NPT male thread pressure ratings

I am looking for some information on the pressure ratings of male NPT threaded connections in Hydraulic systems. I have dug around and I understand the burst pressure of the DOM tube and the 6M female fitting that is being used, but the actual pressure at which we are in danger of NPT threads failing has eluded me thus far. If someone knows where I can find the answer please let me know.

Thanks,

J - Texas