Since it is important not to die, NFPA 70. No multivariable calculus required. https://www.nfpa.org/codes-and-standards/all-codes-and-stand... If you want to know the why, get the handbook too. https://catalog.nfpa.org/NFPA-70-National-Electrical-Code-NE... Alternatively, Epigram 48. Good luck.
I honestly wonder why such a book costs $150. As a layperson, I would have thought that not dying from electric shock must be a pretty timeless body of knowledge. So I’d expect the cost of research, writing and publishing to have been spread out over many decades, in contrast to other literature. How come the book is so relatively expensive? Does electrical code change so often?
What gauge of copper conductor is required for 50m run to a 30A 240v recepticle?
What type of conductors are allowed if that run is outdoors a underground?
How deep does the trench need to be?
What over current protection is required?
And so on.