As I said in my previous post I agree totally with Bort, for small wires by all means use electrical grade solder on small wires where an electrical iron works. However, on bigger wires there aren't many irons that have high enough heat flux to keep up with the big mass of copper that is conducting the heat away from the joint. Virtually impossible to get a well wetted joint that way. The only way I have found to heat big wire (10ga and up) is wit a butane torch. Once you have to resort to that type of direct flame heat, you need a flux that can stand up to the flame. Plumbers flux does, electrical flux (organic rosin) just burns and the joint is compromised. Plumbers solder is made of exactly the same metals as electrical solder so for sure it is plenty conductive of heat and electrical current. Even though the good old lead/tin solders are plenty conductive in their own right, the new lead free plumber solders are composed of tin, copper, and a small amount of silver - all very conducive metals!
As grouch says, if you properly fill the joint with solder all of the flux is washed out of the internals of the joint and can be removed from the surface by careful washing. Have been doing this type of big-wire joint for 40 years so I am pretty sure it works! I don't like putting the solder drop directly on an iron tip, especially flux core solder since the constant exposure of the iron tip to fresh flux eats it away rapidly. By properly fluxing the joint and indirectly heating it so the wire itself is hot enough to melt the solder you end up with a perfectly wetted joint that is stronger than the wire itself every time.