When connecting two wires and soldering the connection, the solder is there to protect the wires from oxidation, not to carry current. The bare wires should be tightly twisted together so they make a firm, continuous contact. Otherwise the connection will heat and melt the solder, sometimes causing fire. That is why commercial wiring never has soldered joints.
Soldered joints are fine for carrying large currents without melting, even without tight contact copper-copper,
for various reasons:
1) Effective electrical area of the joint is many times more than the wire's cross sectional area, a factor of ten or more, assuming wires overlap for some distance, which they should for mechanical reasons too.
2) Solder is a good conductor of electricity, maybe 10% to 15% of copper - so long as the area ratio
makes up for this the resistance of the joint causes no problem. More area makes up for the larger resistivity.
Brass is only slightly better conductor than solder, and its used for connector pins carry substantial current.
3) Solder melts at a much higher temperature than the wire's insulation, so that even if the joint generates
twice as much temperature rise its not the limiting factor for current in the wire, the insulation is.
4) The solder joint is heat-sinked by the copper wire itself, spreading any extra heat generated.
5) The joint zone has at least 41% more surface area per unit length as its fatter, allowing heat to flow out better.
Soldering in the wild with good quality control is a problem, and its much more usual to use other
techniques where quality control is easier to ensure, and which can be done much faster (thus cheaper).
Also soldered joints require proper strain-relief so some sort of connector housing is needed anyway -
a crimped connection is more vibration resistant than a soldered one due to the focussing of
mechanical forces on the point in the wire where the solder ends causing stress-concentration.
Basically crimping two wires together takes seconds and is very reliable. This is why we seldom solder
For an example of soldered joints in real commercial equipment: high power alternators and generators
and motors usually have soldered joints between the incoming phase pigtails and the individual windings -
this forms a compact streamlined junction which is important in motor winding stacks which get hammered flat into place - adding a hard metal crimp joint would risk cutting through the insulation.
Yes solder can melt in a junction running at high current, but that is much more common on pcb's where
the heatsinking effect of the pcb traces is poor and the overlap non-existent. after construction.
Good electrical connections feature a continous metal-to-metal contact, either a soldered, brazed or welded
connection. The high pressure of a screw terminal causes gas-tight cold welds between the terminal
and the wire strands, for instance, as does the high contact pressures in wire-wrap pins and crimped joints.
Standard sprung-contact connectors are usually much less reliable, unless a reasonable large current flows
to cause micro-welding at the contact points. The lack of gas-tight welding allows oxidation, which is why
gold-plating is required to make a 100% reliable sprung-contact connector (especially at signal levels).