Separately derived system 4 pole transfer switch question

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chiefkona1145

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I have a question regarding a new generator being added to an existing building distribution system. The new generator is at the exterior of the building and the feeds from it go underground into the building to a new 4 pole automatic transfer switch; the normal side of the transfer switch is from a transformer connected downstream from the building service. Does this generator need a #3/0 (or #4/0) ground conductor that is bonded to the transfer switch and transformer? The feeds from the generator are parallel (4) sets of 4-1/C 500 KCMIL and each conduit includes a #3/0 ground conductor.

Any help with this including a code reference would be greatly appreciated.

Thanks
 
If the generator's neutral and ground are not interconnected within the generator, you can use a solid neutral and only need a 3-pole transfer switch.
 
Does this generator need a #3/0 (or #4/0) ground conductor that is bonded to the transfer switch and transformer? The feeds from the generator are parallel (4) sets of 4-1/C 500 KCMIL and each conduit includes a #3/0 ground conductor
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If you are switching the neutral it will have to be grounded at the generator. There is no need for another grounded conductor as it would be in parallel with the neutral.

Why switch the neutral? If you do not have a specific reason, use a 3 pole switch. You would need to remove the generators neutral to frame bonding jumper. You would ground the generator using the existing electrical system EGC.
The system neutral would be connected to the generator neutral terminal.
 
bob said:
If you are switching the neutral it will have to be grounded at the generator. There is no need for another grounded conductor as it would be in parallel with the neutral.
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Bob, If you are switching the neutral in the xfer switch and the equipment bonding jumper is made at the generator, then you will need to bring a system bonding jumper along with the grounded conductor.The switched neutral in the xfer switch is isolated.The system bonding jumper is based on table 250.66,so a 3/0 cu will be required.
Rick
 
The 4 pole transfer switch was ordered and is in shipment to the site along with the generator so I don't think using a 3 pole transfer switch is an option. So, the question still exists; do we need to install a separate #3/0 ground conductor to bond the generator, transfer switch and transformer? And if it is required I need to see a code reference that would require it.

Thanks for the input so far - very much appreciated.
 
chiefkona1145,
If you are switching the neutral, then the generator is a SDS and will have to have a system bonding jumper. All of the equipment grounding conductors for the loads will have to connect to the service main bonding jumper for normal conditions and to the generator system bonding jumper for when the loads are served by the generator. Either way you need EGCs to the generator.
Don
 
Bob, If you are switching the neutral in the xfer switch and the equipment bonding jumper is made at the generator, then you will need to bring a system bonding jumper along with the grounded conductor
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RU
Thanks for catching my mistake. With the neutral being switched you are
correct.
 
Here are your code references
Art 100 Definition of Separately Derived Systemss
Art 100 Bonding Jumper, System
250.20 (D) Separately Derived Systems and both FPN's

The key is to determine if the generator is a separately derived system of not. For a building this is determined by the transfer switch. Don't transfer the neutral unless you absolutely have to, for example if you have GFP this will require a 4 pole TS
 
The Main Difference between ?separately derived system? ( and the ?not separately derived system? is that the neutral wire, system grounded circuit conductor (circuit meaning it completes the electrical loop and thus a current carrying conductor-the neutral), is transferred by the fourth pole. While in the ?not separately derived system?, the neutral is fix connected.

The four-pole transferring advantages:
a) All current carrying conductors opens simultaneously.
b) The ground-fault sensing if both the normal and emergency circuit is provided, will operate correctly because each sensing system will independently monitor the residual current in the neutral wire that it is connected to.
c) Because the neutral bonding is separated both for the emergency and normal power, a cable failure on the normal side will not provide a floating neutral situation.
d) Another advantage is, if there is a fault in one of the source, it will not transfer that condition to the other source.

The 4-pole switch also has its drawbacks, including:
a) A potential for abnormal surges due to momentary open-transition switching of all contacts;
b) A potential for abnormal surges due to momentary open-transition switching of all contacts, as well as neutral contact erosion; and
c) Dangerous voltage transients may occur.
 
I see no advantages to transferring the neutral.
95% of transfer switches don't transfer the neutral.
When the neutral is transferred its done on purpose
 
Thank you all for your input and clearing this up for me. The code references are vague on this subject but I will have the contractor install a #3/0 bonding conductor between the equipment and this should solve the issue.

Thanks again and I hope you all have safe and happy day,

chiefkona1145
 
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