BIGGUNDOCTOR, Did you get the specs on the 30kw machine? I might be interested. Might be down your way the end of April. I have been using 3/16" tubing, it is easy to form. When I started I was using 1/4" and read somewhere that folks were making coils with 3/16 so I thought it might be better to use. What determines the size tubing you use. I have a 15 and 25 kw unit.