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Ampacity – New Wire and Old Rules

An oft asked question on the internet is can a person use an extension lead while it is still wrapped around a drum or even just bunched? An electrical contractor I know was overheard telling off someone that they should never plug in an extension lead that is rolled up even without a load. This is possibly a good measure to take as a precaution but in fact without a load the extension lead is really not going to explode in a ball of fire or burn out. In fact the rules are fairly simple and doesn’t require a brain surgeon to understand this. Or does it?

Burnt power plug
Courtesy Mike Senese – link below.

The common train of thought for heating around the drum: The wire has a resistance.  When talking about alternating current we talk about impedance or Z instead of pure resistance or R. When we apply a voltage across the load this wire resistance will impede the current flow and there will be power dissipation. The power dissipation is derived from P = I²R (or current x current x resistance). With this power dissipation there will be heat. Wire resistance is a measure either with direct current (DC – pure resistance) or with (AC – impedance). It’s this impedance which is increased when wrapped around a drum or even in free air and will cause a voltage drop higher than the free ends (not wrapped) which causes the heating. In alternating current circuits the wire ‘resistance’ increases with frequency.   But as your mains current is going to have a frequency of either 50 or 60 cycles per second (known as Hertz) this would be a constant.  The more tightly wrapped your coil of wire the higher the mutual inductance and therefore overall impedance. Because the impedance increases and power is equal to I²R or in this case I²Z,  the power dissipation by the wire increases. Yes, it also supplies a lower voltage to the load because according to Ohm’s Law V=IR or because the resistance (impedance) of the wire has increased there is a bigger voltage drop across the wire causing a lower voltage to the load. Hope this all makes sense?

However, sadly this doesn’t really answer the question because the live and neutral cables are tightly wrapped together or twisted – *this does have a cancelling effect on the inductive-reactive components so although there will be a slight inductive reactance in the circuit (it is after all copper wire) this will not really cause the type of heating often explained on the web. The resistance of the cable, heat dissipation and the lack of cooling taking place around the drum are the main causes of local heating and needs to be avoided.  Ampacity or the lack thereof is the reason for this heat build up, with insulation and ambience playing a very important role in determining what the load carrying capacity of the wire is.

*mains frequencies of 50 and 60Hz has negligible effect on the inductive reactance of cables used in short extension leads. Not so with radio transmission lines where the inductive and capacitive nature of the cabling plays a crucial role in the design thereof.


4 Core Cable

Now where does Ampacity come into this? Well ampacity is just that, the current carrying capacity of the cable, the cable meaning copper conductor as well as the insulation. This current carrying capacity is based on the type of conductor as well as the core diameter of the conductor, the type of insulation and the ambient temperature. A cable can be designed to carry a lot higher current if it is kept cool BUT the core diameter of the conductor and type of conductor has to be taken into account too.

A thin conductor has a lot higher resistance over a known run than a thicker conductor, meaning more losses and more heat. A general rule of thumb when buying an extension cable is to buy the one with the thickest conductor core that you can afford – some cables are sheathed in one insulator jacket some are split with the earth run on the outside. The more free air around the conductor the higher the energy transfer to free air.

I hope this is not confusing.  Therefore by adding all these snippets of information together one will realise that it is not advisable to run your 2KW heater through an extension cable which is rolled up. It is also good practice to always unroll an extension lead before use.  Do ensure the conductors are designed for the current intended to carry – in this regard most electricians will make up their own extension cables for large loads. Many cheap extension leads designed for domestic usage are not designed to run a 2KW heater – just feel the cable after 30 minutes on full load.  In some instances feeling the cable across it’s length there are certain hot spot areas where the cable may even have broken and now make poor contact once the wire is no longer bunched. Very common with vacuum cleaners. That is why industrial cleaners have very expensive power cables – to handle the current and abuse. So to sum up, when a cable is wrapped around a drum the biggest problem is going to be how to dissipate the heat built up? By uncoiling it. How to reduce the heat if you need to keep the cable rolled up? By using thicker cable.

Lastly: Plugs and sockets.

They are not all the same – ensure that the socket and plug is designed to carry the current of the load. Badly connected sockets and plugs also generate heat. Often a DIYer and even electricians do not connect all the strands of the conductor to the socket (or plug) because it can be a nuisance.  By applying a load to this circuit the weakest link is the thinnest part i.e. the poor connection made or where it’s stated that your conductors are 2.5 mm² they are in fact only 1.8 mm², due to this weakest link.  God forbid that contractors take short cuts and use cabling of lower grade than the designed load. Seen the movie the ‘Towering Inferno”?

Some interesting facts:

  • Common conductor (like copper) resistance rises when heated. (which causes more internal heating).
  • Poor connections cause local heating which can and does often cause fires.
  • Down lighting draws large currents – cabling from the transformer to the lamp must be a short as possible (15cm / 6in). They should be individually fused.
  • Power to your town or city is run through cables at a very high voltage sometimes in theorder of 100s of kilovolts – this is to reduce voltage drop and installation costs.
  • Copper theft is often done while the circuit is live. This is a dangerous practice and will kill you eventually. (not for the sensitive viewer).
  • Inductive reactance or X of L (XL) is equal to 2 x pi x frequency x inductance in Henrys.


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