Use {{:PAGENAME}} to transclude a mainspace page. (If you don't use a colon, it calls a template instead.)
This doesn't include a header, so you may want a level 1 header, by adding a single "=" either side of the title, on its own line. (The padding space is optional - i.e. "= Title =" and "=Title=" each on their own line, no quote marks, give an identical output.)
For example:
=Earthing (electrical)=
{{:Earthing (electrical)}}
This will also include categories from the original page (unless they are inside a <noinclude> tag where they occur on the original page).