Transformer Secondary Multiple OCPDs

[ffmatt12]

Hello all, I have been reading this forum for a while, but this is my first time posting! I am working on selecting the appropriate electrical equipment & panels for a project I am working on and had a question about transformer secondary OCPD. All of this equipment will be in the same room (inside of a shipping container).

Main Panel
-------------------
800A 480V Main Breaker fed by POCO
-400A breaker feeding 300 kva transformer
-300A breaker feeding 200 kva transformer
-300A breaker feeding second 200 kva transformer


300kva 416/240 transformer
-------------------
Option 1: Feed main lug panel with (4) 150A breakers (125% of the transformer secondary is only 520A so not sure if this is code compliant)?
Option 2: Feed 500A main breaker panel with (4) 150A breakers
Option 3: Feed (4) 150A fused disconnects directly from the transformer secondary instead of having a panel. (Is this code compliant?)
The 150A disconnects/breakers feed PDUs for computer equipment.

200kva 416/240 transformer (typical for both)
-------------------
Option 1: Feed main lug panel with (2) 150A breakers (I think this is code compliant because 125% of the transformer secondary is 347A)
Option 2: Feed 250A or 300A main breaker panel with (2) 150A breakers
Option 3: Feed (2) 150A fused disconnects directly from the transformer secondary instead of having a panel. (Is this code compliant?)
The 150A disconnects/breakers feed PDUs for computer equipment.


I was wondering if y'all might know if the above options are NEC compliant when it comes to secondary protection of the transformer. Thanks for your input or suggestions!

Matt

 

[augie47]

For the most part your option 1s might not be complaint based on the 240.21(C) tap rules (depends on which rule) PLUS 408.36 requires panels to have a main.
You can supply either MB panels or fused disconnects as long as you comply with the 240.21(C) rules.
I didn't spend a lot of time on the actual numbers but it appears your primary protection meets the 125% rule which should make life easier.

 

[don_resqcapt19]

Hello all, I have been reading this forum for a while, but this is my first time posting! I am working on selecting the appropriate electrical equipment & panels for a project I am working on and had a question about transformer secondary OCPD. All of this equipment will be in the same room (inside of a shipping container).

Main Panel
-------------------
800A 480V Main Breaker fed by POCO
-400A breaker feeding 300 kva transformer
-300A breaker feeding 200 kva transformer
-300A breaker feeding second 200 kva transformer


300kva 416/240 transformer
-------------------
Option 1: Feed main lug panel with (4) 150A breakers (125% of the transformer secondary is only 520A so not sure if this is code compliant)?
Option 2: Feed 500A main breaker panel with (4) 150A breakers
Option 3: Feed (4) 150A fused disconnects directly from the transformer secondary instead of having a panel. (Is this code compliant?)
The 150A disconnects/breakers feed PDUs for computer equipment.

200kva 416/240 transformer (typical for both)
-------------------
Option 1: Feed main lug panel with (2) 150A breakers (I think this is code compliant because 125% of the transformer secondary is 347A)
Option 2: Feed 250A or 300A main breaker panel with (2) 150A breakers
Option 3: Feed (2) 150A fused disconnects directly from the transformer secondary instead of having a panel. (Is this code compliant?)
The 150A disconnects/breakers feed PDUs for computer equipment.


I was wondering if y'all might know if the above options are NEC compliant when it comes to secondary protection of the transformer. Thanks for your input or suggestions!

Matt

I don't understand how you are feeding transformers with a 416 volt primary from a 480 volt service.

The primary breaker for the 300kVA is less than 125% of full load current and may trip on inrush when you energize the transformer.

 

[ffmatt12]

I don't understand how you are feeding transformers with a 416 volt primary from a 480 volt service.

Sorry, I should have said that the transformer primary is 480V Delta and the secondary is a 416Y (416/240)

The primary breaker for the 300kVA is less than 125% of full load current and may trip on inrush when you energize the transformer.

Thanks for this bit, I will look at that, The equipment which the transformer powers would be off when the transformer is powered on and would be gradually turned on after the transformer is energized.

 

[ffmatt12]

For the most part your option 1s might not be complaint based on the 240.21(C) tap rules (depends on which rule) PLUS 408.36 requires panels to have a main.
You can supply either MB panels or fused disconnects as long as you comply with the 240.21(C) rules.
I didn't spend a lot of time on the actual numbers but it appears your primary protection meets the 125% rule which should make life easier.

Thanks for the reply. I'm thinking of going with fused disconnects instead of main panels now since they are more economical, I think with the 200kva transformers I would be fine since the total of the OCPDs is 300A which is less than 125% of the transformer secondary rating (347A) The only think I am not sure about now is with the 300kva transformer...If I use (4) 150A fused disconnects connected to lugs on the transformer secondary, the sum of the OCPDs is 600A which is more than 125% of the transformer secondary rating of 520A. Would I be limited to (4) 125A fused disconnects in this scenario? I found this thread which made me question this https://forums.mikeholt.com/threads/transformer-double-tap-at-secondary.137285/

 

[augie47]

Art 450(3)(B) notes that if you have primary protection at 125% you need not worry about the size of secondary protection as far as the transformer is involved.

 

[petersonra]

Art 450(3)(B) notes that if you have primary protection at 125% you need not worry about the size of secondary protection as far as the transformer is involved.

I'd be afraid of The primary protection tripping at that rating. But probably with a transformer of that size it wouldn't. I mostly deal with relatively small transformers so my experience is make the primary overcurrent protection device as big as possible.

 

[don_resqcapt19]

Sorry, I should have said that the transformer primary is 480V Delta and the secondary is a 416Y (416/240)


Thanks for this bit, I will look at that, The equipment which the transformer powers would be off when the transformer is powered on and would be gradually turned on after the transformer is energized.

Some sources indicate that the worst case for transformer inrush is where there is no load on the secondary when the transformer primary is energized.