Seperately Derived System for Temp Gen with ATS


Senior Member
I'm looking at an application that will have a 480V Delta/Wye transformer feeding the normal side of an ATS with the emer side of the ATS being a 480V temp roll-up generator. The ATS is actually a breaker based MTS with (2) breakers arranged on a common bus in a Switchboard. Emergency side of switchboard will have provision for cam-lock connection for future roll-up gen. The system downstream of ATS will only be (3) phase loads and thus no L-N loads (No neutral switching)

Given this, with the requirements of 250.30 which of the following to I need to adhere to for the requirements of 250.30?

1) Generator is not a separately derived system. Transformer neutral and generator neutral are brought to neutral bus in ATS (Switchboard). Grounds (EGC's) are brought from transformer and generator to ground bus in ATS. A single system bonding jumper between ground and neutral is provided at either the transformer or at the ATS Switchboard (between neutral and ground) but not at the generator.


2) Given that there are not neutral loads served is there a requirement to bring the neutrals from transformer and generator to ATS and provide a single system bonding jumper. Could both neutrals not be pulled and only pull the grounds (EGC's) from both transformer and generator to ATS and install a system bonding jumper between ground and neutral at BOTH the transformer and generator? Unless I'm missing something here the fault current flow seems to work here with no unintended consequences? To me it just seems strange to have to bring neutrals from transformer and generator if they will not be used by loads.



Senior Member
The transformer is a separately derived system. If the neutral of the generator is connected to the neutral of the transformer and is not switched in the transfer between the power sources then it is not creating another separately derived system. The type of loads does not matter.
If you don't pull and connect the neutrals together, other than any connection created through the grounding system, then you have two separately derived systems.