One generator feeding two automatic transfer switches

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
If you are going to replace the transfer switches, please check to see if you can use an ATS that also switches the neutral. That will avoid the problem of the generator connection creating an undesirable parallel neutral path.
 

mbrooke

Batteries Not Included
Location
United States
Occupation
*
I'd need a wiring diagram or instruction sheet. If the GEN is telling the ATS what to do you could end up having a number of issues.
 

Chamuit

Senior Member
Location
Texas
I don't think your arrangement will work correctly if one ATS loses power and the other doesn't.

The Generac controller looks to N1 and N2 for utility voltage. If you connect only one ATS to the yellow conductors (N1/N2), then it completely controls both ATS's in essence. So, unit 1 (ATS) could not lose power and not start the generator for unit 2 (ATS) which lost power, for whatever the reason may be. It would be an odd scenario.

You would have to run the voltage sensing conductors in series through the N1 and N2 fuse blocks so that if either one opened (lost power) the controller would see it. Then it would switch both ATS's via 23/194 (red and black wires).

Just thinking through the circuits.....
 

hillbilly1

Senior Member
Location
Atlanta,Ga
I don't think your arrangement will work correctly if one ATS loses power and the other doesn't.

The Generac controller looks to N1 and N2 for utility voltage. If you connect only one ATS to the yellow conductors (N1/N2), then it completely controls both ATS's in essence. So, unit 1 (ATS) could not lose power and not start the generator for unit 2 (ATS) which lost power, for whatever the reason may be. It would be an odd scenario.

You would have to run the voltage sensing conductors in series through the N1 and N2 fuse blocks so that if either one opened (lost power) the controller would see it. Then it would switch both ATS's via 23/194 (red and black wires).

Just thinking through the circuits.....
The normal situation where one may lose power, and not the other, is if the service breaker on one of the services trips. If the one that trips is the master, the generator will start and transfer both services even though utility is available at the other. If the fault is still there, and the generator breaker trips, both will remain at source two and without power to either service, unless the generator shuts down on a fault. Normal transferswitchs would go back to source one if source two, because the decision is made by the transferswitch not the generator. But with Generac, the generator makes the decision to transfer (at least they did previously) If the other service trips, generator will not start and transfer, but at least it will not transfer into a fault if it’s still there.
 

Chamuit

Senior Member
Location
Texas
The normal situation where one may lose power, and not the other, is if the service breaker on one of the services trips. If the one that trips is the master, the generator will start and transfer both services even though the utility is available at the other.
Correct.

If the fault is still there, and the generator breaker trips, both will remain at source two and without power to either service, unless the generator shuts down on a fault.
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I think you mean if there is a fault at the ATS or downstream? I agree.

Normal transfer switches would go back to source one if source two, because the decision is made by the transfer switch, not the generator. But with Generac, the generator makes the decision to transfer (at least they did previously) If the other service trips, the generator will not start and transfer, but at least it will not transfer into a fault if it’s still there.
Agreed. The controller on the Generac generator closes the transfer relay to change from normal to emergency
 

McLintock

Senior Member
Location
USA
Occupation
Electrician
It really should be that you ATS is your first disconnect. Then you most likely wouldn’t have that problem as it looks like it’s the same service feeders feeding both meters


“ shoot low boys their riding shetland ponies”
 

hillbilly1

Senior Member
Location
Atlanta,Ga
It really should be that you ATS is your first disconnect. Then you most likely wouldn’t have that problem as it looks like it’s the same service feeders feeding both meters


“ shoot low boys their riding shetland ponies”
Looks like that’s what he is already doing, I’m assuming service rated switches. Still same issue, the service disconnect is ahead of the ATS, which if the load faults, trips the breaker in the ATS, starting and transferring just the same. If the main was past the transferswitch, the ATS would not see the power loss. Since this type of ATS is automatic, and there is no “Off” position like a manual switch has, the ATS itself cannot be the service disconnect.
 

kwired

Electron manager
Location
NE Nebraska
Did you put two breakers in the generator? If not it is the main for both when running on generator.
Does it matter outside, of load calculations and assuring there is enough capacity to power any connected loads?

If generator is running and generator breaker(s) is closed there is power at the standby side of both transfer switches, no load is supplied until either/both transfer switch actually transfers, so that transfer signal, whatever logic it follows, is what is most critical
 

hillbilly1

Senior Member
Location
Atlanta,Ga
Does it matter outside, of load calculations and assuring there is enough capacity to power any connected loads?

If generator is running and generator breaker(s) is closed there is power at the standby side of both transfer switches, no load is supplied until either/both transfer switch actually transfers, so that transfer signal, whatever logic it follows, is what is most critical
No, it doesn’t matter, actually having one is better as far as loading. Full potential of the generator can be used because loads may vary greatly between the two services. Just making the point he has one main when running on generator.
 

jaylectricity

Senior Member
Location
Massachusetts
So it works. I sent the 240 volt to the generator. I paralleled the communication and 12v to each ATS. Inspector was fine with it. The only problem will be if only one unit goes out. But the only reason that would happen is non-payment. It's a 2-family, owner lives in one and rents out the other. So I made sure it was the owner's ATS that controlled the generator.

So if the owner doesn't pay the bill, they can run their generator. If the tenant doesn't pay their bill, they'll be without electricity.
 

mbrooke

Batteries Not Included
Location
United States
Occupation
*
I don't think so. The only thing the generator provides is 12v to operate the controls on the ATS when the ATS loses utility power. But I think the ATS senses when it has reliable 240 volts from the generator before switching over.
Alright, I trust your expertise.
 

jaylectricity

Senior Member
Location
Massachusetts
Does it matter outside, of load calculations and assuring there is enough capacity to power any connected loads?

If generator is running and generator breaker(s) is closed there is power at the standby side of both transfer switches, no load is supplied until either/both transfer switch actually transfers, so that transfer signal, whatever logic it follows, is what is most critical
Sorry I didn't respond at the time, but no, it doesn't matter. The generator can supply a certain amount of power. The wires I ran can handle that power. The generator has a circuit breaker to protect the wire from delivering more than it is capable.

Yes, if the generator is not enough, the breaker will trip and the residents will have to reassess what they are able to do during a power outage. But at the very least they can have lights, internet and heat.
 

jaylectricity

Senior Member
Location
Massachusetts
Alright, I trust your expertise.
Oh, I see what you were responding to. Maybe the communication wire didn't need to be paralleled since the (let's call it) master ATS would be doing all the communicating.

My expertise is limited because I don't know if that communication wire is a 2 way wire. I suppose it isn't.
 
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