Whole House - Back up generator

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Npstewart

Senior Member
We are adding a generator to a existing 9,000 square foot house. The house currently has a existing 400A combination service meter/main. Obviously the existing service comes into the meter side of this device, and feeds the main attached to it. (see photo).

I am basically just trying to figure out the easiest and most cost effective way to add a automatic transfer switch to this picture. My thoughts were to add the transfer switch on the left, come out of the meter/main combo to feed it, then come back out of the transfer switch, and back into the meter/main combo to re-feed the main. Would this be possible with a piece of equipment that is a assembly in a meter/main situation?

The catalog # for the square D meter/main combo is a QU12L400SL

Thanks Guys
 

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qcroanoke

Sometimes I don't know if I'm the boxer or the bag
Location
Roanoke, VA.
Occupation
Engineering
Why not put the ATS bettween the Service disconnects and the load?
If you put it in front of the service disconnect it will probably need to be service rated.
 

Npstewart

Senior Member
Because the meter & service disconnects are all in one enclosure. Ideally that would be the best route if they were separate.
 
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qcroanoke

Sometimes I don't know if I'm the boxer or the bag
Location
Roanoke, VA.
Occupation
Engineering
Because the metal & service disconnects are all in one enclosure. Ideally that would be the best route if they were separate.
Yeah, I seen that. You don't have a lot of space to work with for sure. Plus as Ron said you have two service breakers so you would have to have two ATS's. I missed that.
Or just feed one panel with the ats and generator.
 
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Npstewart

Senior Member
See the attachment. This is what I was thinking of doing. One problem is that im not sure there is any knock out holes in the side of this enclosure.
 

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Npstewart

Senior Member
Exactly. It actually has 4 breakers with 2 spares. Maybe I should just call my local square D office and let them know my situation. I certainly don't want the electrician voiding the UL listing on there trying to install this design.
 

masterinbama

Senior Member
For a whole house set up you would need a new stand alone meter socket that feeds a 400 amp main disconnect. From the disconnect you would hit the ATS and then a 400 amp panel to pick up the 4 existing loads.
 

masterinbama

Senior Member
With no main between the service panel and the meter ( I'm sure the 2 are tied together with bussing) it would take 1 transfer switch for each load. What size are you 4 breakers?
 

Npstewart

Senior Member
I think your correct with the bussing. Right now I have (3) 200A breakers for the inside panels and (1) 100A breaker for the pool panel.
 

qcroanoke

Sometimes I don't know if I'm the boxer or the bag
Location
Roanoke, VA.
Occupation
Engineering
Im trying to follow. Can you clarify what you mean by "two breakers".
I only seen 2 breakers from the pic so I thought that was all you had. Then I seen your sketch and Bama is right.

"For a whole house set up you would need a new stand alone meter socket that feeds a 400 amp main disconnect. From the disconnect you would hit the ATS and then a 400 amp panel to pick up the 4 existing loads."
 

Npstewart

Senior Member
Attached is a picture of the manufacturers wiring diagram. I think we came to the correct conclusion here, the existing equipment has to go.
 

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kwired

Electron manager
Location
NE Nebraska
First question is do you want to back up the entire service?

If so you need a large enough generator for the entire calculated load. You will need either multiple transfer switches or change service equipment so there is a single disconnect and transfer ahead of the feeders.

If you only want to back up selected loads you could install another feeder to a panel containing the desired loads and then install a transfer switch and appropriate sized generator and transfer switch to supply just the designated backup panel.
 

Npstewart

Senior Member
We are backing up the whole service, I already did a load calc and came up with about 54KW demand, so I went with a 60KW generator. We are backing up everything because moving selected loads from the panel to be backed up would be un-reasonable.


I ended up removing the existing service equipment and installing a service rated ATS to feed the existing panels. Attached is my new partial schematic.
 

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Npstewart

Senior Member
lol, I thats the real tough part.


If I am using a service rated ATS, the GEC would come out of the ATS and I would need a EGC from the generator correct? Article 250 in the handbook doesen't show a service rated ATS.
 

kwired

Electron manager
Location
NE Nebraska
lol, I thats the real tough part.


If I am using a service rated ATS, the GEC would come out of the ATS and I would need a EGC from the generator correct? Article 250 in the handbook doesen't show a service rated ATS.
A service rated ATS is/contains the service disconnect. The original panel is now supplied by a feeder and you will need separate grounded and grounding conductors to it.

Not trying to be judgemental but you said "moving selected loads from the panel to be backed up would be un-reasonable".

Lets say you only need 15KVA for items that are really needed to be backed up.
Cost of a 16 or 20KVA generator will be considerably different as well as 100 VS 400 amp transfer equipment. Now consider how much fuel the larger generator will use even if it runs with only 20-25% load most of the time when it runs. It will likely run weekly just for 'exercise' with little or no load but will use more fuel doing so.

Just some things to consider.

Most of my residential customers that have ever wanted generator want just about the smallest generator possible yet they wonder why it can't run the entire house when it is time to use it. - Most of time is manual transfer (I use transfer term pretty loosley in this situation) and only 5-6000 Kw generator.

More than once I have had service calls for something not working right while using generator, when I get there they have a double male ended 5-15 cord and are trying to make it energize the entire house panel. The items that don't work right are on the other leg of the panel and are back fed through a 240 volt load such as the water heater.

I once had a farmer hook up a tractor PTO driven generator (directly) to an auger motor so he could get corn out of his bin to feed cattle during an extended power outage. He miswired the motor and made a dead short between L1 and L2 by the way he connected it.

The HP of the tractor was well above the generator capability. When he attempted to turn that "short circuit' on one of the universal joints on the PTO shaft just snapped right off.

Sorry about straying off topic a little there.
 
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