There are many factors related to use proper electric device.
Easy to recall those factors are in the way from Grid line to final equipment. Imagine the lines from to AC outlet. From Main grid line to braker. Then use one copper line(usually not to lose any power by connecting lines) to switch box. Then each of several lines from switch box to Outlet. Plug in electric equipment to Outlet.
To avoid any electric issue, every contact should be good. Power lines must be big enough to carry the currency but not to big to generate loss of power. All is important.
Let assume that I am good enough of electric knowledge and I am selling one electric equipment to 90 years old lady who know nothing about electric world, what would be the proper instruction for her to use equipment safely.
When Mitsubishi compose the user manual, do they guess that buyer is car engineer degree? Or at least buyer knows OHM's Law?, Or, buyer must be aware of what is AC and what is DC? Or buyer will know at leas house voltage is 120V AC / 240 V AC?
Absolute not. They understand average people knows some of basic knowledge but they never assume any knowledge degree of buyer. They compose user manual as if we are at least know nothing about electricity and in that point of view, every safety warning comes out.
If you know the OHM's Law, you are about to know everything for home use equipment. The heat is generated by the Resistance. Line itself has fixed resistance, which we know that every wire should have its own specification. With low wattage, 16AWG is good enough not to generate heat for regular home appliances. When you see the extension cord, it has its own specification. Extension cord is usually one plug and one outlet. Power strip usually have one plug and multi outlet. So the line will be easily checked by its specification.
But usually heat is generated in regular daily life appliance is solely from contact. Even though the line can stay good to use high energy, if the contact is bad, the heat will spread all area. This is the main reason why any manufacturer warn not to use extension cord. Even extension cord manufacturer warn not to connect 2 or more extension cord by themselves.
If the cable has enough spec to carry the power, you don't have to worry to much. But we must check the wall outlet contact, which might be loose over years of use. Usually bad contact of old use will not create sudden heat. it will accumulate heat generally. Also regular house has braker to prevent accident cause of high flow of currant. So if you are uncertain of bad contact, watch it over a time and check heat of the wire. Once heat is sensed, do not use that outlet. Check wall outlet first, if the cord is new.
What if the cable is too long, so voltage drop you are worrying, check voltage drop with the tool, if the wire is sound and has enough amp specification. What I mean is it has enough AWG. If you have tool to check Amperage meter, it will be the best. Volt and amp is good, it doesn't affect charging with long extension cord. The warning in the manual is for the people who do not understand my explanation.
