Eric, I would get in touch with your propane supplier for suggestions on the type of underground supply line to use. They should be familiar with any problems with corrosion, etc. Copper is the material of choice around here for propane but since some areas have corrosive soils talking to a local expert would be wise. Also, there is a CSST product designed for a outdoor use that may be a cheaper choice than copper.

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Jim Luttrall
Mr. Inspector.net, Inc.
Allen, Texas 75002