# Primary and secondary overcurrent protection (transformers)



## Jimmybo (Jan 14, 2011)

I have a 400 amp main disconnect feeding a 277/480v 400amp main breaker panel. I need to install a 75 kva transformer and step down to 208. Install a 208v panel and feed four packing units each 30amp 120/208 3phase, each unit has a 2.41hp motor. The panel will also feed fifteen 20amp circuits for receptacles. What size breaker do I Need to Feed my primary side of the transformer? What size breaker do I need on the secondary side of the transformer? Can I use a 125amp breaker for my primary, and a 200amp breaker for my secondary? Any feedback would be appreciated.


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## BBQ (Nov 16, 2010)

125 on the primary is good, 200 on the secondary is OK, I would likely use a 225 amp main breaker panel on the secondary side, that is what we do here.


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## Jimmybo (Jan 14, 2011)

BBQ said:


> 125 on the primary is good, 200 on the secondary is OK, I would likely use a 225 amp main breaker panel on the secondary side, that is what we do here.


I read article 450 in the nec and it said over current device shall be rated at not more than 125% What would constitute me to go the full 125%


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## don_resqcapt19 (Jul 18, 2010)

Jimmybo said:


> I read article 450 in the nec and it said over current device shall be rated at not more than 125% What would constitute me to go the full 125%


If you have a 125 amp breaker on the primary, you do not need secondary protection for the transformer itself. You do need to comply with the rules in 240.21(C) for the protection of the secondary conductors. If you have a 125 on the primary, you can use the 225 amp breaker at the panel as long as the secondary conductors have an ampacity of at least 225 amps and they are 25' or less in length.


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## Magnettica (Jan 23, 2007)

Jimmybo said:


> I read article 450 in the nec and it said over current device shall be rated at not more than 125% What would constitute me to go the full 125%


Current.


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## brian john (Mar 11, 2007)

Read this.

http://electricalknowledge.com/forum/topic.asp?TOPIC_ID=1371

Then read this

http://electricalknowledge.com/forum/topic.asp?TOPIC_ID=1487


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## Dean S (Dec 19, 2011)

I have a question regarding the secondary conductors of a transformer! The transformer is located in a shed and will feed a disconnect on a wall of another building outside it looks like 240-21(5) will allow me to do this exceeding the 25ft
Am I reading this correctly


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## don_resqcapt19 (Jul 18, 2010)

Dean S said:


> I have a question regarding the secondary conductors of a transformer! The transformer is located in a shed and will feed a disconnect on a wall of another building outside it looks like 240-21(5) will allow me to do this exceeding the 25ft
> Am I reading this correctly


You are looking at 240.21(B)(5), but you need to be looking at 240.21(C)(4), but it really doesn't matter as you have the supply end of the conductors in a building and those two rules only permit the load end to be in a building.


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