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Old 03-16-2009, 07:04 AM   #1
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Generator with 2 transfer switches?

I am trying to roughly layout installing a back up generator for my mother's house. The house has a few complexities beyond the usual residential installation, it is a fairly large house located on a farm, the meter is remotely mounted on pole that feeds 3 weatherheads, 2 going to an outbuilding, and 1 going to pole/panel board mounted 400 amp disconnect that then feeds 75 ft of buried wire going to the house, and also branches off to a 70 amp breaker that is then connected to a buried conduit that runs to the air conditioning condenser units mounted behind a brick wall about 30 feet from the house.. I do not know why the outbuilding has 2 feeds going to it, 1 feed runs the well pump and associated equipment connected to a mostly empty 100 amp panel, the other goes to a 200 amp panel. The generator will be a 33KW Kohler Diesel (diesel was picked as there is a 500 galon diesel tank on the farm) and will be placed behind the brick wall with the air conditioners. Only the main house and the panel operating the well pump will need to be powered by the generator (an single additional 15 amp circuit will be added with the well pump to run the old refrigerator also in the outbuilding, and maybe one for the lights in the pump room). From a physical layout point of view the generator will be located about 40 feet from the 400 amp disconnect, and about 65 feet from the 100 amp well pump panel, these three points are roughly inline with the generator at one end, the 400 amp disconnect pole in the middle and the 100 amp well panel at the other end. My current plan is to install a Nema 3R 400 amp manual transfer switch on the panel oposite the existing 400 amp disconnect (back to back on the board), move the existing buried wires that feed the house from the bottom of the disconnect to the bottom of the 400 amp transfer switch (currently the conduit is bent a little more than 90 degrees to mate with the disconnect, by bending it an equal amount less than 90 degrees it should reach a transfer switch mounted back to back on the panel with the disconnect if the terminals are mounted at the same height as the current ones) Have the 400 amp transfer switch feed the house (the house has 2 200 amp and 1 100 amp panels inside) and move the small 70 amp air conditioner panel to also feed from this transfer switch. Then install a 100 amp manual transfer switch to 100 amp well pump panel. Does all this sound ok so far? Also I would like your thoughts on how to best run the wire from the generator to the transfer switches, is it acceptable to run 1/0 in conduit from the generator to the 400 amp transfer switch, then daisy chain another run to the 100 amp transfer switch? (note the generator hase a 150 amp breaker mounted on it) Also is it allowable to run a pair of control wires for a remote start/stop switch in the same conduit as the current carrying conductors?

thanks for your help, Ike

p.s. what size wire would I need to use to jumper from the 400 amp disconnect to the 400 amp manual transfer switch? The disconnect currently has double wire terminals, with only single conductors going to the house.

Last edited by Isaac-1; 03-16-2009 at 07:11 AM.
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Old 03-16-2009, 02:19 PM   #2
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If you could, break this up in paragraph's, proof read it to make sure it sounds correct and with all the details, leave out irrelevant details, we can probably help you.

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Old 03-16-2009, 02:59 PM   #3
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Originally Posted by Isaac-1 View Post

p.s. what size wire would I need to use to jumper from the 400 amp disconnect to the 400 amp manual transfer switch?
now... i am no expert by any means... but, if you are asking this.. and these questions... i don't think you should be installing this new system.
you need to have this work done by a qualified person...

too dangerous for the newbie diy 'er...
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Old 03-16-2009, 07:58 PM   #4
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I am not particularly a newbie with typical home wiring projects, and in the case of this project I plan to get a friend that is an industrial electrician to help out, I am just trying to do as much of the leg work as possible first. As to this jumper connection to the 400 amp it is much more of a code vs. a safety issue since this is located outdoors and is dealing with wiring that is larger than the utility line feeding the box. Some places online state 500 mcm is large enough for a 400 amp service, others say it is rated at only 380 amps, but can be used with a 400 amp over curent protection as it is the next high standard size, etc. It seems this 380 amps related to calculated load only, and the real world limitation would be the same since the protective device fuse is 400 amps either way. It is much the same for the question of daisy chaining since limiting factor here will always be the generator output.

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Old 03-16-2009, 08:45 PM   #5
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We use 500 MCM copper for 400 amp circuits on a regular basis. Almost all of my work is inspected. Never had it questioned. 2-3/0 copper in parallel would be easier to handle, just make sure that the lugs are rated for 2 wires, or install 2 lugs.

A generator can have as many transfer switches as needed. For example, one of the local casinos I work on occasionally has 4 generators that all synch together (like one big one), and 17 transfer switches. Any transfer switch will start all 4 gens, and they'll keep running until the last transfer switch goes back to utility.

A hospital is required to have at least two transfer switches if the load is greater than 150 KVA. (150 KVA would be a pretty small hospital!). One for the critical branch circuits, and one more for the life-safety circuits.

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