Another Cable Size Question

Discussion in 'Tech/Mech General' started by Baycott, Jun 30, 2015.

  1. Baycott

    Baycott Funster

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    Hi all you techies out there.
    I have just bought a second leisure battery for our new motorhome (not yet taken delivery) and have made sure it is exactly the same as the one fitted.
    It is a Yuasa L35-100 - 12V 100Ah 700A.
    My question is what size cables should I use to connect the two batteries. They sit side by side.

    TIA
    John.
     
  2. mjltigger

    mjltigger Funster

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    A size bigger than the fuse you fit is fine..

    There's all sorts of talk about this and that but really the fuse is the answer. Work out the load and fuse at the top of what you expect to draw then cable one size up.
     
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  3. SomeoneElse

    SomeoneElse Funster

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    The same size as the cable connected to the original battery, unless you changing what you are using the battery for, it that case you need to design for that new use.
     
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  4. Baycott

    Baycott Funster

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    I don't have the van yet. :whistle: Just thought I would get stuff in so I can fit it straight away (y)
     
  5. pappajohn

    pappajohn Funster Life Member

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    Variables.....

    A lot depends where the second battery will be as cable has a resistance and while there may be 12v at the battery it may be less at the original battery if the cable is too small or over a great distance (volt drop)
    if the existing is at the front and the new one at the rear you need to take into account amps load and volt drop.
    if they are side by side you only need to account for amps as the volt loss will be minimal.

    If your present use is around 15amps with everything turned which on you would normally use, then you will need a cable slightly higher in capacity than 15amps.

    If the batteries are close then one fuse may suffice....if they are a distance apart fit a fuse at BOTH ends of the joining wire as if a short circuit happens the single fuse will blow isolating the battery near that fuse but the other battery has no fuse protection so will still be grounded resulting in either a fire or exploding battery.
     
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  6. pappajohn

    pappajohn Funster Life Member

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    <<< cable size calculator >>>

    Just enter amps and length then hit calculate.

    12v at 15amps load over 5 mtrs is 10mmsq cable

    The same volts and amps over 2 mtrs is just 4mmsq.
     
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  7. Moodybrook

    Moodybrook Funster

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    PJ , I just done a couple of checks using your calculater, thanks. Why oh why then do motorhome builders use cobwebs and threads throughout, certainly my van is. I am using 6mmsq and 10mm sq stuff and it looks serious overkill ?? I spent some time in a shipyard so I know the meaning of Clydebuilt, with wiring am I missing something ?
     
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  8. TheBig1

    TheBig1 Funster Life Member

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    Converters tend to use the cheapest cable to just cope with what they fit by design. every £ saved is a £ profit

    as for what size to retrofit, plan ahead. if youre going to fit an inverter, use cable appropriate to the planned load
     
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  9. tonyidle

    tonyidle Funster

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    With low voltage circuits the correct cable size is determined more by the length of the run than by the cable's ability to carry the load. The absolute minimum size is that determined by the current it has to carry - that takes care of safety and ensures that it won't overheat. The actual size that is needed depends on the length of the run and should be sufficient to ensure that there is as little loss of voltage at the load as possible. Cable does have a resistance, albeit low, and over an extended run it is significant and will cause voltage drop.
     
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