Feasibility and procedure for getting graduate admission in CS with non-CS-related engineering background?
Yes, it is possible to get an admission for MS or PhD in a US university having studied Bachelor degree in 'Non Circuit" field. This is more true if you have strong drive, skills and suitable projects, seminars, publications to support the same. However, to get into the top 20s would be a special case though not impossible. 

Try approaching professors from top universities in CS showing them your credentials. If you are able to convince a professor, the battle is more or less won.

One of my friend did his Bachelor in Civil Engg. from a reasonably good non-US university but got admission in Electrical Engg. in a US university of medium rank.