Quote: Just_Chilin wrote in post #10

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A coiled reel of electric cable is essentially one half of a transformer. The current in the cable creates a magnetic field and that impacts the overall impedance of the cable when its coiled (all in the same direction). Its this impdeance to current flow that creates power loss, moreso when the the cable isnt uncoiled in a random or straight-run manner hence the two ratings. Power is converted to heat and its this heat that eventually melts the insulation.

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Sorry but that isn't true. Here's why but it's a bit wordy....

Yes, if the cable was coiled and made from a single conductor that would indeed create an inductor which would have an impedance associated with it and that impedance would be in addition to the resistance of the cable. However, the inductive impedance from the single core coil is incapable of dissipating any power because it is purely reactive. So, no extra heat there. The resistance of the cable is of course not affected by the fact the cable is coiled. Now in a proper situation with two cores in the cable the current at any one instant is actually flowing in two directions; form the live through the load and back the other way through the neutral. Each of these currents does produce a magnetic field and the field from one core is equal in magnitude to the field from the other core... BUT... since the two currents are flowing in opposite directions the two fields are also in opposite directions and so cancel each other out: the result is no net inductance. Forget the Earth - no current is flowing in it - you are in trouble if there is!
So no extra power for two reasons, firstly because a pure inductance cannot dissipate power and secondly because there is actually no net inductance from the coil anyway due to the equal currents flowing in two opposite directions.
So why do cables on cable reels heat up? The answer is simple and not very glamorous I'm afraid. Like every conductor the cable has a small but measurable resistance to current flow and as current flows through the cable it warms up. The actual amount of heat produced (in Watts) is the current squared (in Amps) multiplied by the cable's resistance (in Ohms). Normally when the cable is unwound this heat dissipates freely and the cable stays cool. When the cable is coiled however, there is nowhere for the heat from the inner coils to go (they are thermally insulated by the layers of cable surrounding them which are in turn producing heat) so they get hotter and hotter. If you are drawing a fair current, which you will be with a vacuum cleaner or a powerful appliance on an extension reel then the conductors will get so hot that they will easily melt the insulation.

How much heat is produced?

For, say, a 15m 1.5mm2 extension the resistance is about 0.2 Ohms so if you draw 10 Amps, the power lost in the cable (heat produced) is about 20Watts.

We are ignoring the Earth conductor in all of the above - no current is flowing in it - you are in trouble if there is!

Cheers,
Steve.