We just finished putting our power cable underground, our cables were overhead but the power poles were not behaving themselves, the PEA installed them for us, and we put concrete footings around the bases, but our ground becomes so fluid during the rainy season that they began leaning more and more and the cables drooping more and more.
We contacted the PEA about having them unhook the current cables and hook up the new cables once we were done, and their initial reaction was to question us as to who the electrician was we had doing the job, When our Thai niece told them it was a licensed electrical contractor from America, the fellow she was talking to said "American standards may not be the same as Thai standards".
So our niece asked hi to send us the Thai standards, to which he replied, "we don't have any specific written standards for that".
He said they would have to come and inspect our work before committing to changing the cable over.
We used HDPE and 25 guage NYY cable, as we were unable to source the size of poly we wanted (1-1/4") we used 2 each 1" polys and just ran one cable in each pipe. We dug ditch to 50cm and installed 4 inch metal electrical runner up the side of the house.
When our niece sent pictures of our work to the PEA fellow and said we were ready for the cables to be changed at the meter, his reply was, "go ahead and do it yourself, I can see that you guys did a lot of work". Swapping the cables out at the meter and service in the house went quickly.
Our contractor friend from back home said the use of the NYY cables was overkill, as the 'W" in THW stands for "wet", and that's what code calls for in the rainy Pacific North West where we live, according to him NYY is rated for direct burial. Our cable had been up for the 18 years we have been here, so I don't mind that we bought new cable. The old 25 gauge THW is really stiff and we weren't sure the insulation coating wasn't compromised by the weather here.
It was a lot of work, the ground was rock hard, and finding the correct materials was a pain, but I would do it over again if needed, peace of mind is priceless.