I am planing on installing a 20KW nat gas standby generator.
It will be 250ft from the house ( generator to transfer switch)
Main breaker on generator is 100 amp. The maximum continous load is 75 amps.
I have looked at many on line calculators and get different wire sizes.

Wiring my garage with 4guage copper strand with a 6 guage ground as far as code in ne for continuous 100 amp service. hope this helps, be a good idea to ask about code in your area though

Code is nation wide as in national electrical code

distance is the isue

Im running 100' with mine with air compressor,welder,& whatever other electrical gadjets although not all at once & inspector told me that should not be a problem so long as everything is circuit is protected with the proper breaker. Randy

1/0 is the minimum for 100 amps. I would plan for max output of generator. Due to the length of the run you will be looking at a 3% voltage drop assuming it is a 240v single phase system? A bigger wire will yield a smaller voltage drop but it may become cost prohibitive. 3% is acceptable!

Good luck!

I would see about moving closer, 250' is a long distance away. If generator is a Generac, it is rated at 18KW on natural gas.

I don't want it closer - I don't want to hear the noise. Those things are just too noisy. 250' is it. Gas line and conduit are already in the ground.
All I have to do is pull the wire and hook it up.
Yes I know that it only puts out 18KW as in 75 amps.
The breaker on the generator is still 100 amps so the wire has to be at least big enough to handle 100 amps.

You need 3/0 if you use aluminum cable. 2/0 if you use copper. 1/0 copper is marginal for full load vs voltage drop. You don't want to choke voltage from a generator to your main panel. Motor loads in the home will suffer if your voltage is low. Hopefully you ran a large enough gas line for that distance. Typically homes have low gas pressure; so to compensate you need larger line size. At full load that generator would require 1 1/4 main line for gas volume required. Hope this helps.

2 x 1/0awg black
1 x 1awg white or black with white stripe
1 x 2awg green
This will exceed the amp/load charts for copper wire for power to distance.
The appropriate wire is type THHN/THWN copper buried in 2" sch40 A&U
If you have inspections I don't think your city/county will recognize aluminum on any power supply other than a utility companys service drop.
If the gas run is also 250' I'd guess the btu/hr demand for your gas will require a minimum of 2 psi in an 1 1/2" gas pipe or 3psi on an 1 1/4" gas line with a pound to ounce regulator. If you can't get this high pressure and are stuck with ounce pressure from your utility company you'll probably need a minimum of a 3" pipe at 6 ozi. This generator will have a full sized automobile engine and have a high btu/hr demand.
These long distance applications are better with a propane tank set nearby the generator.
I encourage you to hire a licensed plumber for the gas connections and a licensed electrician familiar with these installs to schedule the service interuption, install the power transfer switch, rewire the line voltage to your existing power distribution center (breaker box) and hookup the associated communication cables which will be required.
I'm shocked you think they're loud, with todays insulated cabinets and muffled exhaust they've never been quieter than they are now.
Note, I'm not an electrician or a plumber but am a general contractor and handle these deals frequently.
Also, I'd encourage you not to place to much demand on the new generator while using it. Even though you have a 100 amp breaker your alternator will be totally tapped by the mid 80's and it's not good to maintain a max rated continous load. You'll hear it if your close as it'll bog down the alternator/powerplant.
In a normal application from utilty provided power the sum of the individual breakers will exceed the main disconnect but in a standby generator aspect the main disconnect tripping amperage will be higher than power output of the alternator. You risk damage to the unit. If I were wiring this at my house I'd replace the 100 amp main disconnect with an 80 amp.

See ya,
Kelly