MLO Panels

[WastefulMiser]

Could you educate me on the section that allows MLO panelboards: Is it 408.36 Exception No. 2? Or is there something else I should be considering?

Also, for all secondaries of transformers, no matter what the distance, OCP is required at the panelboard, unless it is two wire or delta. Is this correct? 408.36(B)

 

[iwire]

Could you educate me on the section that allows MLO panelboards: Is it 408.36 Exception No. 2? Or is there something else I should be considering?

Panel boards must be protected at their ampacity but that dose not mean it has to be done at the panel.

If you have a 200 amp MLO panel supplied from a 200 amp breaker or fuse you have protected the panel.

 

[don_resqcapt19]

...
Also, for all secondaries of transformers, no matter what the distance, OCP is required at the panelboard, unless it is two wire or delta. Is this correct? 408.36(B)

The OCPD might be required before the panel if the length of the conductors from the transformer to the panel exceed what is permitted in 240.21(C).

 

[charlie b]

Also, for all secondaries of transformers, no matter what the distance, OCP is required at the panelboard, unless it is two wire or delta. Is this correct? 408.36(B)

That is not correct. Not at all. That article says that the protection has to be on the secondary side of the transformer. You could put a fused disconnect or an enclosed breaker right at the transformer secondary terminals, then run a couple miles of wire to an MLO panel. Not a great design, perhaps, but compliant none the less.

 

[WastefulMiser]

That is not correct. Not at all. That article says that the protection has to be on the secondary side of the transformer. You could put a fused disconnect or an enclosed breaker right at the transformer secondary terminals, then run a couple miles of wire to an MLO panel. Not a great design, perhaps, but compliant none the less.

Charlie, just to clarify, you don't mean to be code complaint you have to install an OCD inside the transformer, but it has to be located either 10' or 25' away depending on the setup.

What about an outside transformer according to 240.21(C)(4)? Then the secondary cables can be at whatever length as long as it lands into the OCD inside the panelboard, no?

 

[augie47]

You would need to comply with both Articles. 240.21(C) for protection of the transformer secondary conductors, and then, if you are feeding a panel, 408.36.

 

[kwired]

Transformer secondary conductors are considered taps and tap rules apply which is why you usually need fuse or breaker near the transformer.

The panelboard also has to be protected to its rating. If the OC device at the end of the tap is at or below the panelboard rating you have fulfilled this requirement wether it is a main within the panel or not.

If the panel is located close enough to the transformer to satisify tap rules a main breaker panel is used usually because it will cost less this way and will use less space.