Primary & Secondary

[Gene Vodovoz]

If there is a 480V primary, with a 100 KVA secondary, which breaker should be used for the primary service ?

 

[don_resqcapt19]

Anything up to 250% of the transformer primary full load amps. However ratings less than 125% may result in breaker tripping on energization from the inrush current.

 

[electrofelon]

See table 450.3(B). For size of primary you have to weigh the increased cost of the device and conductors to go big versus the chance of tripping on energization if you go smaller. Secondary isn't really a concern because you will have a CPD there anyway to comply with 240.21 and possibly 408.36.

 

[augie47]

(assuming 3 phase) Looks like it would be a 150 amp unless the secondary was protected at 125% then the primary could be A 300 amp

 

[Gene Vodovoz]

(assuming 3 phase) Looks like it would be a 150 amp unless the secondary was protected at 125% then the primary could be A 300 amp

thank you