What is medium pressure gas?

In residential or commercial settings, medium pressure natural gas is often considered anything between 1/2 PSI (14" W.C.) and 5 PSI.  

Historically almost all residential and most commercial gas systems were designed around the use of low pressure. Generally, utilities would provide a meter and regulator set to customers that was capable of accurately delivering and measuring gas energy at 1/4 PSI (7" W.C.).

As the natural gas infrastructure aged and equipment demands increased, engineers were left to solve how to deliver more gas energy through smaller, rigid pipes without the expense of replacing all of the existing piping infrastructure.  

In a contained pressure vessel, like a pipe, natural gas can be treated as a compressible fluid with a variable energy density that depends on a few parameters like:

Pressure: When more molecules of methane (CH4) are compressed into a fixed length of piping leading up to an appliance, the pressure, energy density are increased. By increasing the delivery pressure in existing pipes, engineers are able to increase the energy density of the pipe and the capacity at which it is able to deliver energy.

As pressure in the natural gas delivery system increased to meet the new capacity demands, new building codes had to be written and new products designed.