Neutrals from 2 Separately-derived systems to one transfer switch

[skadee]

Where does NEC dictate whether the neutrals from two separately-derived systems can be connected? We have a 3-P manual transfer switch, fed by 120/208 1-ph, feeding a 120/208 panel. Our design engineer is specifying that the neutrals from the two two source panels are tied together at the neutral bus of the panel fed by the transfer switch.

Logically, I see that there's a potential to energize the neutral bar of one of the source panels with some un-balanced load, when that panel is not supplying the load, but I can't find any articles that nail down the direction to take in this case. The generator section heads down that path when mentioning whether the neutral is bonded to the generator, but this is not a case where we are using a gen set.

Our two sources are typically online 24/7, the transfer switch is primarily a switch that allows us to selectively choose the source feeding critical loads, when one source may be out of service for maintenance.

 

[Smart $]

If the systems are separately derived, then you have to switch the neutrals. Otherwise, you have to make one separately derived and the other not.

Are there [fused switch or breaker] disconnecting means ahead of the transfer switch?

Where are the systems bonded to ground?

 

[skadee]

No OC protection on the neutral, but there's breakers on the un-grounded conductors. I believe the rule that would prohibit this connection of neutrals is the fact that we'd essentially be adding a second system (main) bonding jumper after the first disconnecting means, which isn't permitted. 250.30(A)

Would you agree?

 

[Smart $]

No OC protection on the neutral, but there's breakers on the un-grounded conductors. I believe the rule that would prohibit this connection of neutrals is the fact that we'd essentially be adding a second system (main) bonding jumper after the first disconnecting means, which isn't permitted. 250.30(A)

Would you agree?

It's not explicit, but it provides a good basis, since you are required to have only one system bonding jumper and no other neutral to grounding system connection. Each individual system as described would have two SBJ's.

Another aspect is parallel conductors. Dual SBJ's constitutes a parallel pathway for neutral current, and the conductors do not meet the definition of parallel conductors. And an item of note is the parallel pathway puts neutral current on grounding conductors, which IMO qualifies as objectionable current under 250.6.

One way in which this can be handled without switching the neutral from each system is to make the transfer switch the disconnecting means. The transfer switch would have to be 3-position with a center OFF, and if not fused, a common OCPD would have to be installed immediately adjacent thereto. And of course, a single SBJ would be installed in the transfer switch enclosure.

 

[skadee]

It's not explicit, but it provides a good basis, since you are required to have only one system bonding jumper and no other neutral to grounding system connection. Each individual system as described would have two SBJ's.

Another aspect is parallel conductors. Dual SBJ's constitutes a parallel pathway for neutral current, and the conductors do not meet the definition of parallel conductors. And an item of note is the parallel pathway puts neutral current on grounding conductors, which IMO qualifies as objectionable current under 250.6.

One way in which this can be handled without switching the neutral from each system is to make the transfer switch the disconnecting means. The transfer switch would have to be 3-position with a center OFF, and if not fused, a common OCPD would have to be installed immediately adjacent thereto. And of course, a single SBJ would be installed in the transfer switch enclosure.

Excellent point regarding the parallel paths, thank you very much for the help!

Dan