What is "cable derating"?

wonderwire · 28 Dec 2016

I've been reading about power and lighting cables buried in insulation (yes, I am just that rock and roll).

I understand the 17th Edition IEE Regulations state you must 'derate' the cable if it is buried, by up to 50%.

However, can someone explain what this means in practice? Simply that you don't place more than 50% of the original load on the circuit? Is there another practical consideration / can someone give a real world example?

Thanks

ColJack · 28 Dec 2016

If a cable cannot get rid of the heat generated by the current running through it then you have to lower its maximum rating.

If it's in insulation for example, or bunched with other cables, or in a hot environment.

Likewise if it can get rid of more heat than it's designed for the next you can usually get a little bit more out of it.
That's the reason for different ratings for different installation methods.. if it's clipped to or buried I need something that takes the heat away faster etc..

wonderwire · 28 Dec 2016

Thanks @ColJack, that's helpful - what does this mean in practice?

Is there something electricians actually do to "derate" a cable, or do they simply advise not to plug in high load appliances?

PrenticeBoyofDerry · 28 Dec 2016

A cable's current carry capacity can be effected by the route and method it is installed.
If it is surrounded by thermal insulation, the heat generated by the flow of electricity can not be dissipated, as it can be, when not surrounded by thermal insulation. Therefore it is required that calculation are factored in, so the cable is sized correctly for the method it is installed.

wonderwire · 28 Dec 2016

Thanks @PrenticeBoyofDerry, same question as to ColJack - what does this mean in real life? Is something done to the circuit to derate it, or is the homeowner just told not to plug in 13amps?

PrenticeBoyofDerry · 28 Dec 2016

wonderwire said:
Is there something electricians actually do to "derate" a cable, or do they simply advise not to plug in high load appliances?

A calculation is factored in to derate the cable, or alternatively reference methods tables within BS7671 can be followed, and cable sized from there.

wonderwire · 28 Dec 2016

So if it is a 2.5mm power cable, derated at 50%, it must become 5mm?

ColJack · 28 Dec 2016

Generally it's done at install..
any de-rating is taken into account and the cable and / or breaker sized accordingly.

So you get 1.5mm cables for lights on a 6A breaker.. etc..

PrenticeBoyofDerry · 28 Dec 2016

wonderwire said:
So if it is a 2.5mm power cable, derated at 50%, it must become 5mm?

Does not quite work like that!
2.5mm twin and earth is normally rated at a maximum of 27A, so if you have situation where the cable has to be derated by 50%, then you then need to consider what sized cable can be safely pass the current for the load of the circuit, taking into consideration of the design current and the over current protective device.

charliegolf · 28 Dec 2016

Or is it that a cable rated for 32A derated by 50% because of insulation/heat factors should only be rated for 16A?

ColJack · 28 Dec 2016

wonderwire said:
So if it is a 2.5mm power cable, derated at 50%, it must become 5mm?

Yes and no..
you have a design current ( e.g. A 20A radial ) and you know that the route / method means you have to de-rate any cable by 50% so you pick the cable size that is rated to at least 40A to start with..

PrenticeBoyofDerry · 28 Dec 2016

It maybe worth you reading the following link, it is of an older version of BS7671, but the principles still remain.
https://www.tlc-direct.co.uk/Book/4.3.6.htm

PrenticeBoyofDerry · 28 Dec 2016

charliegolf said:
Or is it that a cable rated for 32A derated by 50% because of insulation/heat factors should only be rated for 16A?

That in simple terms would be true, But it would really be that a cable that requires 16A, should have a cable rated at no less than 32A at a 50% derating factor.

ericmark · 28 Dec 2016

I had a java script program on my phone to work out cable sizes. I am sure there are programs on the internet where you can just bang in figures and get cable size. However in real terms it's not that simple, if we take a length of trunking coming out of a consumer unit packed in tight then the maximum draw for all the cables is 100 amp, and each cable will help remove heat from the few which are hot so in real terms they don't need de-rating.

With cars and trucks many times I have found a wire in free air burnt out, but inside the harness it has been OK. The problem is until you strip harness you don't know it's OK. Where cables are not tight then air can't circulate so you can get pockets of over heating. I had one where some one replaced a 0 - 110 volt transformer with a 55 - 0 - 55 volt transformer so there was no longer a neutral but line 1 and line 2. As a result there should have been fuses or MCB's on both lines, but line 2 had non. So an earth fault caused an overload burning out the cable. On stripping it was found it had melted in odd areas and also melted other cables meaning whole machine needed a re-wire. As a result we tend to over do the de-rating as it's such a huge job if you get it wrong.

winston1 · 28 Dec 2016

ColJack said:
Generally it's done at install..
any de-rating is taken into account and the cable and / or breaker sized accordingly.

So you get 1.5mm cables for lights on a 6A breaker.. etc..

Completely over the top. 1.0mm cable is the norm for lights on a 6 amp breaker even if it is buried in loft insulation.

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What is "cable derating"?

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