September 2008

Stuff it! - or not.....

Yet again time has flown by & I see that I haven't added anything useful (or otherwise) to this publication since July.... I would blame the summer, but it was hard to find one this year. However, we are enjoying a late surge of nice weather at the moment, & I feel moved to add a bit more information.

Something I came across recently prompts me to write about the regulations regarding the number of individual conductors (which means individual wires of type H07V-U or R) that can be put in various size conduits (or "gaines"). The regulations here in France specify that ⅔ (yes, two thirds) of the capacity of any given conduit is left empty. Put another way, the conductors within it must take up a maximum of ⅓ (one third) of the cross sectional area.

There are very good reasons for this; firstly, it is recommended that you should be able to remove old cables from a conduit & replace them. If the conduit isn't over-stuffed & follows gentle bends this should be possible (& initial installation is easier too, if you need to pull in cables after a conduit is installed). Secondly, the current rating of conductors is based on how they are installed, & bunching too many conductors together in a small space reduces their rating as they could get too hot &, in a worst case scenario, melt.

As an example, 3 x 2.5mm² H07V-U conductors occupy a cross sectional area of 3 x 11.90mm² = 35.70mm², inclusive of their insulation.
A 16mm diameter flexible conduit (known as type ICTA) has a cross sectional area of 90mm², so ⅓ = 30mm².
As the cross sectional area of the conductors exceeds that, you cannot use 16mm diameter conduit to run a circuit of 3 x 2.5mm² conductors. The smallest size you can use is 20mm diameter, having a usable ⅓ of 52mm².

Another example concerns running 2 circuits in the same conduit. Our 3 x 2.5mm² circuit above occupies 35.70mm². Add another circuit of the same size & we get an area of 71.40mm². This exceeds ⅓ of a 20mm diameter ICTA conduit, but is no problem inside a 25mm one, with it's 88mm² usable cross sectional area.

I could publish a complete table here, but the sums are better done at this site here: Electys - Outils de calculs

I said earlier that something I'd seen recently prompted me to cover this subject; the something was found whilst checking over a client's relatively new installation. On first glance I couldn't understand why there were so few conduits heading out from the consumer unit. Further inspection revealed that whoever had carried out the installation had managed to get no fewer than 15 x 2.5mm² conductors into a single 25mm diameter ICTA conduit - 9 more than there should have been! Putting the potential overheating aspects aside for a moment, I can't imagine the struggling that must have been involved to get them through in the first place......

I suppose there can be frustration when you seem to be running vast amounts of conduits to the same place, but if you get yourself some bigger conduit this becomes far simpler. Unfortunately the major brico stores don't seem to offer anything bigger than 25mm, unless you jump right up to the red 40mm stuff which is really intended for underground use. 32mm diameter is a very handy size to have in stock.

Having pulled many a cable through many a conduit, it really does make sense to observe the capacity limits, as well as preparing the cables properly. However, that's a subject for another time.