Is the panelboard main breaker considered to be secondary protection for the transformer?

[adionne]

I have spoken with three different PE's on this subject and I have been given three different answers for the scenario below. I am turning to this forum in hopes of getting a clear and precise answer. I have always sized the primary of a transformer feed to 125% in the scenario below but now have a customer challenging the primary sizing of the breaker because of their interpretation of the set up. I am stating to the customer they only have primary protection for the transformer and do not have primary and secondary protection. They are saying they have both because of the breaker feeding the transformer primary and the main breaker of the panelboard which is being fed from the secondary of the transformer so the size the primary protection can be sized up to 250%. I am limiting them to the 125% because I am saying they only have primary protection and not secondary protection because the panelboard main is not considered to be secondary protection for the transformer. They are providing a 150AT breaker on the primary and I am telling them they are limited to a max of 125AT because of the 125% rule based off the NEC 450.3(B) Chart - Maximum Rating or Setting of Overcurrent Protection for Transformers of 1000V and Less for the primary protection using the column for currents 9 amperes and more with primary protection you are only allowed 125% to size the breaker which will not require a separate OCP between the transformer secondary and the panelboard main breaker. Note 1 also allows you to bump your sizing up to the next available breakers size.

We have a 480V MCC with a 150AT breaker (which I say needs to be 125AT) feeding a 75kVA dry-type transformer that is 480V primary with a 120/208V secondary. The secondary of the transformer is feeding a 120/208V panelboard. The panelboard is feeding the utilities (lights, receptacles, space heaters, battery charger, ups input) in the power distribution center we are building.

The customer is stating that the main breaker of the panelboard is the secondary protection for the transformer which is why they say they have primary and secondary protection for the panel which allows them to size the primary up to 250% which would mean the 150AT breaker is fine. I have never considered the main breaker of a panelboard to be secondary protection for the secondary side of a transformer. I have a handbook which shows a picture in 450.5 the overcurrent protection on the secondary side of the transformer external to the panelboard. The purpose of the panelboard main breaker is to protect the panelboard breakers and circuits. It is not protecting the secondary conductors of the transformer. My customer has stated the handbook is not considered to be the code book which I have heard so many times when being challenged on a NEC application. They want me to show them where in the black text of the code book it states the secondary protection of the transformer must be separate from the panelboard where the panelboard main cannot be used or considered to be the secondary protection for the transformer.

 

[augie47]

As for the NEC, as of the 2107, the panelboard breaker CAN serve as the transformer secondary protection.
You would need to meet the requirements of 240.21(C)

 

[ptonsparky]

FWIW, I agree with your customer and it is how I've done them. No inspector issues.

There have been several threads here where the inrush of transformers would trip minimally sized primary protection.

 

[jim dungar]

They want me to show them where in the black text of the code book it states the secondary protection of the transformer must be separate from the panelboard where the panelboard main cannot be used or considered to be the secondary protection for the transformer.

You will not be able to find this in the NEC. It is extremely common to find main breaker panelboards acting as the secondary side protection for transformers, especially when they are next to each other.

 

[mikeames]

IMHO you would have secondary protection if the main braker in the panel was an appropriate size based on the transformer. I would also limit the distance that main breaker is from the transformer almost like a tap but Ill admit I cant find that in the code.

 

[jim dungar]

IMHO you would have secondary protection if the main braker in the panel was an appropriate size based on the transformer. I would also limit the distance that main breaker is from the transformer almost like a tap but Ill admit I cant find that in the code.

The distance limitation is found in 240.21(C) for transformer secondary conductors.

 

[augie47]

I might add, I would consider disconnects ahead of the panels IF you has multiple panels off one transformer so that power could be totally removed from one panel to meet 70E requirements without disrupting other panels..

 

[electrofelon]

I have never considered the main breaker of a panelboard to be secondary protection for the secondary side of a transformer. I have a handbook which shows a picture in 450.5 the overcurrent protection on the secondary side of the transformer external to the panelboard. The purpose of the panelboard main breaker is to protect the panelboard breakers and circuits. It is not protecting the secondary conductors of the transformer.

As others have said, nothing at all wrong with that. I would say that's the way it's done most of the time - you have one device meeting three requirements: 1. 408.36 panel board protection, 2. 240.21(C) conductor protection, and 3. 450.3 transformer secondary protection (if required). Note that if you go with 250% primary protection, you need conductors to match.

 

[don_resqcapt19]

I have rarely seen a separate device to provide transformer secondary protection. It is almost always the main breaker of the panel that is connected to the secondary of the transformer. The only time I have seen separate protection is when the length of conductor would be longer than what is permitted in 240.21(C), but in that case the panel would typically be MLO.

In both case the device on the secondary side is providing the protection for the transformer secondary, the transformer secondary conductors and the panelboard.