MAWP of pipe varies according to the piping code you are building the system to. Each code has a slightly different method for calculation of MAWP.
Judging from your description, it could be any one of the following piping codes, depending on what your system is used for and where it is located :
ASME B31.1 "Power Piping"
ASME B31.3 "Process Piping"
ASME B31.8 "Gas Transmission and Distribution Piping"
ASME B31.9 "Building Services Piping"