3). In this case as long as the ampacity of secondary conductors is at least 66.7 amps and is protectect at its load end at or below its ampacity you are in compliance with the code.
Note, the 100 amp primary breaker is very close to the full load current of the transformer and you could have inrush tripping problems.

 

[coulter]

... This is also in an industrial setting. ...

...We also have another installation coming up which is similar expect the transformer will be a 480v 3phase primary to 120/240v 3phase 4wire secondary. ... ... Are we safe with this scenario or do we need a fusible disconnect located immediately after the secondary conductors leave the transformer ? ...

Possibly you qualify under 240 (Part VIII), Supervised Industrial Installations and OCP on the secondary conductors is not required. See (2005) 240.92.D

carl