Variations of installation conditions along a route

[SgtTrojan]

Where heat dissipation differs in one part of a route to another, the current-carrying capacity shall be determined so as to be appropriate for the part of the route having the most adverse conditions.

So for a 20m cable run where a cable is grouped with 4 circuits for the first 10m and 9 for the second 10m we assume the circuit is grouped with 10 circuits.. but

For a 100m run of XLPE armoured that passes through a thermally insulated wall with a thickness of 50mm we have to assume the entire cable is affected by the 0.88 factor?

I mean, come on, surely the heat will dissipate along its length?

 

[ban-all-sheds]

As designer you are free to make your own decision on something like that if you genuinely believe that you have sufficient information and knowledge to produce a safe solution which uses a different factor.

 

[bernardgreen]

I mean, come on, surely the heat will dissipate along its length?

Yes it will to some extent. And that extent can be calculated. But the formulas are complex invovling many factors.

Common sense strongly and correctly suggests that for

100m run of XLPE armoured that passes through a thermally insulated wall with a thickness of 50mm

the armouring will conduct heat from the "hot" spot to where it can disapate to the air.

But to calculate the maximum thickness of wall before the armouring can no longer conduct and disapate all the heat generated in the wall is complex.

So take the easy and assured option of avoiding the complex calculations when setting the general requirements and regulations.

 

[ban-all-sheds]

Depending on where the wall is it may be practicable to join the cable either side of it and use a thicker section where it passes through in order to avoid 99.95m of oversized cable.....

 

[SgtTrojan]

Yeah, you see! It's ludicrous

 

[ban-all-sheds]

There are many regulations in all sorts of fields of human endeavour which appear ludicrous to those with insufficient understanding of the matters involved to do anything other than say "it doesn't make sense".

If, as designer, you genuinely believe you have both the information and the knowledge to calculate the actual manner in which heat will be dissipated then you may use a different factor.

If you do not have that information and knowledge then you may not, no matter how valid the "common sense" view is. I'm sure everyone would agree that 0.88 is too low, but if you are going to deviate from that you may not arbitrarily pick another value which to you seems sensible. It's a bit like choosing a protective conductor size - you either use the tabulated value or you do the adiabatic calculation, you don't stick a finger in the air and say "oh, Xmm² should be OK, I reckon".

 

[SgtTrojan]

Agreed. There.. it's out of my system now
Thanks

 

[Chri5]

For DI stuff it tends to be less critical and thus common sense prevails,

Take a cu feed in, with 10 circuits, the last 1m of various runs between 10 and 25m away. At the lowest distance 10m, grouping factor should be applied to 10% of that run distance.

The same cables might be in a very well vented sub floor, then in to a warmer air area and finally via part of an airing cupboard. So again you could calculate ambient temps over each section of the route, or apply the highest temp for the whole cable distance.

The same cable has 5 variants of reference method, each making up 20% of the run.........

As I say common sense prevails.

Circuits or sub CU's with low tolerance between cable rating and load will always be the most influenced by external factors due to the cost requirements of the work. If I quoted for 16mm shower feeds and 10mm cookers, and others quote for 10mm / 6mm, then I don't get the job.

It would make perfect sense to over (cable) engineer every element of a job since the labour would be the same and the extra material cost on a 4 bed would represent 10% of the job (guess figures !). But no one would, because it all comes down to best price and the average client wants best work lowest cost work.

 

[ban-all-sheds]

http://www.amazon.co.uk/Electrical-Installation-Design-Guide-Calculations/dp/0863415504

or

http://www.theiet.org/publishing/books/wir-reg/electrical-installation-design-guide.cfm

 

[Adam_151]

Another thing to consider is that xlpe insulated cables are rated to 90 degrees C, but we often have to use the 70C figures for design unless the switchgear is designed for that, but if the short section through the insulation is in the middle of the cable rather than near the ends, we could say its alright for that section to reach 90C and apply the correction factor to current rating for 90C cables.

Its not an exact science and never will be, it comes down to some approximate calculations and some enginnering judgement....

"Engineering is the art of modelling materials we do not wholly understand, into shapes we cannot precisely analyse so as to withstand forces we cannot properly assess, in such a way that the public has no reason to suspect the extent of our ignorance."

- Dr AR Dykes
British Institution of Structural Engineers, 1976.

 

[bernardgreen]

And sometimes they get it a bit wrong

http://www.youtube.com/watch?v=j-zczJXSxnw