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# Loss Of 'power' Using Extension Cables ?

Puzzled001 | 08:53 Mon 07th Jan 2013 | How it Works
Hi, Is there a significant 'loss of power' using a 100m/2.5t&e extention cable. Never thought of it before but a contractor remarked that his SDS power drill seemed 'under-powered' and thought it could be due to the long lead. Any help appreciated.

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I had this explained to be years and years ago, but for the life of me can't remember now.

A 100m 2.5 extension lead won't have the oomph needed, at that length a bigger mm would be needed, even a 4mm might not be enough.

Hope someone comes along with the right information.
Not something I've experienced nor would have expected, but all electrical conductors (super conductors excepted) are imperfect and have some resistance: and given a long enough length will act as a load, warming up and thus losing power along it's length. The result will be a reduced voltage at the end the tool is plugged into and consequent loss of oomph.

As albaqwerty suggests, if you need all that length use a higher rated power cable.
The voltage drop of 2.5mm squared cable is stated as 18mV/A/m (source The Electricians Guide 5th Edition). This is the 'double' voltage drop, allowing for both the live and the neutral return.
A power drill of this type might be expected to draw say 3A.

The voltage drop is thus:
(18*100*3)/1000 volts or 5.4V (or 2%). The power loss is double that, of course
Question Author
Thanks all for your replies. Buildersmate, as ever on the mark, with a power drop of 4% will it cause any significant harm to appliances connected ?
If appliances can not cope with that small reduction without harm you should return them as unsuitable for use.
Using a 2.5 flexible cable of 100m in length supplying a 5a load the volt drop would be ....19mV x 5A x 100m =9.5v. Given that the socket feeding the load could be remote from the intake and the supply could be as low as 215v at the intake it could result in a loss of about 30v....quite significant.
Question Author
Thanks all, again. Better get 'our jenny' to lend a hand .
If you are using the extension cable un-wound from the drum, then at 100m length there will be quite an inductance effect due to the physical arrangement of the cord, restricting current flow.

Normally extension cables are marked, warning against being used at full load un-wound, which can result in over heating and even a fire in extreme cases.
BM....I'm confused about the "18mV being the double Vd"
An easy to use Voltage drop calculator is here;

http://www.tlc-direct.co.uk/Technical/Charts/VoltageDrop.html

My post should have said 'wound on the drum' and not 'un-wound'.
BS: the table I used said 18mV per Amp per metre of cable, yours says 19mV per Amp per metre of cable, so hairs seem to be being split, but we seem to be saying the same thing anyway.
My point was merely that the 18mV was per metre of cable, not per metre of conductor, of which there are 2. Simples.
Hymie //If you are using the extension cable un-wound from the drum, then at 100m length there will be quite an inductance effect due to the physical arrangement of the cord, restricting current flow. /

Incorrect. The active and neutral currents completely cancel the already trivial inductance (at mains frequencies) in the air cored coil.

Cables should not be used wound up because of the poor cooling not anything to do with inductance.

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