I am bidding on a generator job. The service is 800 amps. The generator rep suggested using a 50kw(200 amp capable) generator with an 800 amp switch.
This is for a residence and a smaller family at that. Chances are they never exceed 200 amps.
Can you replace the CT with a transfer switch? I have not done one of this size and am a little unclear of how exactly its all going to come together. Or is the only way to install it next to the ct and pipe out of the ct, mpd, and generator?

Bot:

CT is POCO metering equip & must stay. POCO feed (after CT) and gen to respectice terminations on transfer sw; feeder to panel on 'load' of trans. switch.

800 amp service sounds pretty big for what you refer to as a 'smaller family'. Do they have a load close to 800 amps? 200 amp gen capacity may be a problem that I would look into.

We pulled thier utility records and there kilowatt usuage was low, how low I can't remember.
I put a meter on one of the phases and it was drawing around 70 amps.

Another possibility is you could put a transfer switch before each panel. Such as 4 200A transfer switches if you only had 4 200A main panels. Not saying it's a better way just another option. I don't know how the service is set up and land availible but something elce that could be done is multible generators. Such as 4 18 or 20's hooked to each transfer switch and panel. Maybe bigger for the mechanical panels and les if the panel is a light load. Figure also for the chunk of money the customer is paying they will expect no excuses if they lose power. By having more than 1 gen and transfer sw their will be less chance of a complete failure if a problem dose arise. Also the install may be much easier. Such as not needing a cement pad, crane, over weight permits for the trucks, etc. That's $1-2K less right there. More future maintence also for you. You could even sell this idea to the HO as better being better because all the eggs were not in one basket.

One thing that I wonder is if it can take the A/C load if several compressors were restarted at the same time. Kinda makes you wonder what they came up with the orignal load calc's.

The gen rep you spoke with did he get any details about the square feet, A/C sizes, electric heat, pools, or what ever elce is there? Just kinda wondering if it was a sales person that just guesed it would be good. If it was Generac/Guardian I would not have much faith in them, especialy if it was the sales dept.

A transfer switch at each panel probally wouldn't work as well, this is a 17,000 sq ft house and some of the panels are remotely located.
We had a Cutler Hammer rep come out and look the job over and this is what he specked, however we are now installing a Kohler. The house was built with a future gas line and pad in prep for this future generator.

That's cool having the pad and gas. Make sure you can pipe into the gen in the side. I don't know Kohler but many want the electric to come thru the pad in the bottom of the unit. Also make sure the gas line is of adiquate size. Meaning not just the same size as the fitting on the gen but large enough to flow the max load cubic feet what ever distance to the gen. If you need help I could look it up for you.

You would think the gas meter is big enough if they planed for it but you can't assume. Myself I would just give the customer the flow requirement for the gen and state they were responcable for contacting the gas Co to ensure the prpoer meter size. I used to do that but it got to be too much of a phone tag, unknown cost, billing, ans scheduling issues. Such as some gas Co charge the customer on their next gas bill after the work is done to upgrade the meter. I never got a consistant price and always had to wait a few weeks to get a quote. I played middle man tring to schedle the gas co work to be done on the meter when the customer was available. Also resi gas pressure is 1/4 PSI (5-7" of water column) Some gens need 1/2 psi (10-14"). If this is the case the gas co would have to change the pressure at the meter and then you would need to put a regulator for the rest of the home to reduce the pressure down to 1/4 PSI.

I'm sure you verified the pad size to required size. You might also want to note the concrete specs and the HO is responcable to ensure it meets that.

The ATS has to support the ampacity of both feeds. If the OCP on the utility side is 800A, you need to use an 800A ATS even if the generator can only put out 200A.

With a house that large, I'm betting NEC requires an 800A service, or close to it, so you can't get away with putting a 200A breaker on the input of the ATS, because you'd then fail to meet the service requirements. If you can compute the service size smaller, though, you *may* be able to get away with putting a 400A or 600A service disconnect switch upstream and using a smaller ATS.

What kind of transformer does the utility provide for a house that has to have an 800A service?

Time out!!
The OP clamped a leg and got 70 amps! He does not remember the Peak KW demand from the POCO, but it was 'low'.
The 800 amp service is probably overkill.

Do we all agree:
That the CT equipment MUST stay?
The wiring of the transfer switch is explained above.
The transfer switch must be 800 amp rated IF the service is 800 amps.

PDH:
Our POCO here (NJ) would size the transformer based on load calcs submitted for the residence. 800 amp, single phase, 120/240 could be a 25-75KVA pot, pole mounted.

I clamped my 150A panel and measured 3A per phase. NEC calculations say I'm supposed to have a 200A service. The pole pig feeding the whole block is only 25kVA (104A). Do you think I should trust that 3A is my typical load, and put in a 10A service disconnect and a 10A ATS?

My point is that this is a HUGE house, and I doubt they put in an 800A service for the heck of it. NEC calcs take all the loads into account and tell you what your max loads could be, and the size of the service shouldn't be changed unless it be proven via calculation to actually be overkill. It's a very reasonable size for a house with electric heat, though.

Steve:
17K sq ft is a huge house, no doubt about that. Yes, NEC calcs are how services are sized. No, you should not install a 10 amp service & ATS/MTS. (LMAO)

POCO records were mentioned by me, as it is a reliable record of consumption on existing loads, and is used frequently. I did NOT see the OP's 17k Sq ft data in his reply posts until now.

My point was & is...the original (#1) post was regarding the CT & ATS/MTS placement.

No mention was made as to the loads to be connected to the gen and if a gen panel was being installed. Perhaps an engineer may be needed?

800A into a residential installation??!!

I'd like to see the service wires on that.

Big house or not, no way are you going to need capacity like that.

Mike:
600 amp, single phase is 'normal' on the BIG >10k Sq Ft McMansions here; 800 for 17k Sq ft may be overkill, may not.

Depending on the toys; pool; sauna; jacuzzi's, steamers, etc. the 5 to 10k sq ft range from 300 to 400 amps

One EC has a 21k sq ft maxi-McMansion coming soon....800 amp, 3 phase, 4 wire, 120/208 volt.

Now....don't ask about the bills...or the actual loads! LOL

The only problem with the electric bill method is a steady load of just 30a could be a $700-800 electric bill.

Greg:
We get a KW reading record going back xxmonths or years, not dollars

We have begun this installation and yes the transfer switch is rated and wired for 800 amps. We are installing a fused disconnect for the generator to insure it doesn't pull more than it can handle.
This house also has Geothermal heating and cooling which seems to draw less electricity then convential cooling anyways. I have checked the ampacity and it hasn't gone over 120 amps as of yet.
Now on grounding. As I understand(haven't checked with the AHJ yet)this is now the mpd and should be grounded/bonded as such. Also are you required to drive a ground rod and bond the generator?

Driving a groud rod for the generator will accomplish nothing ... though, I suppose, you might consider it a 'detached structure.'

Bonding generator may create a parallel current path with the neutral and EGC depending on how your transfer switch is set up. Should generator frame be grounded - most manuf. require it. Check out the following reference for your scenario:

http://members.rennlist.org/warren/Gen_Panels_Appl_Note_EN.pdf

The generator frame must be bonded and grounded, but HOW it's bonded/grounded is up to you. Driving a supplemental rod is never a bad thing, but it's probably not required. The neutral grounding depends on whether you have a 3-pole or 4-pole ATS, and where the present neutral-ground bond is. It will most likely be floated at the generator.

Don't make more of this than it is.

The transfer switch is probably arranged to make your installation nearly mistake proof. That is, it will have sets of lugs, grouped together and clearly identified as to main power, generator power, and load. From that point on, it's just "connect the dots."

Likewise, the generator likely has a big ground lug for you to use.

As for the "parallel path" issue ... at this size there ought to be no doubt about breaking the neutral, and having a separately derived system. When the generator is running, there is no reason to remain connected to the PoCo wiring in any manner.