Separately Derived System

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Lady Engineer

Lady Engineer

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New Jersey
Ok, I have a question.

I have a generator which is wired as a separately derived system. The generator is not directly connected to a transfer switch, but is directly connected to the emergency panel via 400A breaker. I have a 4 pole switch, so therefore there needs to be grounding on the generator side. If I ground the system at the generator, I know I need to carry a equipment grounding conductor.

My question, what happens if I decide to ground the system at the emergency panel? Do I still need to carry equipment ground, because we haven't grounded the system until we reach the emergency panel.

What do you think?
 
I'm sure someone with more experience with generators as SDS will have a more intelligent answer, but my thoughts would be since 445.18 requires a disconnect at the generator, that would be your first means of disconnect and a grounding electrode would be there with an equipment grounding conductor required to the premises.
 
Good call, Augie. Now, if the 2 conditions of 445.18 are met, and the disconnect is allowed to be located in the emergency panel, an EGC from the generator to the emergency panel will definitely be needed. Without one, it would be possible that a ground fault in the generator could energize non-current carrying parts of the generator and not trip the OCP in the emergency panel.

One other consideration would be 250.30(A)(1) Excp 2 where the grounded conductor would also serve as the EGC. That's the way I see it.
 
The system bonding jumper must be at the source or first disconnectiong means. If the generator has a circuit breaker then the SBJ would be installed in the generator. And an egc is required from the generator to the e panel
 
Sounds like an oddball setup...

When the switch is open, the generator is an SDS.

When the switch is closed, it is not... if understand the setup correctly.

Assuming the panel is a 3? 4W system, you are not disconnecting the service supplied grounded conductor when you switch over to gennie power. That makes it a non-SDS system when powering the panel, and running a GES for the gennie likely creates a parallel neutral path.
 
Lady,
How do you isolate the generator supply from the normal supply with a single 4 pole switch, or is this switch a double throw switch? However to answer your question an EGC is required no matter where you install the system bonding jumper. If the system bonding jumper is at the generator you need to extend an equipment bonding jumper to the panel to provide a connection point for the panel circuit EGCs. If you put the system bonding jumper at the panel, you need an EGC back to the generator to provide the required grounding of the generator frame and equipment.
 
Lady Engineer said:
I have a generator which is wired as a separately derived system. The generator is not directly connected to a transfer switch, but is directly connected to the emergency panel via 400A breaker. I have a 4 pole switch, so therefore there needs to be grounding on the generator side.
Click to expand...

Lady E
This sounds like the generator is directly connected to an isolated 400 panel.
Is this correct?
 
bob said:
Lady E
This sounds like the generator is directly connected to an isolated 400 panel.
Is this correct?
Click to expand...

Most likely, and the 400a main opens the neutral ( 4 pole switch ) and that makes it a SDS.

The 400a panel then feeds however many ATS's needed. That's my take!!
 
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