Two Generator ATS's in Parallel

[Tuna Fish Electric]

Hi there,
Here is the case scenario. There is a home with an existing 225 amp service. We plan to install a backup generator with ATS. Generac ATSs are sized for 200 amp or 400 amps. It is about $1000 cheaper to install (2) 200 amp ATS's in parallel than it is to purchase and install the 400 amp ATS. I understand that more than one disconnecting means is permissible for a service according to 230.71, however the load side of each disconnect would connect back together at the 225 amp main panel. This seems problematic to me except for the fact that the 225 amp main panel breaker would trip first in the case of an overload as the amperage would be split evenly between the (2) 200 amp breakers at the ATS's upstream. Electrically it should work fine, but according to code, is it a violation to have two separate service disconnects feed the same load? Also, I suppose grounding and bonding would only be permissible at one of the two ATSs.

-Thomas

 

[d0nut]

Do a load calculation for the house and drop the service down to 200A. Since your upstream ATS will need to be service rated, you wouldn't even have to change the existing main breaker in your panel. You will have to replace the conductors feeding the panel and make sure your neutral/ground bond is in the correct location since you are moving where your service entrance is.

 

[Tuna Fish Electric]

Do a load calculation for the house and drop the service down to 200A. Since your upstream ATS will need to be service rated, you wouldn't even have to change the existing main breaker in your panel. You will have to replace the conductors feeding the panel and make sure your neutral/ground bond is in the correct location since you are moving where your service entrance is.

That’s actually exactly what we decided to go with after thinking things through. Glad to hear others are on the same page.

And yes - bonding at ATS as new first means of disconnect, etc. Although sometimes the neutrals and grounds are so overly bonded at panel that we just leave bonding there and remove bonding jumper from neutral to ground in ATS to avoid objectionable current. Depends on where we are, but usually that solves the problem. Not sure if this is a decent use of 250.6(B), but electrically it makes sense, as long as the rest of the service equipment is grounded and has an effective ground fault current path.

 

[texie]

The original plan to parallel 2 ATS would absolutely not be code compliant and would have all sorts of safety issues.

 

[Tuna Fish Electric]

The original plan to parallel 2 ATS would absolutely not be code compliant and would have all sorts of safety issues.

Curious as to which codes it violates and what the safety concerns are. I agree it’s wrong - that’s why I brought it up here after the guys at the shop told me that’s what they’ve been doing.

ATS’s work fine when run in parallel as long as the load sides are two separate loads. The only concern there though is potential for objectionable current on neutral for one ATS if the other was still on when it is switched off.

Now, sharing the same load seems problematic because of how it would trip in the case of an overload or excessive fault. Although, the 225 amp breaker on the panel would clear the fault and ATSs can be made master/slave so as to trip simultaneously - this is not making them mechanically switching together, but it is switching together none the less.

I personally would not install it like this as it’s obviously incorrect - it’s like a family tree that doesn’t branch - it’s just wrong.

Although - from the installers I spoke with, they said they have installed many this way and they were inspected and passed. I personally wouldn’t feel comfortable passing it tho.

 

[d0nut]

Well for one, if you start the generator and transfer one switch, you have now paralleled with the utility feed. If the utility is dead, you are backfeeding it.

Are you sure the installers you spoke to didn't have two separate panels they were feeding?

 

[Tuna Fish Electric]

Well for one, if you start the generator and transfer one switch, you have now paralleled with the utility feed. If the utility is dead, you are backfeeding it.

Are you sure the installers you spoke to didn't have two separate panels they were feeding?

The original question had to do with installing the two ATSs in parallel between meter and 225 amp panel.

Other applications have been installing two 200 Amp ATSs on a 400 amp service that feed two separate 200 Amp panels - One generator feeds both ATSs.

 

[synchro]

Well for one, if you start the generator and transfer one switch, you have now paralleled with the utility feed. If the utility is dead, you are backfeeding it.

Are you sure the installers you spoke to didn't have two separate panels they were feeding?

Similarly, there could be a failure in one of the ATS's such that it's stuck in one position and will no longer switch between power sources. Then, as you mention, the generator can be connected to the utility through the parallel connection of the ATS outputs.

 

[jaggedben]

Other recent threads that discuss the parallel neutral issues with these kinds of setups:

Backfeed main panel from essential panel any code issues

Beside my meter base I have two disconnects 1 is a 200 amp and feeds my main panel. The other is a 100 amp and feeds my essential loads panel. In these disconnects the neutral and ground are bonded together. The essential loads panel is backed up with a 20kw generator with a automatic transfer...

forums.mikeholt.com

Interlock with 2 panels

Is there any known issues with supplying 2 panels from one portable generator using interlock kits in both panels? Would be a 30 amp breaker in both panels with interlock kits and both sets of conductors connected to a 30 amp inlet box. Read a brief article where someone said that shouldn't be...

forums.mikeholt.com

 

[ron]

Sounds like in addition to paralleling ATSs, it would also be paralleling the upstream breakers, which you are not permitted to do unless it is factory assembled.
240.8 Fuses or Circuit Breakers in Parallel.
Fuses and circuit breakers shall be permitted to be connected in parallel where they are factory assembled in parallel and listed as a unit. Individual fuses, circuit breakers, or combinations thereof shall not otherwise be connected in parallel.

 

[Tuna Fish Electric]

Similarly, there could be a failure in one of the ATS's such that it's stuck in one position and will no longer switch between power sources. Then, as you mention, the generator can be connected to the utility through the parallel connection of the ATS outputs.

That is now supposing that both the Electromagnetic Contactor that opens in the case of loss of power AND the signaling wire between ATSs were to both malfunction.

They are designed to be able to be controlled simultaneously.

Again - this is in regards to the two ATSs sharing the same load.

When they have separate loads there would be no potential for back feeding - except for the shared neutral potentially allowing for objectionable current.

Sounds like in addition to paralleling ATSs, it would also be paralleling the upstream breakers, which you are not permitted to do unless it is factory assembled.
240.8 Fuses or Circuit Breakers in Parallel.
Fuses and circuit breakers shall be permitted to be connected in parallel where they are factory assembled in parallel and listed as a unit. Individual fuses, circuit breakers, or combinations thereof shall not otherwise be connected in parallel.

Perfect.
Thank you for the code reference.

 

[kwired]

That is now supposing that both the Electromagnetic Contactor that opens in the case of loss of power AND the signaling wire between ATSs were to both malfunction.

They are designed to be able to be controlled simultaneously.

Again - this is in regards to the two ATSs sharing the same load.

When they have separate loads there would be no potential for back feeding - except for the shared neutral potentially allowing for objectionable current.

Perfect.
Thank you for the code reference.

Failed solenoid in one can leave it stuck in one position yet the other transfers, leaving you with both utility and generator connected together on the load side.

If I actually had load calculation needing 201- 225 amps, I'd probably be installing 2-200 amp panels instead of one 225 to allow for future loads anyway. Could possibly put less desired loads to be backed up on one panel and the generator/ATS on the other panel, which I have done this before in homes with higher capacity services.

 

[LarryFine]

Given the options talked about above, I would opt for the single 200a ATS.