Transformers 450.3 (B)

[kennethjohnston]

My understanding of the code is that a transformer overcurrant protection is
based on the the kva of the transformer and the primary and secondary protection
is then sized. A electrical engineer said that a 45 kva can be connected to a 100 amp
primary and a 150 amp secondary under 450.3b. When I read this I understand it to say
if the transformer has internal protection from the factory. The engineer says I'm wrong.
can I get another opinion on this. I'm not saying I'm right but I don't think I'm wrong.

 

[augie47]

To assure the correct answer we need the voltages, but assuming 3 phase 480 primary and 208Y/120 secondary, the engineer is correct. Your transformer secondary current would be 125 amps and a 150 amp breaker would be within the 125% allowed. With the secondary protected at 125%, the primary can be protected at 250% so your 100 amp breaker is well within the allowable range.
Note that Art 450 only addresses transformer protection. You need to also check Art 240.21 for conductor protection.

 

[iwire]

I agree with Gus.

 

[templdl]

When I read this I understand it to say
if the transformer has internal protection from the factory. The engineer says I'm wrong.
QUOTE]

Yes, agree with your engineer that it is an incorrect conclusion. But, as I recall there is a provision in art 450 that addresses transformer protection should internal protecting have been provided which is not commonly done that may involve an internal thermal device that is capable of tripping an OCPD. The only other transformer that has an internal thermal breaker are those POCO pole mounted oil filled transformers which is a whole different application.

 

[kennethjohnston]

450.3 (b) note 3

450.3 (b) note 3

I can except the fact that I'm wrong , but if so can anyone explain note 3 to me that makes ref. to internal
protection. That note is next to the 250% number. I'm not saying the engineer is wrong but if I rated a 45 kva 480v
transformer at 250 % I could put a 125 amp breaker on the primary side of it. that to me just sounds wrong

 

[templdl]

I can except the fact that I'm wrong , but if so can anyone explain note 3 to me that makes ref. to internal
protection. That note is next to the 250% number. I'm not saying the engineer is wrong but if I rated a 45 kva 480v
transformer at 250 % I could put a 125 amp breaker on the primary side of it. that to me just sounds wrong

The internal protection in my interpretation would be a thermal device that woulh activate a shunt trip in a breaker which would remove power from the transformer.

 

[don_resqcapt19]

If the transformer has both primary and secondary protection, the primary protective device can have a rating of 250% of the primary current, and it would have a OCPD on the secondary that is sized at no more than 125% of the secondary full load current (this can be rounded to next standard size). If the transformer has internal protection per Note 3, the primary device can still be 250%, but an external secondary device is not needed for the protection of the transformer secondary.

Two notes: 1) the primary device cannot be rounded up to the next standard size, and 2) even where the rules in 450.3 do not require protection for the transformer secondary, the secondary conductors will require protection per 240.21(C).

This is much like a motor where the branch circuit OCPD only provides short circuit and ground fault protection and the motor overload device provides the overload protection for both the motor itself and the motor conductors. The inrush on a transformer often requires an "oversized" primary protective device to prevent tripping on energization, however, unlike motors, where the conductors are sized based on the load and not the OCPD, the primary transformer conductors must have an ampacity equal to or greater than the rating of the primary OCPD.

 

[augie47]

Keep in mind the allowance for 250% protection only applies when you have secondary protection at 125%. In essence the secondary protection is protecting the transformer from overload and the primary is more a fault protection.

 

[kennethjohnston]

Thanks Everone I think that give me a better understanding of the intent of the code