It depends on the pipe material you are going to use, if it will be plastic steel ductile iron etc. So if you can answer to me about this I can help you
According to the European standards in metal pipes the nominal diameter is equal to the internal diameter. In plastic and cement pipes the nominal diameter is the external diameter. That is how it is categorized. Hope I was useful.
I WOULD LIKE TO GATHER SOME INFORMATION / OPPINIONS OF GRP PIPES IN COMPARISON WITH OTHER MATERIALS AND WHERE I COULD FIND SOME RESEARCHES FOR THERE LONG TERM BEHAVIOUR AGAIN IN COMPARISON WITH OTHER PRESSURE PIPE MATERIALS.