Generator as a separately derived system

[alixenos]

Hi,

When I spec a generator, and in almost all the projects I worked on, my supervisor indicates that the generator is not considered as a separately derived system.

My question is, what drives this decision? when do consider a generator as a separately derived system?

Thanks for the help.

 

[don_resqcapt19]

Actually it is the transfer switch that makes the determination. If the transfer switch switches the neutral, it is a separately derived system.
Typically not switching the neutral results in a cheaper transfer switch and that is what you see most times.

 

[dkidd]

From NEC 2014

100 I.

Separately Derived System. An electrical source, other
than a service, having no direct connection(s) to circuit
conductors of any other electrical source other than those
established by grounding and bonding connections.


250.30

Informational Note No. 1: An alternate ac power source,
such as an on-site generator, is not a separately derived
system if the grounded conductor is solidly interconnected
to a service-supplied system grounded conductor. An example
of such a situation is where alternate source transfer
equipment does not include a switching action in the grounded
conductor and allows it to remain solidly connected to the
service-supplied grounded conductor when the alternate
source is operational and supplying the load served.

250.30 (A)

(A) Grounded Systems. A separately derived ac system
that is grounded shall comply with 250.30(A)(1) through
(A)(8). Except as otherwise permitted in this article, a
grounded conductor shall not be connected to normally
non–current-carrying metal parts of equipment, be connected
to equipment grounding conductors, or be reconnected
to ground on the load side of the system bonding
jumper.



If the neutral of a generator is solidly connected to an electrical system,
it is not a separately derived system.

 

[LarryFine]

When I spec a generator, and in almost all the projects I worked on, my supervisor indicates that the generator is not considered as a separately derived system.

My question is, what drives this decision? when do consider a generator as a separately derived system?

As stated, it's a non-SDS when the utility, generator, and load neutrals are solidly tied together.

That also makes grounding simpler, because the usual service electrode system is appropriate.

If it's an SDS, each source must have electrodes, and the load neutral floats as with a sub-panel.

 

[Seven-Delta-FortyOne]

At least on residential and light commercial, there is really no good reason to install them as an SDS.

It merely adds a layer of complications that’s completely unnecessary.

I’ve heard that in some of the more complex installations, such as in hospitals, or having multiple generators, SDS have their advantages. But I don’t have experience with those.

 

[wwhitney]

At least on residential and light commercial, there is really no good reason to install them as an SDS.

If you have two or more ATSs connected to two or more service disconnects, then that would be a good reason.

Cheers, Wayne

 

[Seven-Delta-FortyOne]

If you have two or more ATSs connected to two or more service disconnects, then that would be a good reason.

Cheers, Wayne

I would have to draw this out.

I know it was discussed here pretty recently about the possibility of multiple neutral current return paths with 2 service disconnects and one generator.

 

[ron]

As mentioned, it has to do with the ATS selection and whether the neutral is switched (if the neutral is needed for the loads at all, if not then it could be a 3 pole switch). That would drive whether you bonded the N & G at the gen.

The deciding factor for me, whether I specify SDSs, is if there is the need for accurate ground fault detection. When there is no bond at the gen (non-SDS), the ground fault path is squirrely and GFP may mis-operate.

 

[Strathead]

I don't know the nuance of it, but I asked my Engineer for a 7 story hospital project so I think he has good credentials. What I was told is exactly what Ron said. The only reason an SDS generator is needed is when the system has ground fault protection.