Generator as a separately derived system

alixenos

alixenos

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Location
Florida
Occupation
Electrical Design Engineer
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.
 
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.
 
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.
 
alixenos said:
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?
Click to expand...
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.
 
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 said:
If you have two or more ATSs connected to two or more service disconnects, then that would be a good reason.

Cheers, Wayne
Click to expand...

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.
 
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.
 
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.
 
Sorry to bump an old thread but I was wondering if a bonded neutral generator is always treated as a separately derived system regardless of whether a neutral switching transfer switch is used or not.
 
analog8484 said:
Sorry to bump an old thread but I was wondering if a bonded neutral generator is always treated as a separately derived system regardless of whether a neutral switching transfer switch is used or not.
Click to expand...
If your generator has a bonded neutral and can't be changed, then you need to use a transfer switch that switches the neutral, so that you have an SDS. If your transfer switch has a solid neutral, then your system would end up with two simultaneous N-G bonds, one at the generator and one at the service or primary source, which is not allowed.

Cheers, Wayne
 
analog8484 said:
Sorry to bump an old thread but I was wondering if a bonded neutral generator is always treated as a separately derived system regardless of whether a neutral switching transfer switch is used or not.
Click to expand...
It is the switching (or presence of a Neutral downstream) of the neutral downstream that makes it a SDS. If it is bonded upstream, then either leave the neutral upstream (3 Phase loads only) or switch the neutral to avoid multiple downstream N-G bonds
 
Thanks for the responses. I understand using a neutral switching transfer switch definitely makes a neutral bonded generator a SDS. However, to avoid the double N-G bonds issues, I am being told by a company that I should use a non-neutral switching transfer switch with the neutral bonded generator but don't connect the ground to the transfer switch. It doesn't sound quite right so I am doing a sanity check.
 
I’ve always found this to be an odd situation because in a normal dwelling service we are required to have neutral bonded to ground at no more than one point inside of the house, yet it is bonded to ground at many points outside including the meter enclosure, the utility’s transformer, and every other neighboring dwelling that is fed from that transformer. When a metallic service entrance raceway is used between the meter enclosure and the main service panel, that raceway is a parallel path and carrying neutral current. Yet when it comes to a generator we want to change it up.
 
alixenos said:
My question is, what drives this decision? when do consider a generator as a separately derived system?
Click to expand...
We tycpially see the system being an SDS when there is GFPE protection on one or more of the service switches. In those cases the ATS(s) is a 4-pole.
 
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JohnHess said:
I’ve always found this to be an odd situation because in a normal dwelling service we are required to have neutral bonded to ground at no more than one point inside of the house, yet it is bonded to ground at many points outside including the meter enclosure, the utility’s transformer, and every other neighboring dwelling that is fed from that transformer. When a metallic service entrance raceway is used between the meter enclosure and the main service panel, that raceway is a parallel path and carrying neutral current. Yet when it comes to a generator we want to change it up.
Click to expand...
It's not a bond between the neutral and an EGC as it would be after the service; it's more that the neutral IS the EGC in that the service enclosure(s), raceway(s), etc., are considered to be part of the neutral conductor itself.

As earth is not considered to be a conductor, a neutral being earthed in multiple places does not create an objectional parallel pathway.
 
analog8484 said:
Thanks for the responses. I understand using a neutral switching transfer switch definitely makes a neutral bonded generator a SDS. However, to avoid the double N-G bonds issues, I am being told by a company that I should use a non-neutral switching transfer switch with the neutral bonded generator but don't connect the ground to the transfer switch. It doesn't sound quite right so I am doing a sanity check.
Click to expand...
You wouldn't bond neutral to ground in the transfer switch, or land a GEC in the transfer switch, which are things you might do with a non-SDS transfer switch that was also the service disconnect. You *should* run the EGC to the transfer switch and bond it like any other equipment.
 
LarryFine said:
It's not a bond between the neutral and an EGC as it would be after the service; it's more that the neutral IS the EGC in that the service enclosure(s), raceway(s), etc., are considered to be part of the neutral conductor itself.
Click to expand...
That might be a technical way to describe it, but the physical way would be much the same in both situations, no? Electrically, what’s the difference between the metallic raceway between a meter enclosure and a main service panel, and the EGC running back to a portable generator that has the neutral bonded to ground?

I’m not advocating to do it the wrong way. Just saying that this is one of those things that never computed for me.

LarryFine said:
As earth is not considered to be a conductor, a neutral being earthed in multiple places does not create an objectional parallel pathway.
Click to expand...

It’s not just the Earth. Often times these houses are connected with metallic water pipe. That’s definitely a parallel path, we know that when we find that the utility neutral opened and someone working on the water main got shocked when disconnecting it. I commonly work in connected townhouses that they have continuity through their aluminum siding. Etc.
 
Last edited:
jaggedben said:
You wouldn't bond neutral to ground in the transfer switch, or land a GEC in the transfer switch, which are things you might do with a non-SDS transfer switch that was also the service disconnect. You *should* run the EGC to the transfer switch and bond it like any other equipment.
Click to expand...
Thanks for the input. However, I would like to clarify that there is no neutral-ground bond in the transfer switch. Perhaps I didn't describe my situation correctly. Let me try with a picture:

1755705977996.png

As the picture shows, it's a violation to have the neutral-ground/case bond in the generator. So, to avoid code violation, can the EGC between the Generator and the Disconnect be disconnected?

I believe you are saying the EGC *should* be connected suggesting it's not required by code?
 
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