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Cable size?

Discussion in 'General Electronics Discussion' started by Craious, Oct 18, 2017.

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  1. Craious


    Feb 1, 2016
    Hiya, hope this isn't a too basic a question!
    Been trying to workout a cable size for my circuit but I'm not the brightest lamp in the box so am a little confused about some conflicting answers. I did my best to search forums but didn't find anything I could apply - probably my bad so please forgive.

    My circuit is a 12V led driver (36W) driving 10 x 2W GU9 led bulbs (12V) in parallel.
    The driver goes to a junction box (200mm, 8" from it) where it splits to each bulb. Cable length from JB to each bulb is 1m.

    I've used online calculators & most say 0.5mm2 cable but an electrical engineer said 0.2mm2 would suffice.

    I'm torn now as 0.2 would be an easy cable to feed but I'm about worried about the conflicting info.

    May I ask your advice please. Sorry to be a numpty but whilst I'm doing my best as all this really excites my creative streak, I'm not too clever outside making furniture! Lol.
  2. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    It's a matter of acceptable loss of power within the cable. 0.2 mm² cabel will incur higher losses than 0.5 mm² cable, but will work.
    A standard cable size for installation and speakers is 0.75 mm² and should be easy to come by. This cable size will definitely fit your needs.

    If you can't easily buy 0.75 mm² or 0.5 mm² sized cable but have access to 0.2 mm² cable, you can keep losses low with the following setup:
    1) Use 2* 0.2 mm2 cable in parallel (effectively 0.4 mm²) from the driver to teh junction box.
    2) Use 1*0.2 mm² cable from the junction box to each lamp.
  3. Tha fios agaibh

    Tha fios agaibh

    Aug 11, 2014
    You only need about 170ma per 2w bulb, so you could get away with an area as small as about .08 mm2 (28awg) although it's so small it may be impractical to use.
    A wire with an area of .2 mm (24awg) can carry up to 570ma so it's more than large enough to carry the load.

    I would opt for a larger size because the durability of the wire and practicality of the install, rather than just the load it can handle.
  4. Craious


    Feb 1, 2016
    Chaps, thank you kindly for your informative replies. I did actually go for the 0.5mm2 CSA twin flex today funnily enough, reasoning that the larger would be safer but it's really interesting reading the educated reasons & workarounds, thanks.

    Onward & upward, appreciated. Till the next time.........
  5. kellys_eye


    Jun 25, 2010
    Cable sizing is actually quite interesting (for the geeks in us, anyway!).

    They are actually rated by temperature. Many PVC insulated cables are rated at 70degC (for example) so you can load it up as much as you like up to the temperature reaching said level.

    Accordingly, the current handling capacity of a cable is not only set by its length and c.s.a it is also set by the way it is actually run - in free air, clipped to a surface, in conduit, bundled with other cables etc ALL affect it's maximum current carrying capability.

    There are higher temperature-rated cable insulations and, as a result, they can carry more current for a smaller cross sectional area, over longer distances or in 'enclosed' environments etc.

    Even the most mundane aspect of electrical/electronics can have deeper reasoning than first imagined.
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