gas pipe sizing

as a rule of thumb what diversity factor should be used when sizing a 2 psi gas pipe system? say for example you have (5) 130,000 btu input gas fired rooftop units,with a design day in the winter of 12 degrees.it is highly unlikely that all of these units would be firing at the same time.is there a diversity factor that should be used when sizing the pipe?any feedback would be much appreciated.thank you