Proper Generator Size

Status
Not open for further replies.
samuelrock

samuelrock

Member
Location
Morgan City Louisiana
I have been having this discussion with quite a few people and I am getting mixed feed back. I am going to post my methods here and please tell me if I am wrong.

To properly size a generator you must know the load calculation of the house per NEC 220.

If you generator puts equal or more than your calculated figure your install meets 702.4

If your generator puts out less than figure you calculated you must install load shedding as per NEC 702.4


...........................................................

This scenario is set up using a 200A whole home ATS and a 20kw generator.

The load was calculated as per NEC 220 with this excel work sheet www.piertopier.us/hvac/RCO_Automated_Rev2.xls

The CalSheet calls for 152.21A

The home currently has A 200A service.

The customer ordered a 20kw Generator and 200A Service Entrance ATS

In order for this installation to be code compliant you must install load management.

...........................................................

Thanks
Sam
 
Last edited:
augie47 said:
When you are using an Automatic Transfer Switch, as you noted, you are correct. The generator must be large enough to handle the connected load (with load management devices taken into account)

You might find this thread interesting:
http://forums.mikeholt.com/showthread.php?t=159558&highlight=sheding
Click to expand...

Thank you, I was starting to think I was loosing my marbles!

I have asked some highly recognized members of the electrical community and also some home inspectors and I keep getting told load shedding is optional.

From what I read in the book it is only optional if the generator can carry the entire load that is transferred to it.

Common sense should tell you that if the ATS is located in-between the meter base and first load center then the whole house is the load.

If a selected circuit ATS is installed the the load is the sum of the items feed from the selected circuit sub panel.

Sam
 
Yes, load shedding is going to be required whenever the DEG is smaller than the running load.

The ATS should be fine, but you will need a sub-panel, and a new feeder breaker in existing panel.

Set-up shouldn't be too difficult to manage, providing you have physical space.
 
kingpb said:
Yes, load shedding is going to be required whenever the DEG is smaller than the running load.

The ATS should be fine, but you will need a sub-panel, and a new feeder breaker in existing panel.

Set-up shouldn't be too difficult to manage, providing you have physical space.
Click to expand...

This was only an example, I have been a Generac and GE dealer for six years now. I have never sold a unit in my home town because I refuse to install a unit against the NEC code. All the other local guys tell the customers load shedding is optional and that I am only attempting to up sell the job. To top it off I can not get the local inspectors to look for this violation.

I am moving over into insurance and home inspections and attempted to explain to some other inspectors that most generators are installed in non-compliance with the code in hopes they would start looking for this also. I got major push back by the word optional with them also.

Sam
 
Is anybody willing to go with a manual transfer switch instead of ATS and load shedding?
Or is autostart considered a critical feature?

Tapatalk!
 
GoldDigger said:
Is anybody willing to go with a manual transfer switch instead of ATS and load shedding?
Or is autostart considered a critical feature?

Tapatalk!
Click to expand...

The general population today is lazy! Every one wants auto start and don't want to turn items on and off.

I have some installs we done that are portable generators set up with a interlock kit and the customer selects what they want to run. See my video at http://youtu.be/a6Ox3sdwCyA I have done about 50 installs like this in and around Morgan City La. You just need to ensure the generator they are using does not have GFCI on the 240V output or you get a parallel path and it will trip the GFCI.

I have also proposed installing a selected circuit ATS setup that only runs the lighting, Icebox, and A/C unit and the customer responds well XXXX Electrical service says they can do the whole house and not install the load shedding you speak of.

Sam
 
I normally give the customer the following options.

Option 1
A portable generator with a interlock kit and they select what is going to be running at any given time.

Option 2
A selected circuit ATS that will only run the devices they select at the time of install.

Option 3
A whole home setup that will run every thing in the home but have load modules that will shut off the water heater, the dryer, the stove, and possibly all or a portion of the heat strips in the central heater. Customer selects the order they want items to drop out.

Option 4
A large generator that will run every thing just like you are connected to the power grid. This setup will use its weight in gold in fuel cost.

Sam
 
Just to throw some fuel into this discussion :)

For residences, the Article 220 calculation results seem to exceed what the home actually requires.

The example given, with a 200A service and ATS, 152A calculated load, and 20KW generator with no load shedding...violates code but probably works quite well.

Now it may be that this setup works well only most of the time, but fails in corner cases...or it might be that the generator shutting down in overload could be considered part of the 'implied load shedding system' :)

-Jon
 
winnie said:
Just to throw some fuel into this discussion :)

For residences, the Article 220 calculation results seem to exceed what the home actually requires.

The example given, with a 200A service and ATS, 152A calculated load, and 20KW generator with no load shedding...violates code but probably works quite well.

Now it may be that this setup works well only most of the time, but fails in corner cases...or it might be that the generator shutting down in overload could be considered part of the 'implied load shedding system' :)

-Jon
Click to expand...

I run my home that has a 180A load calculation on a 10kw generator. But I manage the loads my self.

I agree that a 20kw for the most part will run a home with 150A calculated load. If the owner knows when they hear the generator start up to not run the dryer and the stove. If the owner only uses what they need at that given moment you will never know of a issue. But this is a owner that has common sense, and how many people have that.

Most of the codes we have and work by are not placed into effect for people with common sense they are placed into effect for the idiots!

For example the first service call I went on for generac was a guy that burnt his generator up. It was during the holidays and his whole family was over. The power went out and he had a shinny new 20kw. His load calculation was almost 200A and no load shedding installed when the generator started up it tripped the breaker. This cat had two central heaters in the house a 12kw and a 8kw plus his wife was cooking food. He goes outside to reset the breaker again but low and behold it trips again. This dude was a sharp one now he took a bit of wood and jammed it in under the breaker so that it could not trip:jawdrop:. End result was a burnt up generator head and no warranty from generac as it was installed in non-compliance.

And people like this is the exact reason we have NEC 702.4 !

Sam
 
Last edited:
I tell you guys what... This is a thread on another forum where I was attempting to tell some home inspectors that a lot of generators are installed in non-compliance. Read it carefully, They just cant grasp the concept of if the ATS is installed before the main panel then the whole house is getting loaded on to the generator.

May be I just suck at communication, But I need some help getting the non electrical type people to understand this. I want to start pushing my local code enforcement to inspect for this but my city inspectors think the same way my friends at inspector journal do. All of these people are knowledgeable people I just need to find a way to explain this better.

Let me know what you think I can do differently.

http://www.inspectorsjournal.com/forum/topic.asp?TOPIC_ID=18397

Sam
 
Status
Not open for further replies.
Top