Cables Older/Newer


Postby billobach181 » Thu Nov 26, 2009 10:41 am

Can you tell me what is the difference in Current Carying ability of the Old 10 mm2 cable and the New 10 mm2 cable? I have a shower at present with the Old Red and Black 10 mm2 feed [8 metres long] and want to know if I can use this same cable, with the necessary RCD, for a 10 Kw Shower or should I run a New Cable?

Both cables have 7 copper strands but I'm not sure if the strands are the same X sectional area.
Regards
Bill Howells
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Postby sparx » Thu Nov 26, 2009 4:30 pm

hi 10mm = 10mm! lectrickery is colour blind.............
current rating for 10mm is anywhere between 32 & 64 Amps depending upon installation method,
10,000w / 230V =43.5A so answer is.....table 4D5 of regs book which needs more info,
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Postby billobach181 » Thu Nov 26, 2009 6:09 pm

Hi Sparks I realise electric is colour blind but what I meant was has the modern cable better current capacity than the older cable due to new technology. It appears to have thicker strands! My installation is more akin to Method 1 than Metod 3.
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Postby ericmark » Thu Nov 26, 2009 8:22 pm

Cables have moved from Imperial to Metric and 7/0.052 is closest to 10mm² at 9.4mm². So for Reference Method 100# (above a plasterboard ceiling covered by thermal insulation not exceeding 100 mm in thickness) instead of 45A it would be around 40A. Not only less cross sectional area, but also cross rubber insulated, so likely 60ºC instead of 70ºC but this is a bit of a guess as I don’t have any books that old.

Modern cables are in the main PVC so rated at 70ºC but there are also Ali-tube or other XLPE (Cross-Linked Polyethylene) or LSZH (Low Smoke Zero Halogen) cables. These are rated at 90ºC so are likely to be 58A in a similar installation.
However although you may have method 100# you could be looking at method #102 (36A) or methods #103 (32A) and to give ratings of cables without knowing installation method would be very miss-leading.

We are not allowed to give links or post pictures on this forum but I am sure you can find Batt or Eland cables web site to get more information.

The problem today is insulation and it is near impossible to tell post fitting what the cable runs through. So really only safe way is never to up-rate any protective device and if it had a 32A type B you leave it with 32A type B.

The circuit breakers and fuses for time a shower is used will take some overload but they also tend to weaken over time so the closer to original sizes the better.

There is also mineral insulated cable used a lot with stone cottages where it can run even hotter but then one must also look and see if the surrounding materials can take the heat. My book give about 7 different values on basic chart and then has a load of corrections to obtain the correct value. But on a personal thought I would not want to run any cables close to maximum and showers are available at below 10Kw and where the routes are unknown one must ask is it worth all the problems to fit large showers or is it better to replace like for like?

Especially when one realises unless like for like you need to pay LABC their £100+ fee under Part P if DIY.

Sparx is of course correct in all he says so no point in me repeating.
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Postby billobach181 » Fri Nov 27, 2009 10:07 am

Hi Ericmark. Thanks for you reply. The existing cable runs accross the hall uninsulated ceiling to the old 8.8Kw shower.straight 8metre run. It has its own Isolator and the old 40amp [wire] fuse all of which has been disabled and will be replaced with RCD Isolator etc.

In view of your comments I will install a new 10mm cable using method 1 [clipped] of BS 7671. All this will be checked by an electrician I just do the donkey work.
Regards
Bill
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Postby ericmark » Fri Nov 27, 2009 5:54 pm

Remember the electrician needs to be registered under Part P or you need to tell and pay LABC before you start. For any spark to register the work he needs to be in control so he needs to tell you what to do even if you do it.
He can't sign for work he has not had control over.
I have seen many fall foul where:-

a) The spark is not registered and he thought the job was under LABC. Often working for registered firm but not personally registered.

b) The spark did not like the work so refused to sign completion certificate. It is his lively hood and if checked and he loses his ability to sign off jobs he is really up the creak.

In theory a firm could employ you to do work for them on your own house but in practice the employment laws and insurance means it can't really be done so any electrician signing for work he has not done is taking a really big chance.

Getting the house holder to dig a trench and then back fill for example is OK as long as he dropped the cable in and put on sand and notice tape. But not to let house holder to put cable in trench and back fill. So be very careful and agree exactly what you can DIY. Too late once done.
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Postby billobach181 » Mon Nov 30, 2009 3:08 pm

Ericmark - thanks for your reply and input. I don't want you to think I'm a total novice. I am a retired Chartered Engineer and used to Design/commission North Sea oil rigs/Chemical plants etc. But I do take your points seriously.
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