My local big box retailer has a decent priced 22kw lp generator/200amp transfer switch deal going right now. I'm considering purchasing it but I'm unsure on the correct way to go about my situation. I have a 400amp service entrance meter panel: enter image description here

Those two 150amp breakers are fed from the double lug terminals which are connected to the service entrance meter. After the breakers they currently go to two manual shutoff switches seen in this picture below: enter image description here

My plan is to remove those 2 service disconnects and replace with the automatic transfer switch.

The transfer switch service entrance wiring diagram shown below is what I would like to use: https://soa.generac.com/manuals/3003428104/10000009574

Since there are only bus bars instead of lug terminals coming from the meter side of the service panel will I need to change to a different service entrance panel to accommodate feeding the two sub panels or is there something made to separate the bus bars from the meter to allow for feeding a transfer switch and then back to the bus bars?

enter image description here

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    You probably can't use the one ATS with both shutoff switches, and you actually probably don't want to because 22kW = 22,000 W = (roughly) 92A, which is less than one of those 150A feeds. Key question: Do those 2 150A simply go to two nice big side-by-side panels in your house, or does one go to the house and one to a secondary building, or to two different places in the house? May 24, 2021 at 19:03
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    Manassehkatz is directing you correctly the transfer switch can only transfer 200a so you could choose one and make that your emergency power system with non critical loads on the other panel. Just a note, I know I would not work with that spike vine and expect the inspector to call out a code violation for working clearances in front of those disconnects (3’ in front of table 110.26 vegetation is considered a grounded surface).
    – Ed Beal
    May 24, 2021 at 19:21
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    Fed into two sub panels side by side
    – D A
    May 24, 2021 at 20:11
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    Since you're using an ATS, your generator MUST be able to pick up the entire load downline of it. 22KW is probably not enough, but to be sure, do a load calculation. Note this load calc will have to be re-done every single time you add new stuff, so adding a hot tub or EVSE may require you upsize the generator, feeder and ATS. I count no less than FIFTEEN 240V breakers in these panels more than I have ever seen in any house... including at least three fat ones with #8+ wire... so, yeah. May 24, 2021 at 21:11
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    @nobody holy smoke andy! I missed that one. Since 125A are available 2-pole, that implies 150 A or more, which on a 150A panel is basically a subfeed lug kit. It may be literally that. So there's a *third subpanel with even more stuff, to say nothing of the 50A load in the meter-main. Somebody isn't being totally honest (i.e. with themselves) about the actual loads to be served. Or possibly a belief that one can choose which loads ATS's pick up (no, not without load shed devices, but you'd go broke putting load sheds on a dozen plus 240V loads.) May 24, 2021 at 23:53

1 Answer 1


Forget about buying this thing. It's an "impulse buy", and that does not work for engineered products like whole-house generators and automatic transfer switches.

It's an Automatic transfer switch

It has a sequenced system which will automatically spin up the generator and then throw the loads over onto the generator.

With an ATS there is a simple rule: The generator must be large enough for the loads to be served. You are not allowed to have a situation where the generator might come online and immediately bog from overload. You must prove the generator is large enough. And the switching equipment must be large enough too!

A "Load Shed" device sits in front of large appliances and automatically disconnects them if the generator seems to be struggling (low voltage or lower-than-spec frequency). Equipment protected by "Load shed" devices can be excluded from the generator calculation, but it cannot be excluded from the switching equipment calculation.

In point of fact, since you currently have 400A service, the 200A transfer switch will be a "choke point" for your power that will prevent you from using utility power to its full capacity.

How do we arrive at the proper size?

You need to do a load calculation on your house and loads

You say you have an all-gas house. However, your service panels are ones from a lavishly electric home. In the 400A meter-main, I see a 50A panel going to ???. In the two large panels, I see no less than fifteen 240V breakers, as well as a subfeed lug kit carrying that panel's entire 150A to yet another, third panel in some other location!

So, the upshot is that all of this needs to be earnestly tallied up, and fed into a proper load calculation. The load calculation will state how many amps/kVA you will need for a "whole house" generator.

I suspect you will painfully discover that your house really does need 400A service, and you cannot shim the house's loads down to 200A -- let alone 92A.

If you are able to eliminate maybe a dozen of those 240V loads (or move them to the meter-main panel breakers, which won't be on generator)... then you may be able to get your Load Calculation under 200A. Then with some Load Shed units you may be able to get the "Load to be Served" under 92A... and then.. you could use that generator.

Having 1 generator feed more than 1 panel is problematic, anyway.

The problem is, only one panel can have a pass-through neutral. All the other panels need to have an isolated neutral, requiring a 3-pole transfer switch. The bargain transfer switch is only a 2-pole.

The way to solve that, after you have reduced the two large panels to <200A calculated load, is to replace one of the main breakers with a 200A breaker... and have the remaining 150A breaker power nothing at all. (or perhaps, another panel which is "utility-side only").

The 200A breaker then feeds ONE "subpanel". That "subpanel" feeds both existing large panels. You stick the generator ATS in between the 200A breaker and the "subpanel" in the normal manner.

I'm putting "subpanel" in quotes because it doesn't actually need to be a subpanel. As long as the wire from "subpanel" to the two large panels is 200A wire (250 kcmil not 4/0), the "subpanel" doesn't actually need to have any circuit breakers at all. A subpanel with no circuit breakers is just a box and some lug splices. "that was easy"

Effectively this will downgrade you from 400A to 200A service, but it will permit use of that bargain generator.

How Harper would do it.

Well if someone held a gun to my head and forced me to use a generator, I'd get a large enough generator for the whole job.

One side-effect is that the generator would be water-cooled (coolant-cooled, to be more precise). I would run that hot coolant through hot-water registers around the house, or one heat exchanger in the forced-air furnace stack... because, hey - free heat!

Or, I'd swap circuits around in the two large panels until one of them only contained loads you would never want to put on generator, and the other one contained loads whose Load Calculation was less than 92 amps. Then, I'd put the ATS on that panel only, and let the other panel just be dead.

  • I bought this house in a short sale, the owners were facing bankruptcy because the house was used as a marijuana grow house. (2nd biggest drug bust house by marijuana volume in the county)
    – D A
    May 25, 2021 at 22:35
  • And those transfer switches were configured in such a way to switch back and forth between meter power vs unmetered (stolen power). Nevertheless Im currently using maybe one fifth the load I’m rated for!
    – D A
    May 25, 2021 at 22:42
  • The growers (let’s call them) had superstrut mounted in racetrack all around inside with HPS grow lighting. And they had a separate small pad poured which housed 5-5 ton AC units to cool all that lighting. And that 150amp breaker fed another panel out at that secondary AC pad...
    – D A
    May 25, 2021 at 22:45
  • I’m located in Florida btw, no heat needed where I’m at. I do need AC though, particularly during week long hurricane caused power outages.
    – D A
    May 25, 2021 at 22:47
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    Holy smokes! Sell all that gear on Craigslist, and don't ask too many questions of the buyer... yeah, follow my "come off one of the breakers to a subpanel, and feed both large panels off that subpanel" plan. Easy peasy. May 25, 2021 at 23:53

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