Main Breaker required?

[Kohlhase]

Customer has a 120/208 Panelboard 42 circuit Main lug is fed from 75KVA transformer 480V pri 120/208 Sec with 90A breaker at Pri. Article 408.36(B) requires secondary protection. The way it is installed by the previous electrician is a code violation.

My customer is asking what is the safety concerns and I told them the 90A breaker will not trip and you will overload the transformer. Am I off base? are there other problems I should tell him?

Thank you!

 

[raider1]

I agree. The reason for the panelboard overcurrent protective device to be located on the secondary of the transformer is that if we only use the primary overcurrent device then it take a much larger fault on the secondary to clear the primary overcurrent device.

Chris

 

[charlie b]

We have to protect not only the transformer's secondary windings, but also the feeder conductors from the transformer to the panel. So unless these conductors were severely oversized by the former electrician (not likely), then you could overduty them, with the result of a short circuit or a fire, without tripping the primary breaker.

 

[buldogg]

Customer has a 120/208 Panelboard 42 circuit Main lug is fed from 75KVA transformer 480V pri 120/208 Sec with 90A breaker at Pri. Article 408.36(B) requires secondary protection. The way it is installed by the previous electrician is a code violation.

My customer is asking what is the safety concerns and I told them the 90A breaker will not trip and you will overload the transformer. Am I off base? are there other problems I should tell him?

Thank you!

408.36B exception refers to 240.21C1, which refers to 450.3 which states that primary only protection is an option. I'm confused

 

[augie47]

Note that 240.24(C)(1) states the secondary is protected by the primary only on 2 wire and 3 wire delta-delta. You have a 4 wire wye secondary.

 

[don_resqcapt19]

408.36B exception refers to 240.21C1, which refers to 450.3 which states that primary only protection is an option. I'm confused

You have to remember that the OCPD rules in Article 450 apply only to the protection of the transformer itself. 240.21(C) applies to the protection of the transformer secondary conductors. 408.36 applies to the required protection of the panel. As Gus pointed out, 240.21(C)(1) is a special case where the protection on the primary of the transformer is permitted to also protect the secondary of the transformer, the secondary conductors and the panel.

 

[buldogg]

Thanks Don and Gus, I missed that detail.