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Help! storage heater cable size?

Discussion in 'Electrics UK' started by dangerousdave1, 10 Mar 2008.

  1. dangerousdave1

    dangerousdave1

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    Hi, I wonder if someone could give me some advice re. old storage heaters already installed when i bought my house. I have 5 x2.55 kw storage heaters which have 1.5mm cable supplying them via 13 amp neon switches. According to information on this site a 1.5mm cable will take up to 3.6kw so theoretically should be adequate even allowing for voltage drop on a fairly lengthly run?
    Other sources however suggest that a minimum 2.5 mm cable should be installed for all storage heaters. What are the regulations regarding this and will i need to replace all these cables otherwise risk overheating and at worst a fire?
    Also i have heard that a 30ma rcd should give some form of protection in the event of overheating. Is this correct and will it suffice for me to install one of these? I don't really want to replace all the cables with 2.5 mm unless its absolutely neccessary! Any advice would be greatly appreciated.
     
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  3. Adam_151

    Adam_151

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    2.55kw is about 11amps

    1.5mm² T+CPC can carry 11A depending on install method, you need to ensure that the volt drop and earth loop at the end are going to be ok though

    Install method wise, it can carry above 11A for all the install methods you are likely to find it in, *except* burried in an insulated plasterboard stud wall not touching the plasterboard (method 103 - 10A)

    Touching the plasterboard in insulated wall (102) - 16A
    Clipped to a ceiling joist and covered with >100mm insullation (101) - 13A
    As above, but less than 100m insulation (100) - 16A
    Clipped direct (C) - 20A
    In conduit in a wall (A) - 14.A

    This assumes no further derating factors are applied, which is the case unless the cable is grouped with others, likely to encounter an ambient temperature over 30C, is in more insulation than the reference method covers, or protected by a re-wireable fuse

    And as long as the run is sub 25m then a fag packet calculation reckons you'd be alright loop impedance and voltage drop wise (assuming BSEN 60898 type B 16A MCB)

    Forget RCDs, they haven't got anything to do with overcurrent protection


    To sum up, 1.5mm² would probly comply in a lot of cases, but if I were installing it, I'd use 2.5mm²
     
  4. Space cat

    Space cat

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    I'm trying to visualize what you've got. Are those switches next to the heaters with short lengths of cable coming out, in which case what sized cable connects the switches back to the CU?

    1.5 sq mm cable is big enough for 13 amps. With your heaters taking about 11 amps, the voltage drop along 1.5 sq mm cable will be about a volt for every four metres.

    I've seen storage heaters wired on individual 20 amp fused radial circuits. These would need 2.5 sq mm cable. I would suggest that you don't need to upgrade your cable because those fuses will not allow you to overload it with bigger heaters. If you were rewiring anyway AND planning to continue using storage heaters, THEN I would put in the bigger stuff.

    The RCD(s) will detect leakage from a heating element to earth long before it becomes big enough to blow a fuse. Whether this is a good thing or not depends upon how you look at it. Insulation degradation is a common problem in old heating elements. Those old heaters, which have been working well for years, might leak more than 30 mA already!
     
  5. dangerousdave1

    dangerousdave1

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    Many thanks Adam for your response. The only problem i can envisage is that 5 of the cables on their final run from the ceiling to the consumer unit are bunched for a distance of about 2.5 metres. These bunched 1.5mm cables supply 2x 2.55kw heaters, 2x 1.7kw and 1x0.85.

    Sorry, i didnt mention the other 3 smaller heaters in my original question because i was only concerned about the larger heaters. The remaining 3 x2.55 kw heaters join the cu from other directions so not a problem with bunching.

    Thanks for your response and advice too space cat. As you`ve probably gathered by now from my above comments the cable between the 13A neon switch and the consumer unit is 1.5mm.

    Another question re. this: Out of interest, do the regulations state that all new 2.55kw storage heaters should be fitted with minimum 2.5 mm cable? If i were to replace the heaters with new ones is there a requirement to replace the cable with a larger one?
     
  6. Adam_151

    Adam_151

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    Are these cables clipped directly to the wall from the ceiling to the CU, if so the rating of 1.5mm² in that situation is 20A, and the grouping factor for 5 cables is 0.6, 20Ax0.6 = 12A (actually, we could *almost* ignore the 850w heater, and count it as 4 cables with a factor of 0.65)

    Assuming these heaters are protected by 16A breakers though, the breaker rating is higher than the cable rating, but thats ok, cbale is protected from overload by the fact that the fact that a fixed heating load is not likely to cause an overload (less someone swaps it for a bigger one!), we should really do a separate calculation to verify fault protection, but in reality it'll comply as long as the Zs is in spec for your B16 breaker

    The regulations require that when work is done, that the capacity upstream equipment (including that of the electricity company) is sufficent for the load and the earthing arrangements are upto spec, while its true that some ratings and factors have been adjusted a tad in BS7671:2008, the way circuits are designed is still the same, but no there is no regulation that states 'Ye shall not install ye storage heaters on ye 1.5 T+E' :LOL:

    I'd install 2.5mm² if I was putting them in, its good practice, but 1.5mm complies in your case as far as I can see
     
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  8. dangerousdave1

    dangerousdave1

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    Yes the cables are clipped directly to the wall at present but i was thinking of burying them or boxing in for neatness...would either of these still be ok?

    Please excuse my ignorance but i dont understand what you mean about the Zs. :confused:

    Yes i have 16A breakers protecting the 2.55kw heaters, 10A breakers for the 1.7kw and a 6A breaker for the 0.85kw. As mentioned earlier all the heaters have a 13A fused neon switch next to them as well.
     
  9. Adam_151

    Adam_151

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    Burrying cables in the plasterwork is still counted as clipped direct, boxing them in changes nothing (however, unclipping them, installing trunking and putting them in that is different - the thermal conducticity of the wall is being used to cool the cables)

    Ensure that burried cables are in a prescribed zone (the most straightforward one is straight up from an electrical accessory, ask if you want details on others)

    That brings me onto one of the more interesting changes BS7671:2008 has brought about, cables buried unprotected in a prescribed zone in a house need 30ma protection (but I wouldn't worry too much about that at the moment)


    5x2.55, 2x1.7 and 1x0.85?

    Thats 17kw of heating load... I hope you don't have an electric shower or anything like that :confused:


    Zs is earth fault loop impedance, if a fault to earth should occur, the impedance of the fault path needs to be low enough that sufficent current will flow to operate the fuse or MCB, unless the cable runs are exceptionally long (I said earlier that anything less than 25m would be easily on the right side), or there is something wrong with the incomming earth, then the Zs at the heaters should be in spec for a B16 breaker
     
  10. dangerousdave1

    dangerousdave1

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    I always wondered what method would keep the cables the coolest. For some reason i thought that burying the cables would make them hotter! They do run vertically from the consumer unit to the ceiling so no problem there.

    Maybe my original idea of fitting a 30ma rcd is not a bad idea after all then? Im worried however that the old heaters may cause it to trip and they dont come cheap so it could be a waste of money!

    Yes 5x2.55, 2x1.7 and 1x0.85 is correct! I had been thinking of installing a triton 300si 10.5kw wireless shower but have so far refrained for the reason you state. I was thinking maybe i could fit a timer switch so that the shower would only operate during daytime hours when the storage heaters are off? I calculated that the storage heaters and shower alone would draw about 115 Amps which is pushing it since the main supply fuse is only 100A..My house is all electric so i also have an electric cooker! Im not sure how the electricity company would respond to a blown fuse and whether i would end up with a hefty bill?

    While on the subject of showers am i right in assuming that it is impossible to suffer electrocution while using a wireless shower especially as the pipe to the shower head would be plastic as is the existing pipework in the bathroom? I have a spare fuseway at my split load CU that i could use. It is protected by an 80A 30ma rcd.

    Im certain that the longest cable run to the furthest heater is less than 25 metres although i do live in a 3 storey house. I (over)estimated that the longest possible cable length would be no more than 23 metres absolute maximum. I calculated this by measuring the longest possible route from CU to heater.
     
  11. Adam_151

    Adam_151

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    Masonry makes a fairly decent heatsink pretty high specific heat capacity and fairly thermally conductive (imagine walking on a stone floor), the worst thing is air!, its no so bad when the air is free to move and the cable can be cooled via convection, but where its trapped, like in thermal insulation (rockwool fibre in itself isn't much of a thermal insulation, but the structure traps a lot of air), naturally youd expect thermal insulation to make the cables warm though :)

    (the jacket on your cold feed cistern in the attic is to keep it cool though, not to stop it freezeing in winter ;) )

    I seriously wouldn't worry about it at the moment

    Aye, heating loads, and lighting and small power would use up most of the 100A, the saving grace is that the fuse will take 150A long enough to shower (... well long enough for a bloke to shower anyway, if you have teenage daughters in the house the fuse might actually blow!), and the heating loads will be on a separate CU to everything else so that there isn't an overloaded CU to worry about... that said, its not good design practice!

    I reckon the cleverest arrangement would be one where the shower dropped out the heating on a bank of contactors for the duration of the shower :)

    They'd probably replace it the first few times, but after that they'd recommend you either reduce your loading or have a tripple phase supply installed (they don't generally install single phase supplies in excess of 100A)

    Those are the ones were the heater unit goes in the airing cupboard, hot water is piped to the bathroom, and control is done by a remote RF unit containing batteries... aye, I'd say they are a bit safer than the normal ones as the electric bit is outside the bathroom, but if your shower is RCD protected and your sup bonding is upto scratch then I don't see that a normal one is unsafe at all

    Cool, what side is the cooker on out of interest?

    Sounds ok, the 25m thing was a very rough thing to err on the side of safety, volt drop bites around 28m and Zs at 38m
     
  12. dangerousdave1

    dangerousdave1

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    I see where you are coming from Adam re. what side of the split load unit the cooker is on. Its on the rcd protected side and probably what you are hinting at is the fact that a 30+ amp cooker and a 40 amp shower in addition to all the other circuits may well overload the 80 amp rcd. Thanks for making me think about that... i will probably be better off installing a seperate rcd unit solely for the shower if indeed i do decide to fit one.

    Many thanks to you for the detailed and very valuable information that you have provided for me...my initial fears re. cable size have been quelled and i can now sleep easy knowing that its not going to cause a fire due to overheating!

    I do have another question re. insulation replacement for storage heaters which i will probably be better off starting a new thread on. Basically im trying to find some replacement insulation panels for the front of the heaters as ive looked inside and the existing panels are in poor condition. I cant find anywhere on the internet where they supply these replacement panels. They seem to supply just about every other spare part but not these!

    Anyway thanks again....your help has been invaluable!
     
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