I’m considering putting in a Generac 20-22kw home generator. If I do I want to run it on propane. I’m not familiar with the underground line sizing. Is there anyone on here that know how the calculation works? I am thinking I would run the propylene line with a tank being approximately 180-220’ away.
The generators full load requirements is 10-12 in water column and uses 3.9 gallons per hour. What would be an adequate run line size?

This is all Greek to me.
I’m thinking it’s 1” line.

Let whoever is installing it figure that out. They're used to doing those sort of calculations and if something is off it's on them and not you to deal with it.

I am not doing a turnkey for over $10k-12k. I can buy a new generator and switch for $5k and an electrician doing side work is under $1k. The propane company provides the tank and will hookup the ends with regulators for $1600 although if I can just rent vs buying that would even be cheaper.. My responsibility would be trench and lines. Saves about $3k.

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High pressure from the tank to the generator will use a smaller line vs low pressure. At my hunting camp my supply is high pressure 3/8 and about 75 feet. On that line I can heat,cook,hot water, 4kw Onan gen set all at the same time off the 3/8 soft copper line. At my house the tank is about 100 feet and on the 3/8 high pressure feed it would run a furnace, dryer and cook stove with no issues.

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Straight from a 250-500 gallon tank.

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I just went thru this on an Onan whole house generator that I installed myself. First, a 1" line is way oversize. That is what you would use for natural gas. First you have to understand how propane gas works. In your large storage tank, the propane is under high pressure. It will be between 50 and 200 psi, depending on the outside air temp and if the sun is shining on the tank. At your storage tank, you will have a high pressure regulator that takes the high pressure and reduces it to 10-20 psi. The line from the high pressure regulator to the generator will be 3/8" or 1/2" copper. Now at the gen, you will have a low pressure regulator that will take the 10-20 psi and it knocks it down to 11" water column. That is like 1/2 psi. very low pressure. The low pressure reg will have a 1/2" npt outlet. From there you can run a small length of black pipe to the gen.

I would call Generac and get the straight poop.