I second the guy above here: The problem I see most often is that when you have to modify existing installations, parts of them will have been done by people wh

I second the guy above here: The problem I see most often is that when you have to modify existing installations, parts of them will have been done by people who didn't know what they were doing, and if you aren't sharp (a professionally trained electrician) you may not notice subtle signs you are working off something that was not originally done correctly. It may not originally have been a problem (ie triggered something), but when you then modify further and for example draw more power, and you don't realise part of the original wiring was done the wrong way, you suddenly have a fire hazard. In my current home, we had a wall that would 'get hot' in a certain area, and it turned out part of the wires used in the wall were too small to carry the current now going through them. That part was clearly not done by a professional.