Dual service entry transfer switches fed from the same meter and generator: Neutral routing

[TJNII]

I'm working on a service upgrade where we're upgrading to two 200A services from a single 320A meter. As part of this install we're installing two service entry rated transfer switches between the meter and the panels to provide generator power during outages. The transfer switches are double pole units that break the hots only, the neutral connection is just a bus. Using the service entry rated switches I don't believe it's possible to directly connect both switches to the generator without a code violation on the neutral. I'm working with an electrician, but he doesn't have an answer to how this issue should be resolved. I also called Generac tech support who was also unable to advise. I'd like a verification from the experts here that my understanding is correct before I engage in expensive or unusual solutions.

• Grounding is left out of scope of this discussion as the neutral alone has code compliance issues, before adding the complexity of bonding.
• Control wiring is out of scope.

Imagine the equipment laid out as so:

Panel 1 <-------- T1 ------- M --------< Utility
                   |         |
                   |         |
                   |         |
Generator >------ GJ ------- T2 --------> Panel 2

M: Meter
T1 / T2: Service entry rated transfer switch
GJ: Generator junction box, to split the generator feed between T1 and T2

• The service disconnects are in T1 and T2, as they are service entry units.
• There are two utility hots and a neutral between M and T1, as well as two utility hots and a neutral between M and T2. The neutrals are connected in M.
• There are two hots and a single neutral from the generator to GJ.

So the quick and obvious solution for the generator side is to split the feed in GJ, running two hots and a neutral from GJ to T1, and two hots and a neutral from GJ to T2. This will require the neutrals to be connected in GJ. However, this will cause what I'm calling a "ring neutral", which will then cause two current paths from T1 or T2 back to the meter or the generator. For example, when on utility power neutral current from T1 can flow to the meter direct via the T1-M connection, or follow the T1-GJ-T2-M connection. I believe this is a definite 300.20 violation, and likely other sections as well.

An alternate idea is to only connect the neutral to T1, running the hots through the GJ-T2 conduit still, and use the neutral through the meter to carry generator current when on generator. Utility power should be fine, and the T1 generator feed will be fine, but the T2 generator feed will not have grouped conductors which is also a 300.20 violation.

A third idea would be to not use the GJ-T2 conduit, only connect the generator neutral to T1, and have the T2 generator conductors parallel the utility conductors through the meter. As the T1-M and T2-M paths are service conductors this would be a 230.7 violation, even if the utility didn't balk.

Converting T2 to switch the neutral as well as the hots would resolve the issue. I called the distributor and there aren't service entry units that switch the neutral. The switches advertised as being able to be parelled in this fashion also don't break the neutral, so I think three must be another option.

I believe the root of my issue is that these are service entry rated switches. If the service disconnects were not in the transfer switches, but in a single enclosure with the ground bond, I believe it would be possible to route the feeds such that these issues go away. Is that what's being done on other multiple ATS installs? My electrician says it's possible to do a multi-ATS setup with service disconnects in the switches, but when I asked how the neutrals are routed he didn't have an answer.

Is there a solution I'm overlooking using these double pole service entry rated units in the grid configuration of my original drawing?

 

[TJNII]

I got hold of a subject matter expert this morning and asked him how he'd do it with two service entrance switches, and his answer was "I wouldn't". He'd do a 400A transfer switch. So I think what I should have done was either:

- Use a 400A service entry switch
- Don't use service entry rated transfer switches, and locate the service disconnects in a enclosure large enough to house the generator junctions.

As for the "why didn't you hire a subject matter expert from the beginning" ... couldn't find one. The SME I called this morning is a 4 hour drive away. Anyway, I think this can be closed, the config in my original post is not the right way to do it. The reason I can't find a easy solution is because there isn't one.

 

[LarryFine]

I'd stick with that you have, having done it plenty myself.

The parallel neutral is unavoidable and is not detrimental.

My first choice is one panel on the generator and one not.

It's simpler, cheaper, easier, and no load-shedding worries.

 

[VirutalElectrician]

Interesting subject. I was just about to post a similar question as I have a similar situation.

I have a 400amp service with a disconnect. This splits into 2x200 amp ATS and then into 2x200 distribution panels.

There is a neutral and ground from each ATS to the disconnect, which has the neutral and grounds bonded. The neutral/ground bonding jumper has been removed from both ATSes.

My question is...Since there is the possibiliity that the meter/disconnect could be disconnected for service, the generator would kick in and supply power. If that were the case, there would be no neutral running between the two ATSs, as the generator neutral just goes to one ATS.

That also means there would be no neutral/ground bond anywhere as well during this situation.

So my question is...Should I bond the neutral between the two ATSes?

This of course would mean that there still would not be a neutral/ground bond anywhere if the meter/disconnect was down for service.

I know this would be a very rare situation, but should be accounted for anyways.

I suppose one could not bother bonding the neutrals, as if this situation where to occur, you could temporarily bond the neutrals, as you'd also have to temporarily put the neutral/ground jumper back in as well.

I believe I read that Generac supports connecting up to 4 ATSes together at one time, if you use a some small gizmo they make handle the 12vdc distribution. They do support upto 2 ATSes just buy splitting the 3 transfer lines and running to each ATS.

I have seen many videos of two Generac ATSes wired from a split 400A service, so this configuration is not uncommon. You'd think Generac would have some notes on the subject.

 

[tortuga]

In my area generally speaking if the transfer switch is a also a service disconnect the install Larry described is allowed, presumable since there is already parallel paths on the MGN system, the concern shown in this 1-line diagram is if you lost a neutral between the 320A meter and one of the 200A panels. Even if there was no generator the GEC would be a parallel path so I think that is why it is allowed.

 

[tortuga]

Interesting subject. I was just about to post a similar question as I have a similar situation.

I have a 400amp service with a disconnect. This splits into 2x200 amp ATS and then into 2x200 distribution panels.

If there is more than one feeder supplied by a common generator then I'd probably use an ATS that switches the neutrals;