Good point Trumpy.
I have a neighbor who is quite capable of carrying out basic installation to an acceptable standard -- installing a receptacle, wiring a switch etc. I've seen his workmanship, and it's fine (better than some supposedly "professional" installations, in fact).
But he freely admits that he doesn't know how to calculate cable sizes, or how to wire more complex arrangements such as heating controls. I'm happy to help him out with diagrams and calculations, and I know that in the case of something being unclear that he'll come back and ask me to go check it for him before he throws the switch.
There are other people to whom I'd be reluctant to give any information, because I've seen the mess they made of something as simple as replacing a switch.
