Not really any limitation to the distance LP Gas can be piped except expense. I'd like to have my tank buried outside my fence, in the recessed entrance area before one gets to my gate. This is so the propane trucks don't have to go through the gate and travel on my driveway.
This will put the tank about 500' from the house and another 150' from the barn. I'll trench down my driveway and use polyethelene piping that's intended to LP Gas use.
The key is to size it properly to avoid pressure loss, just like sizing electric wire properly. The other key is that all propane installations today use first and second stage regulators. The first stage regulator on the tank reduces the tank pressure (which can vary greatly due to temperature, etc) to around 10 psi Then, at the point of use (your house) there is a secondary regulator. If you don't have much plumbing, the secondary regulator may reduce it all the way down to appliance pressure, which is 11" of water column, or approximately 1/2 psi. If you have extensive plumbing, the secondary regulator may reduce it to around 2 psi, and appliance regulators are used for the final reduction.
In my case, I'll have a secondary regulator at my house and one at my barn. From the house, I'll also take off to a picnic pavillion with a 2psi branch and use an appliance regulator at the built-in grill.
There are sizing charts available that show the correct sizes of pipe to use at a given presurre over various distances. The more pressure, the smaller the pipe has to be. I have the chart packed away; let me see if I can find one online.
OK,
here's one. The capacity is measured in BTU's per hour; this is something like amps. You determine total BTUs needed by adding up all the applicances that are likely to be running at the same time. For example, if the furnace has a requirement for 150,000 BTU/h and the water heater for 30,000, you need piping capable of delivering 180 thousands of BTUs. Add a gas range and it might jump to 245 thousands; a gas grill might be 30 to 60 more. You could run your generator on the same pipe off the same tank if you size it correctly; I found a reference for 12KW to 15KW generators that require approximately 200 to 260 thousands. So, a total might easily be 500 to 600 thousand BTU/hr IF everything was going to be running at the same time.
Using the chart, at 10 psi, 1/2" IPS pipe will supply 508 thousand @ 400'. Going to 3/4" will supply 1,072 thousand at the same distance. If you want to go more than 400', I'm sure there are other charts.
Polyethelene gas piping is perfectly safe for outdoor, underground use. It's what the gas companies use. You do need special risers where the piping comes up out of the ground; the polyehtelene must remain buried.
By using one tank for the house and the generator, you may save enough money to pay for some of the pipe (if you plan to buy the tanks). Also, while LP Gas has a long shelf life in the tank, it doesn't hurt to keep it circulating through use, which would be less likely to happen with a separate tank for the generator.
