Tails lengthened to accomodate new consumer unit


Postby d3nb0g » Sat Apr 28, 2007 8:25 am

Hi All
New to the industry with a dilema.

Customer is having the meter box moved which will mean I will need to lengthen the tails by approx. 1metre (total 2M). Original cable is 25mm on a 100 amp 1361 fuse.
Builder wants to run the 25mm tails in the cavity which is allowable under building regs however this does not account for cavity wall insulation in the future which would derate the cable by 0.5 and therefore make the current carrying capacity of the cable too low. But then this is probably the case for 90% of the houses in the uk. Indeed my own house has 16mm tails with an assumed diversity of 40Amps and supplied via a 60Amp 1361 fuse. Derating this by half would mean altering the cable size to 70mm as indicated in Table 4D1A. which would be impossible to terminate in my tiny consumer unit. So a bit of consideration needs to go into how likely the cable will be overloaded and for how long.

Would it be advisable to drop the incoming fuse to 80 amps (from 4D1A 25mm cable ref method 4 will carry 80 Amps) and crimp join the new 25mm cables running to the new box position. If so, can I use heat shrink to protect the cables.
d3nb0g
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Postby sparx » Sun May 06, 2007 6:53 pm

HiSeveral options open as I see it:
First, can't you use a 'Henley Block'? if remaining accessable then this is best way. if not then use heavy duty tube type crimps and ratchet type crimp tool, (not car type squeeze jobs), plus at least 3 layers of heat shrink over crimps, to help with insulation foam and reduce strain on joints try to run in pipe as far as poss. (bath waste pipe good for this, see plumber)
Tails rated very conservatively as actual current drain due to diversity much lower usually than your sums would suggest & at the end of the day what happens later is NOT YOUR PROBLEM! the people adding cavity fill have a duty of care to prevent damage due to their work! sleep well SPARX
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Postby d3nb0g » Fri May 11, 2007 7:07 pm

[quote="sparx"]HiSeveral options open as I see it:
First, can't you use a 'Henley Block'? if remaining accessable then this is best way. if not then use heavy duty tube type crimps and ratchet type crimp tool, (not car type squeeze jobs), plus at least 3 layers of heat shrink over crimps, to help with insulation foam and reduce strain on joints try to run in pipe as far as poss. (bath waste pipe good for this, see plumber)
Tails rated very conservatively as actual current drain due to diversity much lower usually than your sums would suggest & at the end of the day what happens later is NOT YOUR PROBLEM! the people adding cavity fill have a duty of care to prevent damage due to their work! sleep well SPARX[/quote]
Thanks for the advice. Henly block was not possible so I did indeed crimp and join (Crimps were for hire at my suppliers - very handy). My calculations were the worst case scenario of course. With all the main appliances on, range cooker, kettle, washing machine and with under floor heating going pulled 67amps on the tong tester. So come christmas day with all this on and the house full the cats will be sitting in that corner of the house, nice and toasty. Thanks for your help.
d3nb0g
Rank: Labourer
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Posts: 2
Joined: Thu Apr 26, 2007 7:23 am

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