Two tank families
DOT cylinders are portable, used for grills, RVs, food trucks and small applications. They are filled at exchange points or local dispensaries, and must be recertified 12 years after manufacture. ASME tanks are permanently installed on residential or commercial property, larger (120 to 1,000+ gallons), and don't require recertification.
The 80% fill rule
Propane expands and contracts with temperature. NFPA 58 requires tanks to be filled to a maximum of 80% of water capacity, leaving 20% headroom for thermal expansion. A 500-gallon tank therefore holds 400 usable gallons. The 80% rule is universal and non-negotiable — no legitimate dealer fills above it.
Core components
Every residential tank has the same basic fittings: service valve (main on/off), fill valve (where the truck connects), vapor outlet (to the regulator), relief valve (vents pressure if overheated), gauge (typically a dial reading 0–80%), and liquid level valve (used by drivers to confirm 80% fill). See tank fittings.
FAQ
What's the difference between LP and propane?
LP gas (liquefied petroleum gas) is a category that includes propane and butane. In the US residential context, 'propane' and 'LP' are used interchangeably — both refer to the C₃H₈ propane molecule.
Why is propane stored as a liquid?
Under modest pressure (~120 psi at 70°F), propane liquefies. Liquid propane occupies about 1/270th the volume of vapor — vastly more efficient to store and transport. Your tank stores liquid; the regulator converts it to vapor as you draw appliance load.