Optional Stand-By Generator & ATS

[cdcengineer]

I have a homeowner who wants to put 10lbs of sand into a 5 lb bag. She has a 25kW, 120/240V, 1-Ph generator her contractor has ordered which comes with a factory installed 125/2 main. They want to buy a kit (it's all Generac product) to add a 2nd 125/2.

The system has (2) ATS which have load management (load shed) capability. There are (16) integral "critical" loads within each ATS that are always powered whether we have normal or generator power - unless the generator fails. There are feed thru lugs from each ATS which serve downstream loadcenters at 200A. Now the issue, I know the generator can hold the (16) x 2 = 32 proposed critical circuits. The owner wants to try and hold the rest of the loads in the event of a prolonged outage. She describes just trying to have the ability to move from room to room and have lights which she would shut off. Keep in mind tyhat the house was wired before they dreamed this up so the lighting circuits are already located within loadcenters and cannot be easily pulled to the "critical" circuits at either ATS.

Obviously we would overload this small generator if the entire 320A self contained service was being utilized. At this altitude, the generator can only output ~ 75 Amps. But she is convinced that she can manage the use of laods carefully.

They called me in, I do not usually deal with residential, and I really don't love the thought of this. but I'll be darned if I cannot find a reason to shoot it down other than my opinion that this isn't the way to do it. NEC 702.4(B)(2)(b) says that if load management is utilized, than the source must have the capacity to serve the max load. Now it seems as though because the critical laods are designed to be well below the generator's output capacity, than this isn't disallowed for any reason.

Anyone want to jump in on this?

Thanks

 

[templdl]

Not considering the loads can you install the generator/ATS/emergency load panel per code?
I don't think it is your responsibility to second guess the contractor in the size of the genset specified. You have made your comments to the homeowner regarding your concern for the generator's capacity and the loads anticipated. Beyond that is there a safety issue that you are concerned about?
Otherwise do the install according the code requirements and if the generator is overloaded an OCPD may trip and inconvenience the homeowner or the field will collapse on the generator ad it will stop generating.
I personally wouldn't be concerned about the generator capacity, agree with the homeowner that it is large enough to serve her intended loads.

 

[cdcengineer]

I can't see a code violation, no matter how hard I look. It just doesn't sit right with me having (2) 125/2 breakers fed from a 75 Amp source. The generator could constantly shut down and the homeowner will be pointing fingers unless I can clearly convey my opinion.

It's tough because the breakers on the generator won't be tripping, it'll be a shutdown due to overspeed (over-draw) if the load shed is not setup right. And this isn't the intent of the geenrator protection to be repeatedly shutting down.

Unfortunately, sometimes it feels like it's up to us to design things to protect people from themselves.

 

[jim dungar]

I think you are trying to solve a problem that may not exist.
You need to determine the potential loading, everything else is absolute speculation.

For the majority of homes in the US, with a 200A service, I have a hard time believing they draw more than 50-75A at a time. So managing manual load shedding may be an easy task even if the service is a nominal 400A.

 

[cdcengineer]

^ Fair enough Jim. I've just been burned in recent times with people making one claim and than pushing systems beyond the limit.

Thx

 

[jim dungar]

.... people making one claim and than pushing systems beyond the limit.

People do that? Really?:roll:

 

[cdcengineer]

No doubt! It's expected.

 

[templdl]

I can't see a code violation, no matter how hard I look. It just doesn't sit right with me having (2) 125/2 breakers fed from a 75 Amp source. The generator could constantly shut down and the homeowner will be pointing fingers unless I can clearly convey my opinion.

It's tough because the breakers on the generator won't be tripping, it'll be a shutdown due to overspeed (over-draw) if the load shed is not setup right. And this isn't the intent of the geenrator protection to be repeatedly shutting down.

Unfortunately, sometimes it feels like it's up to us to design things to protect people from themselves.

I see your point. All you can do is to point out the possible problem and let them make their choice, that you can't be liable for any warranty of service calls regarding this issue.
Just to a code install as you have done what a professional should do in stating your concern.
What do they say? You can lead a horse to water but.............

 

[cdcengineer]

Thanks for the posts. Have a great weekend. It's Stanley Cup playoff time.

 

[electricguy61]

I just installed a 20KVA generator on a 400amp service / 5500 sq' house.. Actual load on the service was @ 50 amps @ 230v.

I had to show the load calculations to get the permit, and had to shed all A/C loads with the ATS. Very helpful that all the major appliances other then the oven were gas.

	Rated Load​	Demand Factor​	Calculated load​
General Lighting Load: 3va x 5537 sq ft= 16611 va
1st 3000 va @ 100%	3000​	1.00​	3000​
13611va @ 35%	13611​	0.35​	4763.85​
Electric Oven 4kW	4000​	0.8​	3200​
Dishwasher	1200​	0.75​	900​
Furnace motors (3) x 1/4hp + 2086va	2086​	0.75​	1564.5​
Disposal 1/2hp	1200​	0.75​	900​
Small Appliance (2) x 1500	3000​	0.75​	2250​
Laundry	1500​	0.75​	1125​
Calculated load per NEC Article 200			17703.35​
Dryer, Water heater, Furnace, Cooktop = natural gas
AC loads will be shed by Automatic Transfer switch

 

[cdcengineer]

What type of ATS did you use?

 

[electricguy61]

What type of ATS did you use?

I used the Generac 400 amp ATS w/ a Kohler generator. Had to do a run some extra wiring between the genset and the ATS so that all the functions would work (exercise, ats). Also had to install a battery charger at the genset.

In the future, I would seriously consider only using Generac parts including the genset, so that everything was seamless. I went with the 400amp Generac RTS ATS because it was about 30% lower in cost, but the Kohler genset because it was the quietest.

 

[cdcengineer]

400A ATS? I assume this was because it was ahead of the service main? Did you only run 100A feeders from the gen-set since this was probable the OCP at the generator?

The RTS seems like a good way to go because it has the 4 modules plus 2 spaces to dump A/C loads.

Thanks for posting.

 

[iwire]

^ Fair enough Jim. I've just been burned in recent times with people making one claim and than pushing systems beyond the limit.

Thx

Why would that be your concern?

Not trying to be a wise guy but I am a give the customer what they want as long as it is legal.

Let them know it has a certain capacity and they will have to make sure not to exceed it.

 

[cdcengineer]

^ I should probably learn to do the same thing.

 

[electricguy61]

400A ATS? I assume this was because it was ahead of the service main? Did you only run 100A feeders from the gen-set since this was probable the OCP at the generator?

The RTS seems like a good way to go because it has the 4 modules plus 2 spaces to dump A/C loads.

Thanks for posting.

It was an existing 400amp service, so I used the 400amp Generac RTS because it's service rated. And yes, I ran 100 amp wire from the genset to the ATS, because of the 100amp breaker on the genset output. I had some 1/0 XHHW left over from another job, so I used it. Unfortunately, only after running the pipe and wire did I notice that though the genset had a 100amp breaker, the lugs were only good for #2, so I had to make some field modifications for the larger conductors.

Oh well.