Bonding Jumper on Generator

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Designer371141

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Our lead engineer is directing us to remove the bonding jumper at our generator and install conductors from the center tap of the wye connected generator to the first disconnecting means in a different location, once they are there the bonding jumper is to be installed between the equipment ground and the neutral bus. Article 250.30 (2) indicates that the bonding jumper shall be installed at the source when the source and the disconnect are in different locations. There would be no gain in doing this as he has designed the system, only additional cost because of the fourth current carrying conductor in with the feeders, and de-rating. The conductor size has to also be increased.

Am I understanding Article 250 correctly
 
To decide where the N-G bond goes, I often have to draw myself a 5 line diagram and consider where fault current will flow. Is there ground fault protection (or detection) at the generator? If so, that is most important to know where those CT's are in the diagram as it is important to make sure they "see" current they are supposed to "see".
 
This is a stand-by generator to back up utility power derived from the Utility Switchgear. The question is, in Article 250.35(A) it states that unless the generator and the first switchgear are in the same building then the bonding is to take place where the power is derived from, the generator in this case. So, by removing the bonding jumper at the generator are we not in violation of this section of the code?
 
Designer371141 said:
This is a stand-by generator to back up utility power derived from the Utility Switchgear. The question is, in Article 250.35(A) it states that unless the generator and the first switchgear are in the same building then the bonding is to take place where the power is derived from, the generator in this case. So, by removing the bonding jumper at the generator are we not in violation of this section of the code?
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The code logic is a bit messy to follow but 250.35(A) for separately derived (either the transfer switch is switching the neutral or the neutral doesn't even get extended to the transfer switch) says to do it in accordance with 250.30 which says in 250.30(A) do it at any single point on the separately derived system from the source to the first system disconnecting means or overcurrent device ....
This is a design that is performance based and not prescriptive because it has to be done to make it work depending on the application.
 
The reasoning behind this as stated by the lead engineer. He wants a 3 phase, 5 wire system from the engine generator. To remove the bonding jumper and attach it strictly to the enclosure of the generator and the grounds included in the service entrance conductors. In addition he wants the connect the 5th wire (Neutral) to the center tap of the Wye connection and carry that down to the switchgear which is the first disconnecting means and connect them to the neutral, at that point he wants to bond the neutral bus and the ground bus together to meet the requirements. This not only is an out of the ordinary connection, but it increases the size and quantities of feeders due to the de-rating for the 4th current carrying conductor in the conduit. I do not see why remove the jumper in the generator only to move it to the switchgear and do the same thing as for generating the neutral. The neutral is not being carried to any other equipment, such as transfer switch, or other loads. In essence the neutral is not really necessary but for some odd reason he wants to develop it, I am ok with that, its the method that has me concerned.
 
If the grounded conductor (neutral) is not carried down to the transfer switch, then there is no benefit to do the bond downstream of the source unless there is ground fault detection in a particular place that he is trying to be sure "sees" return current during a fault or normal operation.
That's why I was saying to draw a 5 wire diagram to see if there is a real reason. There is no code reason.
The grounded conductor is not carrying current, so no need to be concerned about derating, but 40% fill may be an issue.
There are many articles about 3 pole versus 4 pole ATS's that address this related concept
@electrical/documents/content/ia08700002e.pdf
 
Do you have a 3- or 4-pole transfer switch? If you have the bond at the generator, then the neutral needs to be switched by the TS (so there's only one bond in the system at a time. If the bond is in the TS, the it's not in the generator.

Also, in a 3-phase 4-wire system, the neutral isn't considered a CCC (with a few exceptions).

As Ron said, draw this out with all five leads and look for where duplicate bonds would be and for the fault current paths, that should clarify things.

(Ron and I posting at the same time :) )
 
In general, I prefer to do what the engineer is asking for because it simplifies protection schemes and transfer switches (if applicable). Its not a code violation assuming it is done correctly.
 
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