One generator feeding two automatic transfer switches

jaylectricity

Senior Member
Location
Massachusetts
2-family building. One generator. Needs ATS for each family after the POCO meters.

Generac uses six communication wires. 2 wires to send 240V to generator for sensing purposes. 1 wire to charge battery. 3 wires at 12v.

I believe the 3 wires of 12v are for the generator to supply a relay on the transfer switch in between the time the main power goes out and the transfer switch changes over to generator power. And a wire to tell the transfer switch to change over

So I get the impression that I can send the 240 and battery from just one ATS. Then I can parallel from the generator to the transfer switches.

If you are versed in generac, I did 2 yellow and 1 blue from a single ATS to generator. Then I did red, white, and black from generator to both transfer switches.

Will this work? They didn't blow up when I restored utility power after installation. Haven't tested the generator yet. Any other thoughts?
 

McLintock

Senior Member
Location
USA
Occupation
Electrician
Should work just fine, we do this a lot with one to house and one to garage. Your ATS is basically a disconnect that will transfer from one power source to another power source. As long as one of the ATS is communicating with the gen. It will work


“ shoot low boys their riding shetland ponies”
 

jaylectricity

Senior Member
Location
Massachusetts
Should work just fine, we do this a lot with one to house and one to garage. Your ATS is basically a disconnect that will transfer from one power source to another power source. As long as one of the ATS is communicating with the gen. It will work


“ shoot low boys their riding shetland ponies”
For sure one ATS is communicating with the gen...but is the gen supposed to send a signal to the ATS to let it know, "Hey, we've started generatuing power, it's ok to switch over" or can the transfer switch know that when the utility goes down it will know the generator is running and switch over. that's why I paralleled the 3 wires I *think* deal with that.
 

McLintock

Senior Member
Location
USA
Occupation
Electrician
We have done it both ways, it works just fine, but yes it’s nice to parallel them to be sure


“ shoot low boys their riding shetland ponies”
 

McLintock

Senior Member
Location
USA
Occupation
Electrician
The way I understand is the communication wires only tell the generator to start and the switch will sense the power from the generator and switch over, really no communication from generator to ATS.


“ shoot low boys their riding shetland ponies”
 

Another C10

Electrical Contractor 1987 - present
Location
Southern Cal
Occupation
Electrician NEC 2020
Will this work? They didn't blow up when I restored utility power after installation. Haven't tested the generator yet. Any other thoughts?
If I understand the design correctly being :

1 Utility feeds 2 Independent distribution panels
1 Generator then re feeds 2 Independent distribution panels if overall utility power is lost
1 Generator has controls to monitor 2 independent distribution panels

Problem I would consider:

If someone shuts off the main of Distribution 1 or the meter gets shut off do to service ending , will the generator have a conflicting command being that 2 sets of monitoring wires will have different values, I'd guess not because unless both distribution panels were off at the same time then and only then would the generator say hay, I'm turning on ... now here is another scenario that isn't really a problem but could be an issue, lets say the generator is natural gas fed , not sure really .. and the services of each dwelling is disconnected for what ever reason, I suppose the generator will keep running until someone decides to shut it off ..?

I like the idea, and the work looks good just not sure the design is fool proof.
 

McLintock

Senior Member
Location
USA
Occupation
Electrician
If someone shuts off the main of Distribution 1 or the meter gets shut off do to service ending , will the generator have a conflicting command being that 2 sets of monitoring wires will have different values, I'd guess not because unless both distribution panels were off at the same time then and only then would the generator say hay, I'm turning on ... now here is another scenario that isn't really a problem but could be an issue, lets say the generator is natural gas fed , not sure really .. and the services of each dwelling is disconnected for what ever reason, I suppose the generator will keep running until someone decides to shut it off ..?
Or you have to make sure the generator is shut off before you do any work to the system


“ shoot low boys their riding shetland ponies”
 

paulengr

Senior Member
Or you have to make sure the generator is shut off before you do any work to the system


You do anyway. The POCO and others expect to be able to manually shut off the ATS to avoid back feeding whether or not this is even mechanically possible. More than a few linemen have been killed when someone wires a generator directly into a distribution panel without opening the main. It is supposed to be very obvious how to turn it off so that even a local volunteer fireman can figure it out.
 

jaylectricity

Senior Member
Location
Massachusetts
Ok so yes... all disconnects are within 10 feet of each other. There should be no confusion.

One assumption is that if the power goes out for one panel it's probably out for both panels in a situation where backup generator is needed.

Maybe marking the one ATS as the ATS that will signal the generator is necessary. But otherwise any qualified person will have all disconnects obvious to them
 

McLintock

Senior Member
Location
USA
Occupation
Electrician
Ok so yes... all disconnects are within 10 feet of each other. There should be no confusion.

One assumption is that if the power goes out for one panel it's probably out for both panels in a situation where backup generator is needed.

Maybe marking the one ATS as the ATS that will signal the generator is necessary. But otherwise any qualified person will have all disconnects obvious to them
Also mark which panel the ATS goes too


“ shoot low boys their riding shetland ponies”
 

kwired

Electron manager
Location
NE Nebraska
I don't work enough on these to know how they are wired, but I believe the ATS has a control relay that is powered from the utility side of the switch to tell the system whether there is utility power present. Loss of that signal would initiate startup sequence for the generator. You might need to series both those circuits to one genset and if either one goes down it starts the genset, if you paralleled them it may not detect just one switch losing normal power. Would have to have both switches with line side power before it even considers transferring back to normal power and shutting genset down. As I said I believe this is powered from the line side of the switch and if you manually open the switch it will not start the genset as long as there is still line side power. Still a good idea to disable the generator so it doesn't automatically start and transfer while you are working on something if that is why you manually shut it off.

When it comes to transferring back to normal power, I believe most the time the transfer switch must see normal power for a predetermined amount of time before it will initiate transfer back to normal power, but I don't know if the transfer relay is powered by the genset or by the utility when it does this transfer. Whether or not the normal power detection is done via series or parallel contacts can also effect how transfer back to normal power will be handled.

I can see each unit individually responding to generator power to transfer to standby mode, though most the time it would be pretty much simultaneous transfer.
 

hillbilly1

Senior Member
Location
Atlanta,Ga
I haven't installed a Generac in several years, but the old style used two wires for voltage sense (240 volt), two wires for the charger (240 volt) and two wires for the transfer (12 vdc) relay. They must have changed the way the switch communicates if they are using three wires now, so I don't know how the communication works now.
 
I'm a GENERAC dealer and we do this all the time it seems like maybe this is being over thought you only need to parallel 2 wires (23 Transfer White, 194 Red) because the master transfer switch is already charging the battery and phase sensing

To the question of power loss on the slave transfer switch you are correct it would not turn the generator on or transfer but the entire point of the system is for power loss not an incompetent forgetting to push the off button on the generator before working on one of the mains or downstream equipment.

I can't think of any event where one meter would have power and the other wouldn't have power that would need a back up generator because obviously it could be fixed by poco or an electrician.
 

kwired

Electron manager
Location
NE Nebraska
I can't think of any event where one meter would have power and the other wouldn't have power that would need a back up generator because obviously it could be fixed by poco or an electrician.
Doesn't pretty much all power loss events it is supposed to cover need either an electrician or POCO to intervene?

If you lose power to one of the transfer switches it is fairly obvious the problem is likely on site, but if generator doesn't kick in and transfer at least that one circuit the owner is likely asking why. This discussion is primarily for optional standby systems, but if it were a required standby system you may need that one switch to transfer by design in a similar situation.
 

hillbilly1

Senior Member
Location
Atlanta,Ga
I'm a GENERAC dealer and we do this all the time it seems like maybe this is being over thought you only need to parallel 2 wires (23 Transfer White, 194 Red) because the master transfer switch is already charging the battery and phase sensing

To the question of power loss on the slave transfer switch you are correct it would not turn the generator on or transfer but the entire point of the system is for power loss not an incompetent forgetting to push the off button on the generator before working on one of the mains or downstream equipment.

I can't think of any event where one meter would have power and the other wouldn't have power that would need a back up generator because obviously it could be fixed by poco or an electrician.
There is the possibility of a fault tripping the main, and smaller transferswitches do not have transfer into fault protection, so if the generator breaker also trips, the second transferswitch along with the first have already transferred, they will remain transferred until the generator shuts down. That is the downfall of the Generac method of controlling the switch. Others when source two fails, if source one is still available, will transfer back to source one. Generac’s will only do it if the generator fails. Of course, these scenarios would be rare, and in a residential system, not that critical.
 

jaylectricity

Senior Member
Location
Massachusetts
I'm a GENERAC dealer and we do this all the time it seems like maybe this is being over thought you only need to parallel 2 wires (23 Transfer White, 194 Red) because the master transfer switch is already charging the battery and phase sensing
So the black wire, I believe it's labeled "ground" (and I don't remember the number) is unnecessary to parallel to both ATS?
 

jaylectricity

Senior Member
Location
Massachusetts
I haven't installed a Generac in several years, but the old style used two wires for voltage sense (240 volt), two wires for the charger (240 volt) and two wires for the transfer (12 vdc) relay. They must have changed the way the switch communicates if they are using three wires now, so I don't know how the communication works now.
Yes, it's up to six wires now. Even an old generac we're working on had four wires. Tried to make it work with an aftermarket battery charger, but I think we're going to end up replacing the transfer switch now. Didn't want to do that because they had #10 copper and four control wires in a half inch pipe. Looks like we're going to have to rewire the whole thing.
 

jaylectricity

Senior Member
Location
Massachusetts
There is the possibility of a fault tripping the main, and smaller transferswitches do not have transfer into fault protection, so if the generator breaker also trips, the second transferswitch along with the first have already transferred, they will remain transferred until the generator shuts down. That is the downfall of the Generac method of controlling the switch. Others when source two fails, if source one is still available, will transfer back to source one. Generac’s will only do it if the generator fails. Of course, these scenarios would be rare, and in a residential system, not that critical.
There is no main for both. The service conductors enter a 2 position meter/main where each space has their main breaker after each meter.
 
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