I'm no expert but from what I remember you are drawing to much current thru a wire that's not thick enough.
I remember in high school science we made a gauge with a piece of timber and a wire. The more current we put thru the more the wire would bend so we could mark how far the wire bent in relation to current. Too much current and the wire would get hot and eventually break in 2 with a puff of smoke.
Wires are rated for current capacity based on the gauge of the wire. So this means depending on the size of the wire would determine how much current and in turn how much voltage drop that is seen with using a particular size wire.
There are 2 main reasons a wire will get hot. The first reason is the cable is under rated for the amount of current and the 2nd is if there is a short caused by a dodgy connector or water or incorrect wiring, because the short will normally cause more current to flow down the wire than what the wire is rated for.