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Cable size?

Discussion in 'Electrics UK' started by Terrywookfit, 15 Sep 2007.

  1. Terrywookfit

    Terrywookfit

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    I am fitting a weather compensator to control a heating boiler!!
    The instructions recommend a twin cable of 1.5mm with a max length of 35m for wiring the out door sensor!!
    As the run of cable to the sensor will only be 3m max would 1.00mm cable be adequate???
    The resistance of the thermistor varies from 480-560 ohms!!
    What is the resistance per metre of 1.00mm/1.5mm cable??
    Or am I missing something out of the equation??
     
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  3. John M

    John M

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    In all probability a 1mm cable would do ....... However if your really strapped for the money to get 3 meters of 1.5mm sq send me your address & I'll give ya £2.50 :rolleyes:
     
  4. Terrywookfit

    Terrywookfit

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    Thank you for your kind offer!! I wish there were a few hundred more people like you about!!
    Ps I accept paypal!!
     
  5. John M

    John M

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    If its 240v 1.0 mm will do it ........ If its low voltage like 3v or somthing silly it probably would need 1.5mm to get it 35 meters & back ...... Which is weired I know, but there ya are.... Nip to one of those back street scruffy electrical shopz they will sell ya 3/5/a million meters of cable @ a sensible per meter cost...... B&Q & co will just defile you & probably poison your cat too :eek:
     
  6. John M

    John M

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    W0t sorta boiler are you fitting anyways?

    Its ok I'm not a member of the "Ooooh ya can't do that" brigade
     
  7. Terrywookfit

    Terrywookfit

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    Viessman 333!!

    Out door sensor only gives a resistance reading to the interface so I just wondered what ohm per meter difference there is between 1.5 and 1.00mm cable!!
     
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  9. Spark123

    Spark123

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    Are you really in Antarctica?
    According to the on site guide a 1m piece of single 1mm² cable is 18.10 milliohms @ 20ºC.
    1.5mm² cable is 12.10 milliohms @ 20ºC.
    (Remember to double the figures for there and back!!)
    For different temperatures as a guide you can adjust the resistance by mutliplying the figure by 1+(0.004 x difference) for an increase of temp or dividing by 1+(0.004 x difference) for a decrease in temp.
     
  10. John M

    John M

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    How much are they ........ I'm planning a system based on the 200 ......... Ummmmm :confused:

    Ball park kinda thing ;)
     
  11. Space cat

    Space cat

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    Let's do the sums. So the on-site guide says 1.5 sq mm wire is 12.1 milli-ohms per metre. This means that the 35m length - that's a 70m round trip - is 0.847 ohms. Your thermistor's range is 80 ohms so the maximum length of 1.5 sq mm cable will introduce an error of about one percent of its full scale range - but that's not important right now.

    A 3m length of 1 sq mm cable has a round trip resistance of just 108.6 milli-ohms, considerably less than the maximum permitted length of 1.5 sq mm. It won't just work; it'll work better!

    Just to cover all the possibilities, what about current rating? Could it be that the 1 sq mm cable won't be up to the job? Now this size cable is actually good for 11 amps but we'll go with 5 amps anyway. If that thermistor really did take 5 amps it would put out about 13kW. It would light up like an arc lamp!

    We can safely conclude that 1 sq mm cable won't have any significant effect on your sensor circuit and it won't be overloaded. I say go for it.
     
  12. John M

    John M

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    W0t bout a bitta door bell flex then ;)

    :LOL:
     
  13. securespark

    securespark

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    Or telecoms cable? :evil:
     
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