help please


Postby the man » Mon Mar 07, 2011 7:08 pm

i am doing a home course b-tech, and there is one question bugging me

the question is, design a ring main circuit from scratch, and show all the design stages of cable calculation, i have got so far and i am stuck, so can you please help..........this is what i have got

max wattage for ring main = 7200W

so 7200 / 230 = 31.3 Amps

32 Amp rcd reqd

correction factors

32 / 0.88 x 0.85 = 32 / 0.748
= 42.7 Amps
the osg gives 2.5mm when clipped direct 27 Amps

so please how do i continue, do i half the current (42.7 / 2 = 21.3Amps)

please help.

the cable length is up to me to decide what length i should use. i was going to use 30 meters, to work out VD
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Postby ericmark » Tue Mar 08, 2011 1:59 am

The whole problem with a ring main is it follows some odd guide lines.
For example it is considered that the 32A is used as follows:-
20A from centre point
12A evenly spread around whole ring
So for loop impedance and volt drop the design amps is taken as 26A not 32A.
The cable where 2.5mm is used is considered as being rated at 20A so total of 40A.
So you have to allow for this and by time you use the formula in BS7671:2008 working on volt drop I think it works out at around 106 meters.
So correcting for volt drop the 18mV/A/m becomes 16.5mV/A/m
So Ct = 0.9177627 with tp = 70 and Cg and Ca = 1
In real terms no one would want to work to the limit and with a 100 meter role of cable per ring main by time it is trimmed at every socket likely it will comply.
However I have had many a heated argument as to the use of 4mm cable to reduce volt drop and loop impedance. The problem is regulations say where there is a change in current carrying capacity there should be a current limiting device i.e. fuse or MCB clearly using 4mm cable for part of run does not fit into the rules yet it makes sense and I have in the past used 6mm or more to do long run then dropped to 2.5mm around local area of ring main. This however does not follow rules.
Same applies to house where it has been extended and some rooms have plastered walls and others have bare stone. With the latter using 1.5mm mineral cable allows it to be hidden in the mortar of the stones making a very neat job but where plaster is used then return to cheaper 2.5mm twin and earth or Ali-tube cable. However it does not strictly comply although nothing wrong with it.
In the real world we do it. We know it is safe. But for an exam it is another story.
The other point is side to side split instead of upper and lower floor split. By splitting side to side you use less cable than splitting up and down and technically there is nothing wrong with it. However since the norm is up and down there is a chance that some one will not prove dead and will try working on a live socket. Point is of course it would be their fault not yours but it could be argued that stickers should be used on every socket to identify which supply it was fed from in same way as when three phases are used.
The college lecturers will receive guidance on what is expected and will in turn pass it on to you. However once you have the B-tech you will know enough to make up your own mind. I would hope you have some lecturer who you can ask questions and I would ask them rather then other electricians as they get the feed back we don't as to what is expected.
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Postby kbrownie » Tue Mar 08, 2011 4:13 am

Normally RFC are pre-designed for domestic properties
But assuming this is not the case and taking in to consideration that I am guessing what question was asked, these are my calcs on the info given.
Not sure why you have come to conclusion over 2.5mm cable?

Step 1) find design current
Step 2) find overcurrent device
Step 3) Correction factors
Step 4) select cable

Step 1) =31.3A
Step 2) =32A
Step 3) so I assume ambient temp=1, grouping factor=0.85 and thermal insulation=0.88
=32A/(0.88x0.85)=42.78A
Step 4) ref method c plus grouping factor(depending cable type) gives you 6mm cable.
(7.3x31.3x30)/1000
gives voltage drop of 6.85
so your stuck with 6.00mm
Last edited by kbrownie on Tue Mar 08, 2011 5:49 pm, edited 1 time in total.
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Postby the man » Tue Mar 08, 2011 5:39 pm

thanks guys for your help. tbh i have posted this question on another lets say not so friendly forum,and the flack i got was beyond belife. the two that answered thought i was bogus, and it was a redicules question, the two answers i have recieved from your kind selves is much appriciated, and many thanks for taking time to answer.


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