Electrical Induction, what are the cable limits before it becomes a problem
Induction isn't the problem, even at 50Hz, and certainly not at DC, when winding a few metres of wire onto a spool. What IS a problem is heat : a wire in open air cools reasonably well, and the heat is spread out over its length. On a spool, the heat is concentrated and the inner turns of wire are insulated by the outer layers. Draw too much current - even though the wire is within its normal ratings - and ... well you don't want its insulation to melt.

If your equipment takes 7.5A, this may matter. If your wires are rated for 7.5A but the equipment takes a couple of amps or less, you'll be fine.