Shower Instructions Can I Use A Bigger Size Cable For My Electric Shower?

Can i use a bigger size cable for my electric shower? - shower instructions

My instructions say that showers use a cable 6 mm, I have installed and used. But I often had problems with replacing the switch (cord) Last Minute (IVE three times) and the heating cables and creating a very bad smell. The ends of the Son of melted insulation, etc., and I reterminated and it happened again. Can and should I use a cable a little bigger? The race is only 10 meters, free standing (not channel) in a loft cavity. and the shower has its own dedicated consumer unit