Solar panel cable sizing

Discussion in 'Solar Power' started by scotjimland, Sep 4, 2011.

  1. scotjimland

    scotjimland Funster Life Member

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    I raised this topic on another thread but as it is so important thought it worthy of it's own post..

    There is no point in spending ££££ on panels and charge controller then skimping or fitting incorrect (too small) sized cable .. failure to do this and your panels will under perform and disappoint ..

    This is so important with a low voltage system.. where you MUST take into account the

    * System Voltage
    * Max. Current
    * Acceptable Volt drop
    * Cable length

    To illustrate how cable length affects volt drop and the cable size required..

    An 80watt panel with two different cable length runs..

    Panel voltage = 17V
    Max current = 5A (panel watts/voltage)
    Acceptable volt drop = 1%
    Cable length = 3mt

    Cable required = 4mm2

    Increase cable length to 6mt and you have to increase cable size to 16mm2

    To calculate use this handy cable sizer ... http://www.solar-wind.co.uk/cable-sizing-DC-cables.html
     
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  2. Stewart J

    Stewart J Funster

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    Another point to consider when buying/fitting cable is that the cable from the panel itself should be UV resistant where it is exposed to sunlight on roof, as this is more expensive and often stiffer to work with fit a suitably sized junction box in a convenient location where cable enters MH, say a wardrobe or overhead locker. Ordinary cable covering will degrade over time when exposed to sunlight often leading to cracking and allowing water ingress..

    Then as Jim has posted then use a suitable heavy cable from this JB.
     
  3. andyman

    andyman Read Only Funster

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    Why are the cables for solar panels rated so low. 4mm is rated at 25amps.
    Can someone please explain
     
  4. scotjimland

    scotjimland Funster Life Member

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    The volt drop not just the current carrying capacity that must be taken into account when sizing cable.. voltage loss is critical .. and MUST be considered when sizing low voltage cables ..

    Consider an 80watt panel, only gives about 5amp max..

    if you sized this on current alone you would think 1mm2 would be more than adequate .. but the volt drop would be horrendous and the current available to charge would be drastically reduced..
     
  5. Wildman

    Wildman Read Only Funster

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    It is because it is DC and suffers from voltage drop much more than A/C due to the way free electrons behave.
     
  6. Douglas

    Douglas Read Only Funster

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    I prefer to over engineer the solar panel cables Jim on the grounds that although the voltage drop may appear small at the point of 80% charge the battery charging voltage is getting so close to the panel voltage, the charging rate slows down, therefor even small voltage drops have a much bigger effect than imagined. So I use auto 30 amp cable. and on the link from the batteries to the inverter I use light starter cable

    Doug...
     
  7. scotjimland

    scotjimland Funster Life Member

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    shortcircuit has just pointed out an error.. :Blush: thanks :thumb:

    that should say 10mm2 .. NOT 16mm2
     
  8. wizzer59

    wizzer59 Funster

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    Having Just bought this will all the suppied cable be ok or should I invest in better? Or what cable will I need from the controller to battery?:thumb:
     
  9. wizzer59

    wizzer59 Funster

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    Another daft question:Blush: Is it better to have the controller as close to the panel as possible? or as close to the battery as possible? if you get my drift:thumb:
     
  10. Stewart J

    Stewart J Funster

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    Controllers are best sited as close to the batteries as possible usually in the compartment many like mine have a temperature sensor which determines the temp of the compartment for more efficient regulation :thumb:
     
  11. wizzer59

    wizzer59 Funster

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    Sorry Guys didn't realize you had answered most of my questions on my other post:Doh:

    Mind you still can't believe the cable suppied with my panel aint good enough, if I'd known what I know now I would have bought everything separately:Sad::Sad:

    Tim
     
  12. Techno

    Techno Funster Life Member

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    Have you considered that cable specific for solar use is most likely a lower resistance than regular automotive cable? I always use 4mm tinned double insulated UV protected and unless the van has good wiring to the batteries from control panels that have a solar input then I will go direct to the batteries via independent regulator. Direct to the batteries is favourite as it negates any control panel faults that may arise
     
  13. scotjimland

    scotjimland Funster Life Member

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    Irrespective of panel rating and cable length ????
     
  14. Techno

    Techno Funster Life Member

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    If it got beyond 4 metres I'd have to reconsider but can usually get there in less depending on where the panels can be sited.
    On my own van the regulator is in the overhead locker next to the control panel and output connected across common negative and common of the battery selector switch. The internal wiring (Autocruise) is probably no better than 4mm to the batteries but as I have 200 watts I have surplus to cover a little loss.
    [​IMG][​IMG]
    All the panels are in parallel so only classed as 4mm from the junction box into the van
     
  15. Gromett

    Gromett Funster

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