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Topic Title: Current Carrying Capacity for 90deg XLPE ref method 100
Topic Summary:
Created On: 02 October 2017 04:39 PM
Status: Read Only
Linear : Threading : Single : Branch
 Current Carrying Capacity for 90deg XLPE ref method 100   - synapse - 02 October 2017 04:39 PM  
 Current Carrying Capacity for 90deg XLPE ref method 100   - AJJewsbury - 02 October 2017 05:07 PM  
 Current Carrying Capacity for 90deg XLPE ref method 100   - weirdbeard - 02 October 2017 05:57 PM  
 Current Carrying Capacity for 90deg XLPE ref method 100   - mapj1 - 02 October 2017 07:27 PM  
 Current Carrying Capacity for 90deg XLPE ref method 100   - synapse - 05 October 2017 10:30 PM  
 Current Carrying Capacity for 90deg XLPE ref method 100   - ArduinoXR - 06 October 2017 01:22 PM  
 Current Carrying Capacity for 90deg XLPE ref method 100   - leckie - 06 October 2017 05:23 PM  
 Current Carrying Capacity for 90deg XLPE ref method 100   - OlympusMons - 06 October 2017 07:37 PM  
 Current Carrying Capacity for 90deg XLPE ref method 100   - Alcomax - 06 October 2017 08:45 PM  
 Current Carrying Capacity for 90deg XLPE ref method 100   - leckie - 06 October 2017 09:07 PM  
 Current Carrying Capacity for 90deg XLPE ref method 100   - AJJewsbury - 07 October 2017 08:21 AM  
 Current Carrying Capacity for 90deg XLPE ref method 100   - OlympusMons - 07 October 2017 10:52 AM  
 Current Carrying Capacity for 90deg XLPE ref method 100   - AJJewsbury - 08 October 2017 09:49 AM  
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 02 October 2017 04:39 PM
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synapse

Posts: 22
Joined: 22 March 2013

I can't see a table for current carrying capacity for 90deg rated cable buried under insulation (ref method 100). The only table I could find was for 70deg cable.

Have I missed something?
 02 October 2017 05:07 PM
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AJJewsbury

Posts: 15855
Joined: 13 August 2003

Nope, you're right there isn't one. I guess the powers that be think there's little call for using T&E (if the white sheathed XLPE insulated variant) at a conductor temperature above 70-degrees (given that most if not all domestic accessories have a terminal temperature limit of 70 and a general desire to keep sheath temperatures below 60 for burn reasons).

The official answer is probably to use the 90 degree tables and the (limited) information in reg 523.9 - although to sanity check you could maybe look at the re-scaling between 4D5 and 4D2A and apply that to 4E2A.

- Andy.
 02 October 2017 05:57 PM
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weirdbeard

Posts: 3116
Joined: 26 September 2011

Originally posted by: synapse




Have I missed something?


Have you checked that the equipment connected to the 90.C conductor will be OK at this temperature?

-------------------------
:beer)
 02 October 2017 07:27 PM
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mapj1

Posts: 9388
Joined: 22 July 2004

Its not totally silly to ask though, as you can have the ends cooler than the middle of a long run, - specifically if some of the cable run is enclosed in insulation and/or grouped, then you cant touch it, so the only concern about heat is one of setting fire to the building, or cable insulation failure.
Of course there needs to be a reasonable length at each end where it is split out so that it can cool down, before reaching a 70 degree terminal.

-------------------------
regards Mike
 05 October 2017 10:30 PM
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synapse

Posts: 22
Joined: 22 March 2013

many thanks for comments - following the table if it existed would demonstrate compliance. Given the amount of insulation stuffed in new homes using the 70deg table means oversizing cables.
 06 October 2017 01:22 PM
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ArduinoXR

Posts: 34
Joined: 16 August 2017

Originally posted by: synapse

many thanks for comments - following the table if it existed would demonstrate compliance. Given the amount of insulation stuffed in new homes using the 70deg table means oversizing cables.


Not a bad thing in my opinion. If the client isn't particularly cost sensitive there is nothing wrong with this.
 06 October 2017 05:23 PM
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leckie

Posts: 4329
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It wouldn't make any difference if such a table existed. The cable may be rated at 90 degrees but as has already been pointed out, the terminations in anything T&E is likely to be connected to are not. so you have to calculate based on the values given for a 70 degree rated equivalent cable. So you should use the table for normal T&E. You still get the benefit of Low smoke and fume. You could take into consideration what Mike says regarding the ends being cooler that a particular section, but that would need some careful thought.

Regarding increasing cable sizes, well you if the increase is required by the calculation for the installation method then so be it. But it is often fairly simple to avoid this by careful routing and cable spacing. I for one am not a fan of trying to cram larger conductors into terminal. Mind you with the 18th looking into energy saving including IsqR losses of conductors, this might become the norm. I am already seeing specifications asking for spare capacity on cable sizes of about 30%.
 06 October 2017 07:37 PM
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OlympusMons

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You don't have to increase the csa of the whole cable, just the bits of it that are in contact with the insulation.
 06 October 2017 08:45 PM
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Alcomax

Posts: 231
Joined: 12 November 2009

You don't have to increase the csa of the whole cable, just the bits of it that are in contact with the insulation.



That is a whole lot of joint boxes then.

Given that sparky's are the only ones that give a monkeys about de-rating of cables, extra insulation after the event by others cannot be predicted. The only routes of some certainty are sheathed and plastered. Everything else is up for grabs, subject to builders / badgers law.
 06 October 2017 09:07 PM
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leckie

Posts: 4329
Joined: 21 November 2008

Yes, I suppose we really should wire lighting circuits in 2.5mm
 07 October 2017 08:21 AM
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AJJewsbury

Posts: 15855
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You don't have to increase the csa of the whole cable, just the bits of it that are in contact with the insulation.

The conductor temperature doesn't change sharply at the surface of the insulation though - the cable outside the insulation acts as a heatsink lowering the conductor temperature for some distance into the insulation while the higher temperature within the insulation increases the conductor temperature outside the insulation for a similar distance. The actual distances involved will depend on a lot of factors including the cable c.s.a. but typically you'd be looking at about half a metre each way before getting to nominal operating temperatures.

A Wiring Matters article from 2006 dealt with this - http://electrical.theiet.org/w...t-ratings.cfm?type=pdf (note that BS 7671 rating have been revised since then).

The bottom line being you need to have about half a metre of cable outside of the thermal insulation before an accessory.for the approach to work properly.

- Andy.
 07 October 2017 10:52 AM
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OlympusMons

Posts: 51
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If the csa is increased at the point where the cable enters insulation, the cable within the insulation should not be appreciably hotter than that outside the insulation. So no need to have 0.5m tails of the larger csa outside of the insulation before attaching an accessory like a JB. But I think the approach you are referring to is more in relation to using 90deg cable as opposed to increasing csa as in my post.
 08 October 2017 09:49 AM
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AJJewsbury

Posts: 15855
Joined: 13 August 2003

But I think the approach you are referring to is more in relation to using 90deg cable as opposed to increasing csa as in my post.

Indeed - sorry for any confusion.
- Andy.
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