When using an Electric Hook Up (EHU) cable it should be fully unwound from a reel to prevent over heating.
Even copper conductors have some resistance (Ohm's) to the flow of current (Amp's), and that resistance will hinder the flow of current resulting in some power (Watt's) losses as heat. You will also get a voltage drop across along the length of the conductor.
The recommended EHU should have cable with conductors of 2.5mm2 cross sectional area (CSA)
A length of copper conductor of 2.5mm2 cross sectional area will have a resitting of 7.41 milli-Ohms (mR) per meter length.
So each conductor in a 25M long EHU will have a total resistance of 1,852.5 mR or .185 Ohms.
As the EHU has both a Live and Neutral conductor, both of them will have the same resistance, and becasue the same current flows through both teh Live and Neutral we have to add the values together giving us a total of 2 x .185R = .37 Ohms total resitance in the EHU cable.
The heating effect of a Resistance is dependent on the current flowing through it and at full load that would be 16A.
Ohms law tells us that Power in watts will be Current x Current x Resistance:-
16A x 16A x .37R = 94.72 Watts. That nearly as much heat as an old style 100W filament bulb produced!.
Imagine an enclosed cable reel with that much heat being pumped into it, it wold soon start to heat up. Most EHU cables use a PVS insulation which is only rated to 70C, after which it starts to soften and the insulation may start to breakdown.
IF teh EHU is fully unwound the heating effect is only 94.72W/25M = 3.8 W per meter, and in the cable will easily dissipate that amount of heat to the open air and ground it lies on.