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DIY Forum >> Electrical Questions >> will this be alright? https://www.askthetrades.co.uk/cgi-bin/yabb/YaBB.pl?num=1098546631 Message started by trowelhead on Oct 23rd, 2004, 4:50pm |
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Title: will this be alright? Post by trowelhead on Oct 23rd, 2004, 4:50pm Been to a customers today to price laying a patio.(great weather >:()The customer has already laid the base(solid concrete, 12 inches thick!) and underneath has run the swa to feed his shed.(2.5mm only) The swa is to be jointed into the utiliy room ring via a fcu,on this dedicated ring is washing machine and tumble dryer.Measurements as follows:- cu to fcu point=15m fcu to edge of patio = 4m patio to shed =20m All the elderly chap wants is to run 1 light and 1 double socket for his woodworking hobby,nothing to serious at all. Question is,would this set up be alright for the full length in 2.5mm? If not, could he do an external/underground joint from the edge of the patio and joint the 2.5 to two 2.5's (he has got enough left over,so therefore making the 20m run from the patio to the shed 5mm) If none of the above look like a good idea then i'll have to do what i don't want to do,dig up his concrete base :o Please bear in mind i'm only a humble brickie so if none of my ideas sound sensible,dont shout to loud! :-[ :-[ Thanks in advance. |
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Title: Re: will this be alright? Post by sparkyjonny on Oct 23rd, 2004, 5:03pm You can't use two lengths of 2.5mm as a 5mm conductor. However, the total run of 39m, all of which is wired in at least 2.5mm is sufficient to take a 3kW load (13A fuse in FCU). The volt drop is 8.1v (according to tlc), which although some would argue is not ideal, is perfectly safe, within the regs, and will not present a problem. Fit a consumer unit in the shed, make sure you earth the armour of the SWA well using glands (get a spark in if unsure), and that the shed has RCD protection (either in shed or house). Good news that you won't have to dig up though! |
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Title: Re: will this be alright? Post by trowelhead on Oct 23rd, 2004, 5:15pm thanks jonny,as long as i dont have to dig up and as long as his drill turns i'm happy :) All the works are to be done by his sparkie (or himself,not me),i just didnt want a phone call after i'd finished asking me to dig up and re-lay the patio!I'll tell him the info and put the ball in his court,thanks again. |
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Title: Re: will this be alright? Post by trowelhead on Oct 23rd, 2004, 5:17pm As i've said ,being a brickie and not overly tech minded can you tell me why 2x2.5 cables together will not work as a 5mm conductor? |
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Title: Re: will this be alright? Post by sparkyjonny on Oct 23rd, 2004, 8:24pm They would have to be exactly equal lengths of cable, as if one was slightly longer, the majority of current would travel through the shorter, lower-resistance cable, potentially overloading it. You've also got the issue of two seperate supplies terminating at one point. The only option would be to treat each cable as a seperate supply, with an FCU for each, but you'd have to label the fact you have two incoming supplies in the shed. This is feasible, but 13A should be sufficient for his needs anyway. Glad you noticed the comment, i won't mention plumbers...!! lol! |
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Title: Re: will this be alright? Post by supersparky on Oct 23rd, 2004, 8:29pm its not acceptable in any situation to double cable up weather they are the same length of not(exept rings and HV dist), 2.5mm is fine for the shed in this situation if you wanted to be future proof you could stick some duct in along the surface- ie just cut a channel ss |
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Title: Re: will this be alright? Post by HandyMac on Oct 23rd, 2004, 11:14pm wrote on Oct 23rd, 2004, 8:29pm:
You are the expert so I'm not questioning the advice - but I thought the regs required cables to be buried at some depth and covered with warning tape etc? Andrew |
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Title: Re: will this be alright? Post by JerryD on Oct 23rd, 2004, 11:21pm Yes, regs require (apart from earthed mechanical protection such as armouring) cables to be buried with warning tape or covers, all suitable identified. Also cables must be buried to a depth to avoid being damaged by any forseeable disturbance to the ground. (reg 522-06-03) What does that last sentence actually mean?? Typical regs!! ;D |
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Title: Re: will this be alright? Post by supersparky on Oct 23rd, 2004, 11:29pm were talking at cross purposes here look at my reply then see the reg you quoted lol its SWA so will have earthed protection it will be buried to a depth to prevent damage in the future...its a patio and although this could be disputed, its not likly to be dug up every 5 mins and the 'warning tape or other cover' hence i said ducting ss |
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Title: Re: will this be alright? Post by HandyMac on Oct 23rd, 2004, 11:52pm Thanks ss - just curious, not the sort of job I get involved with (but who knows who might want a patio in the future?). Andrew |
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Title: Re: will this be alright? Post by Lectrician on Oct 24th, 2004, 8:43am The regs to recognise doubled up cables, but it is beond the scope of most people to fit such a system - and I wouldn't want to see it anyway. |
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Title: Re: will this be alright? Post by supersparky on Oct 24th, 2004, 2:29pm same lec, as i see it, electrics need to be less prone to major faults as its oftern installed and forgotten if one of the doubled up cables came out then its higher load on the others, and the chances of a T+I being done on time which might pick it up is unlikly ::) not something id do *Thinks of his 4 core...2 core shed supply.....not to be done by anyone* ss |
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Title: Re: will this be alright? Post by qw on Oct 29th, 2004, 3:59pm Doubling up cables...I can't believe I am reading this....Noone here can be any sort of qualified electrician, otherwise the penny would drop that they are talking about a perfectly standard ring main here...just configured in a slightly different way...no wonder you can't rely on the so called experts to do anything right... |
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Title: Re: will this be alright? Post by billythekid on Oct 29th, 2004, 4:02pm *hides out the way for this one* |
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Title: Re: will this be alright? Post by L.Spark on Oct 29th, 2004, 4:06pm wrote on Oct 29th, 2004, 3:59pm:
And where did you train?.. I strongly question your judgment! :) |
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Title: Re: will this be alright? Post by supersparky on Oct 29th, 2004, 4:31pm do you even know why 24 amp rated cable is allowed on a breaker exeeding the rating of the cable? its not a ring cct its a submain as it supplys a consumer unit. it is also serving one point only not multiple points. ss |
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Title: Re: will this be alright? Post by trowelhead on Oct 29th, 2004, 5:03pm I must say that i asked a sensible(to me anyway ???) question and by consensus of opinion (ie the majority,if not all replies ) said or replied in the same way to it,so as far as i am concerned these respondees (where did i get that word from ::)) are correct. Now to all those, whom i gratefully respect, for answering,(as i've said before,i am just a bricky with VERY basic knowledge of electricity ) can you explain what is the difference in what i thought could be done (connecting 1 feed to 2 cables to the shed) as opposed to a ring as qw says is all that i thought of! As i have said,this isn't an 'argumentative' question,it's just not being a sparky,i can see where he's coming from! (even if the current crop of replies is telling me he's talking tripe) SO what exactly is the difference? |
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Title: Re: will this be alright? Post by L.Spark on Oct 29th, 2004, 8:28pm Quote:
19.5A in a wall or something, ring mains were under threat because it was thought that if cables were run in insulation or similar the cables would not be able to take the load. However there was many tests and it was mutually agreed that the 2.5mm cable can take 20A in those circumstances Quote:
I can't ;) |
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Title: Re: will this be alright? Post by JerryD on Oct 29th, 2004, 11:05pm Can I ask a quick question here? If the cable is rated at 20 amps, how is it that you can use a 32a MCB? My understanding is: Design current </= Protective device </= Cable rating I presume there is some 'correction' to be applied because it is a ring? I need to know, my exams are soon :'( |
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Title: Re: will this be alright? Post by sparkyjonny on Oct 29th, 2004, 11:47pm A cable rating in a ring must exceed 2/3 of the rating of the protective device (there's a specific reg). This is due to the assumed typical spread of a load. It works in practice, but if you think through the theory and want a 100% safe system, then radials are the way forward. Mind you, if you start thinking of this, issues such as double sockets not being rated at 26A are raised, along with many other faults permissible within BS7671. |
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Title: Re: will this be alright? Post by JerryD on Oct 30th, 2004, 12:01am but 2/3 of 32 amps is 21.33 amps, still more than the 20 amp rating of the cable. How is this allowed? |
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Title: Re: will this be alright? Post by L.Spark on Oct 30th, 2004, 12:17am wrote on Oct 29th, 2004, 11:05pm:
Hello Jerry What was asked had no reference to a ring circuit. Hope your exams go well for you and you get the mark you want. |
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Title: Re: will this be alright? Post by L.Spark on Oct 30th, 2004, 12:21am wrote on Oct 30th, 2004, 12:01am:
I want to put this to rest ;) but to awnser your question, ring mains are an exception where the IEE regulations set out the maxium floor area surved, the type and size of cable that can be used, and maxium cable length This allows a ring main of 32A using 2.5mm Twin+Earth Cable + 100M of Cable in total. You could as an alternative have a Radial of 50m of 2.5mm T+E on a 20A CC Or Radial of 100m with 4mm Twin Cable, with a CB rated at 32A.. |
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Title: Re: will this be alright? Post by JerryD on Oct 30th, 2004, 12:27am Thanks for the info. I hope this question comes up in the exam!! :) |
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Title: Re: will this be alright? Post by plugwash on Oct 30th, 2004, 7:06pm wrote on Oct 30th, 2004, 12:01am:
burried in a normal non insulating wall counts as method 1 (the same as clipped direct) and the rating of 2.5mm in that situation is considerablly more than 20A (though not as high as 32A) |
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Title: Re: will this be alright? Post by sparkyjonny on Oct 31st, 2004, 3:32pm 22 or 24A from the top of my head? |
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Title: Re: will this be alright? Post by dingbat on Oct 31st, 2004, 5:13pm wrote on Oct 31st, 2004, 3:32pm:
Table 4D5A says clipped direct has CCC of 27A |
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Title: Re: will this be alright? Post by ban-all-sheds on Nov 15th, 2004, 11:52am http://www.iee.org/Publish/WireRegs/Amd1andRC.pdf |
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Title: Re: will this be alright? Post by L.Spark on Nov 15th, 2004, 12:19pm Read that awhile back, the findings was the important bit 8) |
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Title: Re: will this be alright? Post by ban-all-sheds on Nov 15th, 2004, 5:02pm But back to the plot - what kind of dork goes to all the trouble of burying SWA, knowing that it will be covered with concrete and paving, and only puts in 2.5mm? Stupid git. |
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Title: Re: will this be alright? Post by plugwash on Nov 15th, 2004, 5:53pm or in my case 1.5mm the answer is those who see the current ratings in the sheds and don't know about the concept of voltage drop (or don't realise its significant) also i still think you can get away with a lot more voltage drop than the regs allow for |
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Title: Re: will this be alright? Post by sparkyjonny on Nov 15th, 2004, 6:31pm Yep, for a typical shed load (light, 500w heater, occasional lawnmower) then it's not as big a issue as it's made out to be. Remember volt drop is determined by the drawn current, and is calculated from the design current, not rating of the protective device, so under normal low current loads, the volt drop may well be acceptable. |
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Title: Re: will this be alright? Post by The_Trician on Nov 15th, 2004, 7:07pm And there was I thinking that volt-drop was affected by the electrical resistance of the cable too........! TT |
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Title: Re: will this be alright? Post by sparkyjonny on Nov 15th, 2004, 8:33pm Nope, thats totally irrelivant TT!! lol! I meant the variable which can change after installation. |
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Title: Re: will this be alright? Post by L.Spark on Nov 15th, 2004, 9:09pm wrote on Nov 15th, 2004, 5:53pm:
That's not the point Quote:
And they seem to forget about expansion.. ::) Quote:
Well quite a few people it seems, its got to be better than 1.5mm unprotected flex tho ;) |
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