sizing breakers for a single phase 15kva transfomrer

[rlorenz2]

I am looking for clarification on how to size the primary and secondary protection for a single phase 15KVA 480-240/120 V transformer.

Is this correct?

Primary:
15k/480*1.25=39.06A So next standard size is 50A Primary Breaker

Secondary
15K/120*1.25=125A So next standard size is 175A Secondary Breaker

 

[augie47]

Not exactly:
Take at look at 450.3(B),
If you are protecting the transformer by the primary OCP, your 39.06 is correct, however the next larger breaker is a 40 amp
If you are protecting the transformer by the secondary OCP and you are using the 240 secondary then the figure would be 15000/240 = 62.5 x 1.25= 78 amps and a 80 amp breaker would be used.
(In the event you are relying on secondary protection, your primary OCP could be 250% or 70 amp.)

Also keep in mind those numbers are for transformer protection. You need to reference Art 240 for conductor protection.

 

[rlorenz2]

Not exactly:
Take at look at 450.3(B),
If you are protecting the transformer by the primary OCP, your 39.06 is correct, however the next larger breaker is a 40 amp
If you are protecting the transformer by the secondary OCP and you are using the 240 secondary then the figure would be 15000/240 = 62.5 x 1.25= 78 amps and a 80 amp breaker would be used.
(In the event you are relying on secondary protection, your primary OCP could be 250% or 70 amp.)

Also keep in mind those numbers are for transformer protection. You need to reference Art 240 for conductor protection.

Thank you for you timely response.

I am a bit confused on your primary sizing when relying on a secondary

15k/480*2.5= 78amps would not the next size be 80amps for the primary?

 

[petersonra]

Thank you for you timely response.

I am a bit confused on your primary sizing when relying on a secondary

15k/480*2.5= 78amps would not the next size be 80amps for the primary?

note 1 in the table does not apply for 250%, so 78A is the max CB you can use. I don't think there is typically a 75A Cb so you have to use a 70A CB.

 

[rlorenz2]

note 1 in the table does not apply for 250%, so 78A is the max CB you can use. I don't think there is typically a 75A Cb so you have to use a 70A CB.

That makes sense thank you for you help