Bonding Jumper, Isolation Xformer for an Industrial Control Panel

[richwaskowitz]

I want to make sure I am applying article 250 correctly here. I have an industrial control panel fed from a 200 amp 3 phase straight 480 v bus drop. Inside that panel I have a branch circuit supplying the primary of a drive isolation transformer mounted to the floor adjacent to this panel. The 480Y/277 (solidly grounded) volt 3 phase secondary feeds back into the same panel and is then distributed to various VFDs and other motor circuits. As there are no 277 volt loads, no current carrying conductor is brought back to the panel with the phase conductors.

Am I correct in understanding 250.30 (A) in that I should take a system bonding jumper (identified with the color green), sized according to 250.28(D) from the X0 terminal in the transformer, run it along with the secondary phase conductors and connect it to the main ground bus in the control panel?

If this is correct, can I then bond the transformer enclosure to that X0 lug also?

 

[richwaskowitz]

Add Sketch

Add Sketch

I am adding a sketch in order to try to clarify my question. Is the scheme correct as pictured? Does conductor #1 get sized according to 250.28(D)? What about conductor #2? What are the proper terms for these two conductors?

 

[raider1]

Conductor #2 is the system bonding jumper and would need to be sized in accordance with 250.30(A)(1)

Conductor #1 would be a equipment bonding jumper and be sized in accordance with 250.30(A)(2)

Chris

 

[richwaskowitz]

Mr. Raider1

Mr. Raider1

So are you saying that my scheme is OK then? Specifically, that the equipment bonding jumper (#1) is connected through the ground bus in the industrial control panel which in turn is connected to the ground system by means of the bus and is not directly connected to a grounding electrode of its own.

 

[augie47]

IMHO, As part of a "industrial control panel", the grounding should be determined as part of the listing and labeling of the panel and determined by the applicable panel Code, not by the NEC.
If one were to use NEC specs for the control panel then they would be sized per Chris' post. If the transformer is >1000va, then I would think there would need to be a connection to the appropriate grounding electrode per 250.30(A)(3)

 

[richwaskowitz]

Not part of ICP

Not part of ICP

An externally mounted transformer and the wiring between it and the panel is not part of an industrial control panel ans is therefore not addressed in UL508A. It is field wiring and subject to NFPA 70, the NEC.

 

[augie47]

An externally mounted transformer and the wiring between it and the panel is not part of an industrial control panel ans is therefore not addressed in UL508A. It is field wiring and subject to NFPA 70, the NEC.

I'll take your word for that even though I have installed many and industrial control panel that came with the transformer "removed" and pigtails in place where it was once part of the panel. If you are determimned it falls under NFPA70 the I think 250.30(A)(3) should be followed also.

 

[raider1]

So are you saying that my scheme is OK then? Specifically, that the equipment bonding jumper (#1) is connected through the ground bus in the industrial control panel which in turn is connected to the ground system by means of the bus and is not directly connected to a grounding electrode of its own.

You still need to have a grounding electrode conductor connected to the secondary of the transformer. Take a look at 250.30(A)(3)

Chris