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#1
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Correction factors in running cable
I am running standard twin & earth PVC cable. Various cables will be run
from the CU it as follows: clipped to the wall for a short space then under the downstairs floorboards in free space then up the wall buried in the plaster (no conduit) then run between the joists (between downstairs ceiling and upstairs floorboards, laying above the ceiling) At one point, two cables will be passing through the same hole in a standard size joist. The cables will not be grouped together at any point other than passing through the joist - they will be side by side when going up the wall. Does the thermal insulation factor come into this anywhere (e.g. when buried under the plaster?). Does the grouping factor come into account for where the two go through the joist together? Thanks Roger |
#3
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Correction factors in running cable
wrote in message ... I am running standard twin & earth PVC cable. Various cables will be run from the CU it as follows: clipped to the wall for a short space then under the downstairs floorboards in free space then up the wall buried in the plaster (no conduit) then run between the joists (between downstairs ceiling and upstairs floorboards, laying above the ceiling) At one point, two cables will be passing through the same hole in a standard size joist. The cables will not be grouped together at any point other than passing through the joist - they will be side by side when going up the wall. Does the thermal insulation factor come into this anywhere (e.g. when buried under the plaster?). Does the grouping factor come into account for where the two go through the joist together? Thanks Roger Grouping can be ignored if the point at which the cables are running together is less than 1.5 mtrs. When two cables are popping side by side through a joist the grouping factor is totally negligible to the final circuit as the effect is so small that you'd need very accurate test gear to find it. --- http://www.basecuritysystems.no-ip.com Outgoing mail is certified Virus Free. Checked by AVG anti-virus system (http://www.grisoft.com). Version: 6.0.687 / Virus Database: 448 - Release Date: 16/05/04 |
#4
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Correction factors in running cable
In message ,
wrote: [...] The cables will not be grouped together at any point other than passing through the joist - they will be side by side when going up the wall. Does the thermal insulation factor come into this anywhere (e.g. when buried under the plaster?). Does the grouping factor come into account for where the two go through the joist together? As others have said, grouping doesn't really come into the equation through a hole in a joist - but don't try to squeeze cables through a too-small hole, eh? Apart from anything else it'll be a pig of a job pulling them :-) As no-one seems to have covered the point, buried in plaster counts as practically the same as clipped direct, so no derating needed there either. One thing you haven't mentioned is Voltage drop, but again in general you'd have to have a very big house, or an extremely tortuous wiring route in order to hit problems with that domestically. You haven't mentioned thermal effects either - none of this route is to be alongside heating pipes is it? As for the wider comment someone made about domestic circuits being generally over specified anyway, this is of course true (you wouldn't expect anything else on uk.d-i-y, would you? :-). 2.5mm2 cable for example is rated for a full-size ring (32A MCB, 20A cable) even when buried in conduit in a thermally insulating wall. This is installation method 6. Buried in plaster is method 2, and is to be treated as method 1- clipped direct. For completeness: (From table 4D5A BS7671 or 6F in the OSG) Cable Current capacity Size method 1 method 6 Comment (mine) 1mm2 16A 11.5A Generally used on 5A/6A lighting 1.5mm2 20A 14.5A 6A or 10A lighting 2.5mm2 27A 20A 20A radial or 32A ring These next two are the ones you need to be careful about: 4mm2 37A 26A 20A or 32A radial 6mm2 47A 32A Showers and cookers up to 45A It is more usual to use: 10mm2 64A 44A for very heavy (10kW, 45A) showers -- Martin Angove: http://www.tridwr.demon.co.uk/ Two free issues: http://www.livtech.co.uk/ Living With Technology .... DOS Tip #3 : Don't use DOS. |
#5
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Correction factors in running cable
wrote in message
... I am running standard twin & earth PVC cable. [...] The cables will not be grouped together at any point other than passing through the joist - they will be side by side when going up the wall. Does the thermal insulation factor come into this anywhere (e.g. when buried under the plaster?). No, for all the cases you've mentioned the 'clipped direct' (M1) ratings would apply without any derating for thermal insulation. Cables embedded in plaster count as 'clipped direct' unless the plaster is a special thermal insulating type (containing polystyrene beads). If the cables in the wall are touching (side-by-side), the grouping factors (Cg) are 0.85, 0.79, 0.75, 0.73 for 2, 3, 4, 5 cables (falling to 0.70 for 9 cables). However if you space them out so as to have one cable width's clearance between them the Cg becomes 0.94 for 2 cables and 0.90 for 3 or more. With 2 widths clearance no grouping has to be applied. Does the grouping factor come into account for where the two go through the joist together? I'll take issue slightly with what the others have said here since, if you are passing through several joists on a run, the cables will tend to remain bunched between the joists. The 'bunched' Cg for two cables is then 0.80. For a 30/32 A ring circuit you need to achieve 20 A installed rating (or 27 A with a rewireable fuse). So the Cg can be as low as 0.74 except for the rewireable fuse case, where no grouping at all is allowable! It sounds as if you ought to get yourself a copy of the On-Site Guide (new brown one). HTH -- Andy |
#6
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Correction factors in running cable
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#7
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Correction factors in running cable
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