First off, let me say that it seems odd to see a master electrician ask some of the questions you have.

I will address a few of the topics very generally-

(1) for a house with dual 200A panels, you will probably end up either putting in a 12 to 20 kw set with a generator subpanel, or using a much larger set (perhaps 35-50kw) with either a 400A transfer switch or dual 200A switches. It is a good bet that the 400A or dual 200A switches will have to be service entry rated.

(2)You will need at least a 200 gallon propane tank for a 12kw, and maybe 500 or more for a 35kw+ set. It is far easier/cheaper to get a stationary one than a DOT approved portable in these sizes.

(3) YES, you should get this inspected. (you knew that before you asked)

If you have never done a standby generator install, the 400A service you describe is a bad one to learn on. I respectfully suggest that you get in person advice from someone who has done such an install before.

Stay as far away from Generac/Guardian and the other sellers who rebrand that junk as you can. Spend more money and get a Kohler or at least a Cummins/Onan, you will thank me later.

What part of KY are you in?

I live in the Lexington area...I know being a master and asking some of those questions makes you wonder. But they did just adopt the 2008 NEC last September here...(I used to live in wisconsin and they stayed current) I dont do any residential and have never done much with Generators...

I know I had to get a permit...shouldnt of asked!

A couple of us guys were not sure if I could get just one ABT and let it back feed from one panel to another... Didnt say it was the right way, just wondered if I could do it...

Can you recommend a 12K to 18K Generator? Should I stay with LP or Diesel?

Where you from??

Given the size of your house, you will almost certainly have to some sort of load shedding or install a generator fed subpanel if you limit the generator size to 18KW or less. My house is probably half the size of yours, with a 2.5 ton HVAC, and I have to interlock the water heater, air handler strip heat and shed my garage heater to keep automated loads below 15KW. I would need a much larger set to avoid load management, it isn't unusual for automated loads to push my usage to 25+KW. Right now my house is drawing 22KW.

Diesel vs. LP/natural gas is dependent on your own ideas and resources, money being one of them. Diesel sets cost a LOT more. I'm 150 miles west of you, and was glad to have a diesel during the ice storm 2 years ago. I have about 140 gallons on site, and my neighbors have maybe 500 gallons on hand most of the time. When someone figures out an easy, relatively safe way to obtain and transport LP with a few 5 gallon buckets, length of hose and a pickup truck, I may change my mind.

As far as recommending a brand, I will say that you get what you pay for, and that Caterpillar, Kohler, Generac and Cummins/Onan all sell LP and Diesel sets. Drive around Lexington and you will see three of those brands sitting behind police, fire and EMS stations a lot more often than the remaining one, there is a reason for that.

Do a search on Cummins-Onan and Kohler. You can buy CO at Costco and order Kohlers on-line (see link below). Before you make your decision (both are high quality) check your local area to see who is an authorized service center. While both companies offer great products, if they break you don't want to pay a service tech to drive long distances to get to your house. I live in Northern VA and Onan service comes out of Richmond which is at least 4 hours away. So I went with Kohler since local companies can provide service. I do all the genset standard maintenance which is quite easy, but if something big goes bad I will call in a pro to fix.

I have a 600 amp service into my house - three 200 amp panels. When we were building I decided that I wasn't going to backup the entire house - only bare necessities. My rationale was cost-benefit on the genset size, cost to operate, the likelihood and typical length of outage, and my worst case scenario for wanting the backup.

My worst case is an outage in the winter with freezing temps. I have geothermal heat pumps and put in wood burning fireplace (insert) that ia rated as a heater to serve as my backup heat. The fireplace needs a small amount of power to run the fan and I use vornado portable circulating fans to move warm air around the house. Wanted refrig's backed up, phone-internet, ability to cook (coffee a must or I will die) , water, a few outlets, some lights, etc. To do this I needed no more than a 12kW generator. The Kohler 12kW uses about 2.5 gallons of propane per hour at idle speed which is also 50% power output. When we lose power my genset never needs to accelerate beyond idle spreed. At 2.5 gal/hour I can run for 2-3 days assuming I let it run continuously. I have a 500 gallon burried propane tank but assumed that it would be at 40% full when we got the outage which is 200 gallons. These are all assumptions that I made when deriving my needs. You can get smaller above ground propane tanks (e.g. multiple 100 gallon tanks) and that might work just fine depending on what your backup needs. You can also manuallly turn the genset on and off to extend the operating life if you know the outage will be longer than the fuel you have.

I had my electricians install a sub-panel when the house was being built and put all the circuits I wanted backed up into that sub panel. The sub panel was located on the interior foundation wall close to where the generator was going, which is on the down wind side of the house. For placement of the genset you must condsider generator exhaust and our local code dictated distances that the genset had to be away from any combustable structure and windows. I like the seperate sub panel becuase over time (and after a few outages) I have moved a few circuits from the main panel to the backup sub panel. If you get an ATS with a load center, they limit the number of circuit breakers you can have (12 - 16 is typical).

I installed the generator and automatic transfer switch (without the load center) about 6 months after the house was completed and it was pretty easy to put in. The ATS was installed right next to the sub-panel. Sub-panel feeder cable was moved out of subpanel and into ATS. ATS gets the load feed cable from the generator and then a short cable run from the ATS back to supply the subpanel. So then the ATS choose either house power or generator power to feed the sub panel.

The worst part of the install was drilling through the foundation wall to run two pieces of conduit - had to rent a hole saw with water cooling and it was a tough spot to drill. Code required that the generator feed cable be in a seperate conduit from the power supply cable and generator control cable. Your genset will have a battery that needs to be charged so you need to provide a 15amp circuit to the generator to power the charger plus any other accessories like a cabuerator heater and battery warmer. BTW - I recommend if you buy a Kohler and live in a cold climate that you get both warmers.

Search this forum as well as the HVAC forum for reviews on Generac gensets before you buy one. Also beware of gensets sold by big box stores because the units they sell may not be the same as the units you get from a dealer or distributor. One clasic case was the Kohler 12RES-L (sold buy Lowe's) vs. the Kohler 12RES. The -L version did not have the electronic speed control circuit (to lower cost) which is essential to maintain the frequency of output to 60Hz +/- acceptable tolerances of many electrical components. So if you bought the Lowe's unit you might have issues operating frequency sensitive devices like computers.

The gas company did the supply hookup and pulled permits for that. I did the rest myself and saved about 4-5 thousand dollars over the quotes I got for turnkey installation.

BTW - if you buy a Kohler they will mail you a packet of info telling you that if you don't have an authorized dealer do the Startup Service, it "may" invlaidate the warranty. Dealers wanted 500 dollars to do this for me which i found outrageous. I then carefully read the checklist and most of it was check this or that. The only adjustment I had to make was the output voltage and frequency and for that I had to purhcase meter from Lowe's that provides a frequency measurement. Out of the box the Kohler was almost dead on, only a slight tweak was required.

Kohler really tries hard to force people to go through dealers for everything. They do not let you buy service manuals from Kohler. If you hunt around you can find parts dealers on line that will selll you everything you need. I use these guys and they are great:

http://www.mpgservice.com/products.htm

You can order the service manual from them and it is worth the 30 bucks.

Also look in the manual and buy spare parts you will need. The Kohlers have 2-3 small fuses and if they blow you will want a spare. Two of the fuses are right on the front control panel. One of the fuses is on the starter motor power feed and you have to take the top cover off to reach it.

Good luck

Here is a link that might be useful: Generators On-line