two consumer units


Postby apprentice08 » Wed Apr 16, 2008 5:51 pm

Hi all i need some advice so if anybody can help, it will be well appreciated.

i have been asked to run and electric wire for a shower and put some spotlights.

the house has two consumer units one next to the meter and second one is about 13 meters away.

The second consumer is powered by a 6mm twin and earth cable, white sheathed oldskook , which runs from the first consumer unit and is on a 50amp mcb.

I have no means of running a swa, other wise i would have fed the second consumer unit with a 16mm swa.

The shower is a 9kw and second consumer units has lights and power sockets running off it.

if i run the shower off the second consumer unit, i doubt enough power is coming through a 6mm twin and earth.

What can i do can i double 2 6mm wires and but them on an mcb in the first consumer unit to feed the second one?

if i do double them then the lenghth of both cables will not be the same difference will be about 5/6 meters.

Old 6mm cable runs through joice and is about 12/13 meters long.

i can run a second 6mm cable which will run through the loft and will be buried in wall about 3 meters but length will about 15/16 meters, and atleast 2 meters will be outside on the external wall.

Please can some body help me as i need to get it certified as well so, need to do it to legal standards.
apprentice08
Posts: 6
Joined: Mon Apr 14, 2008 6:39 pm

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Postby ericmark » Wed Apr 16, 2008 10:45 pm

You should not take my word for it but work it out for yourself but some pointers 6mm² cable is listed as 7.3 mV/A/m since V/A = Ω then that’s 0.0073Ω per meter so with 16 meters we have 0.1168Ω and 12 meters 0.0876Ω since 1/R = 1/R1 + 1/R2 the total ohms = 0.050057 with the max of 50A that relates to volt drop of 2.502857V since A = V/R the 16m cable will have 21.42857A and 12m cable 28.57143A so differential is 14.28571% and you are allowed 10% so it will fail. So have I passed?
I use excel to work things out like that. Although you could cancel out many things and do it all in one lump by doing each part any mistakes are easier to see. So if A1 = Cable length 1 and A2 = Cable length 2 and A3 max amps and A4 mV/A/m then C1 = A1*A$4/1000 (The $ fixes the square so I can drag and drop) = Resistance of 16 meter cable and C2 = A2*A$4/1000 = Resistance of 12 meter cable and C3 = both = =1/(1/C1+1/C2) next volt drop D3 = volt drop next Amps so D1 = D$3/C1 for 16 meters and D2 = D$3/C2 next %difference in amps = (D2-D1)/50%. This way one can easy change the base input so 14.6 meters to 12 meters = 9.774436% Out of interest (A1-A2)/(A1+A2)% gives exactly the same answer as I said the formula will cancel out a bit first. But since you may measure the cables using an ohm meter I have used both methods. Excel or OpenOffice.org Calc are very useful tools I do want to know what marks I get I assume you college homework?
ericmark


Postby apprentice08 » Thu Apr 17, 2008 6:08 pm

well eric so whats the solution?

i am stuck on this job, can i use two 32amp mcb each accomodating 6mm cable?

I undertsnad the length of cables is not same. the difference is about 5/6 meters.

What advice can you give on resolving this issue?.
apprentice08
Posts: 6
Joined: Mon Apr 14, 2008 6:39 pm


Postby apprentice08 » Thu Apr 17, 2008 6:11 pm

hi eric thanks for the info

what can i do to resolve this issue, can i put each wire on seperate mcb 32amp?

please give some advice as am stuck on this job.
apprentice08
Posts: 6
Joined: Mon Apr 14, 2008 6:39 pm


Postby ericmark » Thu Apr 17, 2008 9:36 pm

I think you need to forget rule book and use common sense. 28.6 amp is well within the ratting of 6mm cable so in real terms the trip will open before any overload takes place. Sorry I though it was a college homework. If you do want to use separate overloads for each cable use a two pole breaker so both will trip together i.e. double pole 32 amp.
The route is important have you looked at cable ratings? Instead of 6242Y look at Twin & Earth 624B 300/500V BS7211 XLPE/LSZH Low Smoke sometimes called 6242X Batt cables website will give you ratings by using 90 degs instead of 70 you can get away with single 6mm with some routes and 10mm will easy do job.
We all break the rules from time to time look at 110 volt cable on split phase supply i.e. normal 110v in UK 55-0-55 colours of cores should be Brown and Black but nearly every one you find is Brown and Blue.
Your on site you need to make the executive decision! If you do go for two cables I would measure the actual amps with a clip on. If it were me I think I would abandon original cable and run one 10mm
ericmark


Postby apprentice08 » Fri Apr 18, 2008 5:14 pm

[quote="ericmark"]I think you need to forget rule book and use common sense. 28.6 amp is well within the ratting of 6mm cable so in real terms the trip will open before any overload takes place. Sorry I though it was a college homework. If you do want to use separate overloads for each cable use a two pole breaker so both will trip together i.e. double pole 32 amp.
The route is important have you looked at cable ratings? Instead of 6242Y look at Twin & Earth 624B 300/500V BS7211 XLPE/LSZH Low Smoke sometimes called 6242X Batt cables website will give you ratings by using 90 degs instead of 70 you can get away with single 6mm with some routes and 10mm will easy do job.
We all break the rules from time to time look at 110 volt cable on split phase supply i.e. normal 110v in UK 55-0-55 colours of cores should be Brown and Black but nearly every one you find is Brown and Blue.
Your on site you need to make the executive decision! If you do go for two cables I would measure the actual amps with a clip on. If it were me I think I would abandon original cable and run one 10mm[/quote]

Thanks eric, you are a topman, i come to a decision that if i put both 6mm twin and earth 6242Y on a single 50 amp mcb they should be ok.

i appreciate that there is a difference of 5/6 meters difference in length but i think they will be ok.

2 6mm twin and earth 6242Y should run decent amount of power to the second consumer unit.

its only gonna have some sockets lights and shower running which is 9.5 kw, no cokker, so the likely chance of all these being all on at same time is slim.

Thankyou you have been very helpful, without your help i was lost.
apprentice08
Posts: 6
Joined: Mon Apr 14, 2008 6:39 pm


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