Is it logical that Enel's power is ineffective against Luffy?
No, it isn't logical, but it was deliberate. TVTropes has a trope which perfectly summarizes this kind of phenomenon: "Acceptable Breaks from Reality". 


  A Willing Suspension of Disbelief is a must for almost any work of fiction. There are certain elements of story or gameplay where realism would simply make a work tedious, difficult, or confusing for the audience. Thus there are ways in which works will be blatantly, unabashedly unrealistic, and nobody really minds.


The manga deliberately takes liberties with certain inconvenient real-life physics rules for the sake of drama or balance; Most Logia Devil Fruits would have been way overpowered and/or impractical if the manga followed the laws of physics faithfully. Akainu's magma powers, for instance, would have made him unapproachable because his magma would have vaporized everything without even needing to make contact. For the same kind of reason, Luffy is completely immune to electricity; Enel would have won way too easily otherwise.