Bonding Jumper

[Brandon Loyd]

I have an MCC section with a xfmr and panel inside. The xfmr has a bonding jumper/grouding electrode attached to the MCC ground bus, which has a ufer or building steel ground itself. The panel above that the xfmr feeds has a bonding jumper from its ground bus bar to its neutral bus bar. The panel ground bus also has a ground to the MCC ground bus. I have heard it is ok to install your bonding jumper either in the xfmr or in the first panel. But is it ok to have both? Is the MCC's ground bus a suitable grounding electrode for the neutral return path? Or should it always have its own seperate path?

 

[ericsherman37]

Kinda confused here but...

Your transformer secondary is a separately derived system. 250.30(A)(1) says something along the lines of, "System bonding jumper connection shall be made at at any single point between the separately derived source and the first disconnecting means or overcurrent device."

If you have a bonding jumper at your transformer already, then the one at the panel isn't necessary as long as you can hook it up to the EGC or whatever.

Exception #2 to the above reference says "A system bonding jumper at both the source and the first disconnecting means shall be permitted if doing so does not establish a parallel path for the grounded conductor." If all your stuff is in one big MCC enclosure then it might be okay.

I don't have much experience with that stuff yet though, so let one of the wiser sages here correct me if I'm wrong

 

[ericsherman37]

Is the MCC's ground bus a suitable grounding electrode for the neutral return path?

Just noticed this part. Neutral return path shouldn't have anything to do with your grounding electrodes. Neutral return path just wants to get back to its source - the transformer. Sounds like the transformer secondary neutral is bonded to the MCC ground bus which is connected to the grounding electrode system. Okay... that's good. The panel's ground bus connecting to the MCC ground bus is good too... that's where fault current goes if a fault condition occurs (where it hops onto the main bonding jumper back to the source). If the panel's neutral bus has a direct connection to the MCC ground bus, then that seems kind of weird to me... it'd be an additional fault current path. But if it's all in one enclosure and was installed that way by the factory, then so be it I guess. Again, I don't have much experience in this area yet.

 

[masterinbama]

I have installed a lot of 480 volt MCC's over the years that are factory assembled just as you describe this one.

 

[don_resqcapt19]

The bonding at both the transformer and the panel is a code violation.

 

[masterinbama]

The bonding at both the transformer and the panel is a code violation.

I agree with you Don. My point is that all of the MCC's I have installed with a transformer/ panel section came from the factory wired this way.

 

[pete m.]

I have an MCC section with a xfmr and panel inside. The xfmr has a bonding jumper/grouding electrode attached to the MCC ground bus, which has a ufer or building steel ground itself.

The NEC recognizes utilizing the grounding electrode brought to the MCC as long as it complies with exception number 2 to (1) and (2) found in 250.30(A)(4).

As far as having a system bonding jumper at the transformer and at the panel I agree with Don. This is a violation of 250.30(A)(1)

Food for thought... just because a piece of equipment is listed by an NRTL that doesn't mean it "trumps" the NEC.

Pete