Transformer Secondary Protection

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GarrettGalyen

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I came up with an issue with an inspector. The Inspector is stuck on NEC 240.4(F) Which states that transformers that are not delta-delta or single phase are NOT considered to be protected by primary OCP.

But, According to NEC 240.24 {C}2 Primary OCP is permitted. And it doesn't matter what type of transformer it is.

This is also reinforced by 450.3 and table 450.3(B).

I have a 480V board with a MCB that is feeding a transformer (delta-wye) via ocp. The transformer is feeding a 208V board within 10' routing distance.

Does there need to be OCP on the secondary side?
 
GarrettGalyen said:
I came up with an issue with an inspector. The Inspector is stuck on NEC 240.4(F) Which states that transformers that are not delta-delta or single phase are NOT considered to be protected by primary OCP.

But, According to NEC 240.24 {C}2 Primary OCP is permitted. And it doesn't matter what type of transformer it is.

This is also reinforced by 450.3 and table 450.3(B).

I have a 480V board with a MCB that is feeding a transformer (delta-wye) via ocp. The transformer is feeding a 208V board within 10' routing distance.

Does there need to be OCP on the secondary side?
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Can you describe what type of loads will be fed out of the LV Panel?

JAP>
 
Please remember that there are five things that need overcurrent protection:

  1. The primary conductors that bring power to the transformer.
  2. The primary windings of the transformer.
  3. The secondary windings of the transformer.
  4. The secondary conductors from the transformer to the downstream panel.
  5. The downstream panel itself.

The primary side feeder breaker can handle items 1 and 2. It might also be able to handle item 4, depending on the type of transformer and other considerations. That is where 240.21(C)(1) comes into play. Most of the time, a main breaker in the downstream panel will handle items 4 and 5. It can also handle item 3, in those instances where table 450.3(B) requires protection of the secondary windings.
 
GarrettGalyen said:
But is the following not true?
In a typical electrical room where a 480V panel is serving a 208V panel via transformer within 10' secondary protection is not required.
Click to expand...

You can make an argument that as long as the ampere rating of the device(s) on the secondary side do nor exceed the ampacity of the conductor,you would be in compliance with 240.21(C)(2) but, the argument is moot since the FPN in that section refers you to 408.36 which requires a main in panels other than service panels meeting the 6 disconnect rule.
 
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