In theory, at least with the Kohler equipment, you could have two auto transfer switches. Each would have separate utility feeds to their main breaker, and the generator feed would be split to each of their generator standby breakers. You would only need to wire one of the transfer switches to the starter relay on the generator. Once the generator was fired up, the both auto transfer switches (even the one not wired to the starter) should switch over to the generator breaker.

Conversely, once the transfer switches detect utility power again, they will both automatically switch back, with the the one wired to the starter being responsible for shutting down the generator.

My proof of concept on this was when I had my Kohler auto transfer switch first installed, but didn't have the standby generator yet. So the transfer switch was not wired to any starter relay or any such thing. But hooking up a manual-start gasoline generator to the standby breaker and firing it up was enough for the transfer switch to automatically switch over. When utility power came back, it automagically switched back and I just had to shut down the gasoline unit myself.

All that said, I opted to save $2K and do the 2nd meter (the geo meter) on a manual transfer. And that is accomplished with an ultra-cheap sliding metal interlock on an otherwise standard breaker panel. The interlock simply forces you to flip off one breaker (either the main or generator breaker) before the other one can be flipped on.

I figure that power outages that last longer than a few hours are rare enough that I can be bothered to do the heating system manually. Of course, this is in New Hampshire where we had the Ice Storm of '08 followed by the Wind Storm of '10 a mere 15 months apart. Down for four days and two days, respectively. PSNH is finally trimming back trees and branches in the problem stretch that feeds my neighborhood, so hopefully the odds are getting better.

I have a Generac Guardian set up for auto xfer.

What David describes sounds right. One of the Xfer switches would start the genny based on loss of utility power for a certain time (mine is 40 seconds if memory serves). The Xfer switch won't actually xfer until genny is up and making stable power. The other xfer switch should go along for the ride.

For those truly bored, the back-story on my generator install experience...

During the geo planning, I figured it made sense to put in a generator if I was going to be doing all that electrical service entrance work anyway. Do it once and get it over with. I settled on Kohler's new, way-cool, integrated transfer switch, which is a full 200A main panel with the transfer switch built right into it. Swap out the old panel and put the new one in, which simplifies things quite a bit and solved some space issues.

This thing comes, to put it mildly: "some assembly required". I figured I would save some billable electrician hours by wiring up the internals myself and having him cross-check my work afterward. Probably took me twice as long it as it would him, but it was straightforward enough and I got to see how it was put together.

Since I run servers on a Comcast Business line here, I can't afford a long outage. So a week prior I bought a decent 6KW portable gasoline generator and figured I'd just run extension cords to the critical areas while the service entrance was down for the various items being installed. That was an inverting unit with a 220V outlet that can be patched directly into the house if desired. So near the end of the service entrance work, I was pretty jazzed by how cool the new panel was and the fact that I had a genny that could patch straight in. My electrician was able to stay a bit longer, so I ran down to Home Depot to get some 10/4 wire and a power inlet to cover me until the propane generator was installed, "just in case".

So here we have the rather unusual and unlikely scenario that I actually own a generator. And even more of a fluke that I had bothered to install an inlet, since it was to become obsolete in a few months.

Two weeks later: the Ice Storm of '08. Talk about dumb luck.

And it gets better.

Power goes out at 9:30PM. Where am I? In the basement, sharpening a chain saw. So now I have to navigate an obstacle course of sharp, pointy objects in the dark to get to a flashlight. But hey, I've got the genny and the fancy inlet, so I'm good, right?

The kids (3 and 5) are still not put to bed for some reason, and they are freaking out by the lights going out (to this day I don't understand this). They absolutely will not let Mommy abandon them. Which is fine, except the guy who replaced our garage doors and openers never bothered to balance the springs properly. Not usually a big deal, except when you have no power and have to open the door (and have it stay open) so you can pull the car out so that you can wheel the generator out where it won't kill you with CO poisoning.

So I really need mommy's help, except that she's stuck with two freaked out kids. And here I am saying, "Please, I just need you for 30 seconds and I can get the lights back on!" What a circus. So she manages to convince them that a flashlight will have to replace mommy for one minute, and she's finally able to help me keep the door open long enough to get everything where it's supposed to go.

And it was quite magical to have the lights come on 10 seconds after the generator started. Don't know why, but I hadn't figured that the auto-xfer switch would just do its thing without me having to run down to the basement after the generator was fired up. But it sure was nice to have the kids calm down. :-)

Funny story - glad it worked out

My kids are at the moment 3 and 5 as well - fun ages

Posted By DavidYon on 05 May 2010 09:39 PM
In theory, at least with the Kohler equipment, you could have two auto transfer switches. Each would have separate utility feeds to their main breaker, and the generator feed would be split to each of their generator standby breakers. You would only need to wire one of the transfer switches to the starter relay on the generator. Once the generator was fired up, the both auto transfer switches (even the one not wired to the starter) should switch over to the generator breaker.

Sorry diverted for awhile.
Anyway, seems I would have to employ a third isolator. If one generator feeds two panels, how are they isolated from one another when the generator isn't on?
joe

When the generator is off, switch A connects line A to load A and switch B connects line B to load B. So there is no connection from A to B to be isolated.

Agree with jonr, the ATS boxes will naturally provide the isolation you're looking for.  With utility power, the generator is not connected to anything at all, it may as well not exist.  The main breaker in each panel connects to a different power meter, so those are isolated.

With generator power, the ATS boxes will first turn off the main breaker, so now they are connected to absolutely nothing.  Again, you have isolation.

Then, and only then, will the ATS boxes turn on the breaker that is the input from the generator.  So yes, now your two sets of branch circuits are tied together, but so long as the generator has sufficient power to run everything, that shouldn't be a problem.  At this point, the feeds to the utility power have long since been disconnected, so you still have isolation from the perspective of the utility company.

Now, if for some reason there is a problem with having the two sets of branch circuits ever being tied to a single power source, then the transfer switch technology doesn't matter---you'd need two generators.  But I'm having trouble wrapping my head around why you'd ever care about that, since ultimately the two panels are tied to a single power source, it's just that the power source happens to get split and come in through two separate meters.

Here's the ASCII version of the circuit on utility power:

Meter 1 ------0     0---------+
               \              |
                \             |
                 O            |
                 |            |
                 |            +-------- Generator
               Load 1         |
                              |
                              |
Meter 2 ------0     0---------+
               \
                \
                 O
                 |
                 |
               Load 2

Here it is again on generator power:

Meter 1 ------0     0---------+
                   /          |
                  /           |
                 O            |
                 |            |
                 |            +-------- Generator
               Load 1         |
                              |
                              |
Meter 2 ------0     0---------+
                   /
                  /
                 O
                 |
                 |
               Load 2

Posted By jonr on 11 May 2010 12:05 AM
When the generator is off, switch A connects line A to load A and switch B connects line B to load B. So there is no connection from A to B to be isolated.

Agreed.
Thinking faster than typing last night.
What I'm contemplating is isolating relays that allow geo to run off meter B or generator panel (never both). ATS and panel ~$650.....couple relays and contactors ~$50
j

I've made my own like that and they worked fine although they had with no delay or motor start capability .

IMO, the load side should have surge suppressors. When you have an inductive load (eg, a motor) that is suddenly not connected to much of anything, it produces back emf and a large voltage spike. While the motor may be designed for this, other loads on the same circuit may not be.