Gas meter thread sizing standards
Residential, commercial and industrial gas meters have their own standard thread sizes. The gas meter is connected to customer piping through a swivel and nut, which has a dedicated set of thread sizes. These thread sizes were originally named for the amount of gas designed to flow through them in terms of gas lamps, for example a 30-Lt. meter can provide enough gas for 30 lights and was referred to in the late 19th century as a 30-light-gas-meter. These sizes are typically 10Lt, 20Lt, 30Lt, 45Lt, or 60Lt, though smaller and larger sizes are available. The thread sizes are slightly, about 1/16", larger than the nearest size NPT size, in order to accommodate the appropriate inner diameter within the swivel.