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Correction Factors for Ring Main

Postby JMGWEY » Tue Nov 23, 2010 9:48 pm

I am designing a new ring main for the kitchen and am looking at the route of the cable for correction factors.

If the cable TO a socket is routed with the cable FROM the socket under the same capping, is this classed as two circuits and needs a correction factor of 0.8 or is it classed as one circuit and needs no correction factor?

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Postby ericmark » Fri Nov 26, 2010 4:40 pm

It is two cables and needs some allowance as a result. However it is rare to use singles and once one adds extra insulation the much of the calculations shown in the on-site guide is not really valid.

There are a few problems in that to radiate heat needs some contact with a heat conducting material and placing two lengths of twin and earth in oval conduit because they are a tight fit will likely radiate the heat better than same cables in two separate oval conduits.

In other words it is not an exact science and one has to use some common sense. Where one feels one may be close to the limits then XLPE (Cross-Linked Polyethylene) or LSZH (Low Smoke Zero Halogen) seem to be the answer. Rated at 90 degs rather than 70 degs gives that extra leeway. The same with Ali-tube cable which has added advantage of not needing conduit or capping and they can be dressed with ease very like mineral cables.

I have questioned the wiring of kitchens many times and if we follow the regulations than fixed items over 2kw should have a dedicated supply and so washing machine, tumble drier and dishwasher should not be on a ring main but should have there own supply. However this is rarely done.

So if the kitchen is wired correctly as to supplies to fixed appliances then there will be no problem as to ring main overheating.

However one can't have rules for when rules are broken. And we all break the rules to some extent so what I am saying as designer you must make the executive decision and decide what rules you are going to break.

It seems odd to me that Ali-tube cable is specially designed for wiring the modern house yet the 17th Edition still gives the values for 70 deg twin and earth cable. We have to make a compromise and one can't always follow the rule book to the letter.

For example with split phase 110volt we should have cores coloured Brown, Black and Green/Yellow remembering there is no neutral. However try buying it and you will find it very hard to locate. Nearly all coloured Brown, Blue, and Green/Yellow. One should therefore sleeve the blue with a black sleeve. Yes and we are really going to do that. However no one is going to tell you it complies.

So sorry this is something you need to work out for yourself.
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Postby sparx » Sun Nov 28, 2010 8:42 pm

hi, very commendable to be trying to use correction factors BUT WHY are you?
If you read the rules for ring final sub circuits it says (& always has said)
if you stick to the parameters set
ie one ring for each 100m2 of floor area,
2.5mm2 cable,
no more on spurs than on the ring,
32A mcb,
no heavy fixed loads,
balanced spread of outlets over the ring

then 'No further calculation are required'! (appendix 15 of the big red book of fun).

The IET designed such circuits to save us having to redo the maths every time so why look for work?
Not knocking your principles but it's just not required.
regards Sparx
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