Generac generator undersized?

D

DavidEF543

Member
Location
North Carolina
Occupation
Electrician
A friend of mine just bought a new Generac generator and automatic transfer switch from Lowe's. The listing says it is a 22kW generator and the ATS is 200A rated for whole house power. He's doing the installation himself and asked me about wiring because he has 4/0 cable but the generator only has 2/0 sized lugs. Doing the math, 22kW is really only 100A at 220V.

The NEC says that if your generator is unable to serve the whole load that it is connected to, then you're not allowed to use an ATS, right? But this was sold as a set with a 200A ATS and he's expecting it to power his whole house. This will be an optional standby generator.

What's the deal? Is this legal? If so, how should connection be made to the generator, since the 4/0 aluminum cable has already been run and connected at the ATS. Also, if he isn't allowed to use the ATS as I suspect, could he legally just not connect the control cables and use it as a manual transfer for inspection purposes?
 
Sounds like you'll need to load shed module to keep the load under 100 amps. Why can't the lugs be changed?

22000/240=92 amps
 
If your friend didn't do a load calculation or look at metering for his house before purchasing the generator, I would be very concerned about his ability to do the installation. I am sure there are a bunch of other important items that have been missed. For example, is the ATS service rated? Where is the neutral/ground bond made? What is the size of the output breaker on the generator, and why would you need a conductor with more ampacity than that breaker? Is it running off of natural gas or propane? The output on natural gas will be less than on propane. If it is propane, is the tank large enough to provide fuel to run the generator without freezing?
 
If the calculated load i
infinity said:
Sounds like you'll need to load shed module to keep the load under 100 amps. Why can't the lugs be changed?

22000/240=92 amps
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maybe, maybe not. Depends on the loads available. I put in a 20 kw last year, all appliances propane including heat. One A/C unit. 20 kw was more than enough. The lugs are probably on a 100 amp breaker in the unit. A splicer/reducer would be the easiest way to downsize the wire. Usually there is a lot of room where the breaker is, but I haven’t done a Genercrap in a long time.
 
My understanding is that with an automatic ATS the Gen has to be able to supply the entire load. With manual transfer the gen can be any size. Of course I could be wrong.

I suppose he could put in a sub panel with a manual interlock and move the "emergency circuits" to the new sub panel. That defeats the ATS.

Generac should know the code on this better than anyone. Maybe he should call them.
 
I personally think ATS's are a extremely over rated and the most common cause of generator overload. In a typical residential application rarely does a homeowner want to run every load on their 600A residential service and charge 3 electric cars.
A Generac ATS can be modified to a manual transfer switch by adding a toggle switch.
 
Just because you have a 200 amp panel doesn't mean you have 200 amps of load to run.

A majority of 200 amp transfer switches (manual or auto) are not supplied with a generator capable of delivering sustained 200 amps. NEC requires an automatic transfer arrangement to have capacity of handling the actual connected load but does allow load shedding methods to be used to keep the running load below the rating of the standby source, which can result in more total connected load than the rating of the generator, but will never run more than the rating of the generator if set up properly.
 
Eddie702 said:
My understanding is that with an automatic ATS the Gen has to be able to supply the entire load. With manual transfer the gen can be any size. Of course I could be wrong.

I suppose he could put in a sub panel with a manual interlock and move the "emergency circuits" to the new sub panel. That defeats the ATS.

Generac should know the code on this better than anyone. Maybe he should call them.
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The entire load will not be 200 amps. Either a spreadsheet from the poco giving peak load, or a calculation of the normal loads. 100 amps will run most normal houses. Electric emergency strip heat, or EV chargers are the big killers of a smaller generator.
 
kwired said:
NEC requires an automatic transfer arrangement to have capacity of handling the actual connected load but does allow load shedding methods to be used to keep the running load below the rating of the standby source,
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The problem I had with the Generac load shedding modules is they were a just a basic 24V contactor in a Generac branded enclosure and they are electrically held 'on' when the transfer switch is on utility power. So basically you have a bunch of 24V contactors normally 'on' 24/7 waiting for a power failure to turn them off.
I have had issues with them buzzing and or failing.
And now if your on the 2023 NEC those load shed modules are not a article 750 Energy Management System.
 
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Normally the generac cable is a #3 copper.

you run whatever you want.
As far as loading, did anyone do any calculations? Just because you have a 200 amp service doesn’t mean you will ever use that. The POCO can tell you your Peak Demand.
Also, they have instructions. I’m assuming he read them?

Look up on the pole at the transformer or the padmount transformer.
What size is it? It would need to be a 50kVA feeding ONE house to supply 200 amps.
You can not size a generator by what size XF is feeding the house. We are allowed to push these to 200% of rating for a while.


Overhead service? My bet is #2 al triplex.
UG service? Could be 2/0 from the utility.
Also, that rating is with Propane. NG will have a lower rating, down to about 18kW.

My bet is a #3 copper would be fine.
The NEC says if it can’t supply the whole load, not the breaker capacity.
 
Okay, so assuming the load calculation shows that the generator is sufficient, what should he do to connect to the 2/0 lugs? splice the 4/0 to a piece of 2/0 in a junction box at the generator?
 
DavidEF543 said:
Okay, so assuming the load calculation shows that the generator is sufficient, what should he do to connect to the 2/0 lugs? splice the 4/0 to a piece of 2/0 in a junction box at the generator?
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The inspector should ask for a load calculation to determine if the generator is large enough for the entire load. Those lugs cannot be changed to #4/0? Yes it can be spliced or use pin adapters.
 
DavidEF543 said:
Okay, so assuming the load calculation shows that the generator is sufficient, what should he do to connect to the 2/0 lugs? splice the 4/0 to a piece of 2/0 in a junction box at the generator?
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You run conductors sized according the the overcurrent device in the generator. 22 kVA generator will likely have either a 90 or 100 amp breaker.
 
If the load calculation shows that the generator is undersized, then can the ATS still be (legally, according to code) installed as a manual transfer switch by disabling the automatic control? The generator does have a auto/off/manual selection interface in the controller. And the automatic transfer system relies on a set of control wires run between the generator and the transfer switch.
 
DavidEF543 said:
If the load calculation shows that the generator is undersized, then can the ATS still be (legally, according to code) installed as a manual transfer switch by disabling the automatic control? The generator does have a auto/off/manual selection interface in the controller. And the automatic transfer system relies on a set of control wires run between the generator and the transfer switch.
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Yes, but the engine exerciser would be disabled. Best way is to load shed any excessive loads automatically. There are several ways to do it.
 
DavidEF543 said:
If the load calculation shows that the generator is undersized, then can the ATS still be (legally, according to code) installed as a manual transfer switch by disabling the automatic control? The generator does have a auto/off/manual selection interface in the controller. And the automatic transfer system relies on a set of control wires run between the generator and the transfer switch.
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Yes, I think you do it one way for Generac and another way for all others.

@hillbilly1 was it you that said for Generac there is a 12V signal (#23 wire) that goes from the genny to the xfer switch and if you interrupt that signal with a toggle it does not auto transfer to generator and the generator still starts and stops with utility sense lead?
 
tortuga said:
Yes, I think you do it one way for Generac and another way for all others.

@hillbilly1 was it you that said for Generac there is a 12V signal (#23 wire) that goes from the genny to the xfer switch and if you interrupt that signal with a toggle it does not auto transfer to generator and the generator still starts and stops with utility sense lead?
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Can’t remember for sure the number, but on the older switches, it powers a cube relay (you will see it easily) that controls the transfer. I think the newer ones have a printed circuit board with load shed outputs on it. May still have the cube relay.
 
Generac 22kW has a 100A MCB at the generator. There is no reason to run 4/0 between the generator and the ATS. I use #3 from the ATS to the gen.

I happen to know for a fact that the rxsc/rxsw200A3 ATS have lugs that will fit at least 3/0, because I land them all the time.

If the 4/0 al doesn’t fit, there is plenty of room for a Polaris in the ATS.

22kW is probably my most common unit, and I install them on houses with 200A services all the time. I also install 18kW units on 200A services. Just because the house has a 200A service doesn’t mean it needs it, or draws 200 amps. A proper load Calc will tell you what it needs. Sometimes it will need a 38kW liquid cooled. 😲👍
 
tortuga said:
Yes, I think you do it one way for Generac and another way for all others.

@hillbilly1 was it you that said for Generac there is a 12V signal (#23 wire) that goes from the genny to the xfer switch and if you interrupt that signal with a toggle it does not auto transfer to generator and the generator still starts and stops with utility sense lead?
Click to expand...

That is correct. Or just don’t land wire 23.

The ice cube relays are no more. I still run into them quite often on service calls, but the new ATS have a board.

They have 4 “A/C load shed” slots, that are basically 24 volt relays. If you want to install a contractor and use them, you can. It’s just a lot easier to use a load shed module. Generac says they have fixed the issue with the boards burning up, but I don’t know. There is another brand of SMM that I believe @Birken Vogt posted that is supposed to really good. I’ll have to go try and find it.
 
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