Ahh, but the whole power plant is not built to just one code. Sorting out which code applies where can be difficult on some of the older plants where record keeping and even drawings are scarce. ASME Section I for the boiler proper, B31.1 for the steam piping, API for oil storage tanks, AWWA for condensate and raw water tanks and circulating water piping, D1.1 for the structural steel and pipe supports, HEI specs for the condenser, ASME VIII for the feedwater heaters and deaerator, D14.6 for forced draft and induced draft fans,...
I have no problem with folks trying to learn about which code they must follow, or which code would be best when you have the option to choose. That's how you learn to be better at your job.