"Heavy Duty" is in the eye of the manufacturer! I consider 22 gauge to be standard duty; most manufacturers consider 22 gauge to be heavy duty. I consider 20 gauge to start the heavy duty size but that is just my opinion. I use 6 volt or higher batteries and don't think the standard wire many servo manufacturers supply will allow full usage of supplied voltage. I agree that probably the length of the extension is a better determinant of how "heavy" the wire should be than any thing else. I make up my own extensions, soldered to the servo on one end and new, crimped plugs on the other end.