Transformer OCPD question

[MarkieD]

Looking for help on Breaker size. I have a 45KVA 480 to 380 auto transformer that's feeding a single piece of equipment with a 29A load. I've done the calculations based on article 450.3B and 240.6A and the numbers just seem crazy to me. The 45kva transformer's rating is 54A and my load 29A. Can I just protect the primary side with a 60A breaker and feed it with #6's and protect the secondary side with a 40A breaker and feed it with #8's? Do I even need secondary protection? I believe I do based on what I read. I came up with 135A max protection on the primary side and 85A on the secondary. I believe those are maximums so not sure if my thought process is the way to approach this. Any thoughts and help would be greatly appreciated. I apologize if this is all over the map.

Thanks Mark

 

[jim dungar]

Generally for transformers:
1. Choose and locate an OCPD to protect your load.
2. Size and protect your secondary conductors based on the rules in 240.21(C). This OCPD is allowed to be the same device chosen in step 1.
3. Size your primary OCPD based on 450.3. If you decide to exceed 125% of the transformer rating, you may need to resize the secondary OCPD and your conductors.
4. Size the primary side conductors, based on the OCPD chosen in step 3.

 

[MarkieD]

Thanks Mark

 

[augie47]

One word of caution:
When selecting your secondary overcurrent device and conductors note that 240.21(C) does not allow the "next size up" (240.21) rule to be applied. Your secondary conductor ampacity must be equal or greater than the protective device.

 

[wwhitney]

When selecting your secondary overcurrent device and conductors note that 240.21(C) does not allow the "next size up" (240.21) rule to be applied. Your secondary conductor ampacity must be equal or greater than the protective device.

Are we aware of any good technical reason for this limitation? 230.90(A), which is a similar situation for services, does allow the use of 240.4(B).

Cheers, Wayne