Conduit cross sectional area

[Gtfast]

Hi all
As per the 6.5.6.3 Multicore cables in wireways.
When working out the cross sectional area of conduit does one use the inner or outer diameter .This is in order to acertain how many multi core cables one can insert into the conduit .

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[AndyD]

Surely the reg can only be referring to the space available which is the cross sectional area of the void inside the conduit ie work out the available area from the internal radius (half internal diameter) using Pi R squared.

[Gtfast]

Thanks Andy
That is my understanding.

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[Gtfast]

What does one do with regards to stove connections where 6mm flat twin and earth is used in 20mm conduit ,which seems to be the norm .According to calculation it is very border line in fact the cable is a bit big.20mm conduit is 90.75mms with internal radius 8,5mm and the cable is 93,8mms with values 13,4mm *7mm as per Aberdare specs.When doing a COC how does one deal with this.It seems 25mm conduit should be used or change cable to single cores or am I missing something.This also effects if 20 mm flexible conduit is used from isolator to stove 25mm pvc flex conduit should be used or use 6mm single cores

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[AndyD]

..........20mm conduit is 90.75mms with internal radius 8,5mm.........

If a 20mm conduit has an internal radius of 8.5mm then;
8.5 squared = 72.25,

multiply that by pi,
72.25 x 3.14159 = 226.68mm˛

I don't think you need to be that accurate, you can round up to 227mm˛ to make the calculations easier.

If the CSA of the 6mm cable is 93.8mm˛ then the conduit will be 41% filled.

6.5.6.3 Multicore cables in wireways
To determine the size of wireway needed to accommodate cables of different
sizes, add up the overall nominal cross-sectional area of each cable and
ensure that this total area does not exceed the following percentages of
cross-sectional area of the wireway:
a) 40 % for conduit;
b) 35 % for ducting; and
c) 45 % for trunking.

So the conduit would be 1% 'overfilled'....would this be something I'd personally fail on a CoC?...Probably not, I think the discrepency is too small to warrent remidial action. If the radius of the conduit is actually 1/100th of a milimeter larger than you measured (8.51mm) then it would be within the 40% full requirement. I wouldn't consider a conduit being 1/100th of a milimeter too small to be something worth quibbling about.

[Gtfast]

Thanks Andy
With you on that one.

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[Dikela]

Hi colleagues, I read your comments above and am still lost a bit. My confusion is around Reg 6.5.6.3 which talks about overrall nominal cross sectional area of each cable. How do I work overall cross-sectional area of 2.5mm2 flat twin & earth, for example? Because 2.5mm2 referers to each core inside that insulation sheath, but excludes insulation sheath itself.

[AndyD]

Yes, the 2.5mm˛ refers to the CSA of the copper conductor. For the cross-sectional area of the entire cable including the insulation and sheath you'd need to refer to the manufacturers data sheet.