Solar shed lighting setup - correct approach?

Discussion in 'The Projects Forum' started by Upex, Jul 23, 2015.

  1. Upex

    Thread Starter Member

    Nov 10, 2013
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    Hi all,

    Been awhile but I'm now embarking on my next mini mission - to get some solar lighting up in and on my mother's shed. I therefore wonder if you gurus can please offer any steer, suggestion, considerations or corrections so I can head off on the correct path.

    I think the set up should be (crude picture attached):

    Solar panel (have an 80w one),
    charge controller,
    battery (going to trial on an old 120ah car battery, but have some deep cycle leisure batteries to swap it out for when all set up and working),
    fuse (thinking 10 amp be ok? But struggling to find dc fuses and holder, other than car type blades, will they be ok?),
    switch (looking to have a manual option, ie on/off, but also a light sensor - will detail this for guidance later, after I get the general approach confirmed/corrected),
    lights (have 10w 12v smd lights, planning on using 7 or 8, 3 internally and 4/5 externally)

    Is the above (attached) ok or am heading towards trouble?

    As well as the switch question, I'm also unsure on cable size, as calcs saying up to 10mm, but ill be looking to split the internal and external on 2 separate feeds, internal circa 10m from battery to last light, external will be a bit more, say 12 to 15m from battery to last light.

    Any input would be great.

    Thanks, Upex P_20150723_124543.jpg
     
  2. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    An 80W / 12V solar panel will put out about 6.7 amps max, that is low enough to where you can get away with directly connecting it to your battery. We've done something similar here for several years now on a lighthouse in the middle of a bay: we might be burning thru deep discharge batteries (which you shold use) faster that we should but nothing noticable.

    How long will the lights be on each day? you want to limit the discharge on the battery to about 25% (normal) or 50% (occasional) and 100% never. Your capacity is fine, but you also need time to charge them the next day, and with short cloudy winter days you may not top the battery off each day, which eventually leads to that dreaded 100% discharge.

    But your basic scheme seems sound.

    Automotive fuses will work OK but they will never meet any code requirements should you need to get the work inspected. 10 amps sounds OK for the 70W of lights.
     
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  3. Upex

    Thread Starter Member

    Nov 10, 2013
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    Thanks Ernie.

    I have the 80w panel surplus from a failed previous build, and controller just in case of dark discharge, so ill use them for now, likewise the car battery.

    Once all done and working without issue, ill get another panel and already have some decent size leisure batteries to. Using scraps to build until right, as it were.

    Regards use, couple of hours in morning and couple during evening, give or take, when its darker at night/morning.

    Would it ultimately be best to have the internal and external lights fed from same battery (or batteries) and same panel(s) so that uneven use between in and out lights spreads the load evenly, or have 2 separate set ups? One for internal and one for external? I assume the former as no wasted juice if one set isn't used much and the other is etc.

    Thanks for confirmation I'm heading down an ok set up, wanted to make sure I had the concept of bits in the right place and whatnot.

    Thanks for the fuse confirmation to, I do a good bit of work on cars so bits are plentiful and was really struggling to find specific dc breakers or fuses etc.

    Thanks for the help thus far, much appreciated.
     
  4. nsaspook

    AAC Fanatic!

    Aug 27, 2009
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    Looks good to me but I would put a protective fuse on the charge controller wire to the battery (would also be a CC disconnect) or connect it to the protected side of the main fuse. Separate switch/lamp setups with a master on/off switch seems like a good idea. I have a similar type setup that powers a low power 'night light' bulb at all times so I can see the main switch when it's dark.
     
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  5. Upex

    Thread Starter Member

    Nov 10, 2013
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    Thanks nsaspook,

    Do you mean put a second fuse between charge controller and battery, on the positive or negative lead?

    Thanks, Upex
     
  6. nsaspook

    AAC Fanatic!

    Aug 27, 2009
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    Positive lead close to the battery to protect the charge controller wiring/circuit from a possible short with full battery current.
     
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  7. Upex

    Thread Starter Member

    Nov 10, 2013
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    3
    So, would my maths be correct as:

    7 x 10w lamps will draw just shy of 6 amps per hour (so ill assume 6 amps for ease)

    So at 6amps, 5hrs use would drain 30amp hours from battery. And at 8 hrs use it'd be 48amp?

    So using 5hr usage, would need to replace 30amp, which on the 80w panel, would require 4.5hrs of strong sun per day?

    Is the above roughly thereabouts?

    Don't think 5 hrs will be the normal usage, likely between 2-4, so thinking the 70w of lights will roughly be ok with the 80w panel for the time being, as the drain should be less than above?

    Happy to take any corrections.

    Thanks, Upex
     
  8. Upex

    Thread Starter Member

    Nov 10, 2013
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    Would it be another 10 amp fuse, or 7.5 or?

    Cheers, Upex
     
  9. nsaspook

    AAC Fanatic!

    Aug 27, 2009
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  10. Upex

    Thread Starter Member

    Nov 10, 2013
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    Its only a cheapo mppt CC, but 10amp sits right in my head, and I can always try a 7.5 and see how it goes, obviously blowing the fuse is an inconvenience at most, so no dramas if it does pop, ill stick the 10 in instead.

    Thanks for your help nsaspook.

    Regards wire size, I'm thinking 2.5mm cable (will use twin and earth as cheap and plentiful) will be about right? It'll be tacked on the shed wall, uncovered, and the interior should be circa 10m long and carry 30watts, the outside ones could be up to 15 meter and carry 40watts, that sound about right?

    Thanks all, Upex
     
  11. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    What gauge is 2.5mm? With low voltage systems you want to use very heavy cable or you loose much of your power in your wiring.

    I'd use house wiring.
     
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  12. nsaspook

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    Aug 27, 2009
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    Calculate the wiring sizes to keep the resistive losses under a few percent at each current loop leg of the circuit. What's really important (and a reason to oversize wiring there) is to keep the voltage drop from the charge controller to battery as low as possible. 0.5 volts is not much of a loss when driving a light but it's a world of difference in the SOC of the battery. Some of the better charge controllers have remote voltage sensing to comp for charge wire voltage drop at the battery.

    For AWG sizes but it's easy to find the metric size equivalent.
    http://www.windsun.com/Hardware/Voltage_Calc.htm

    For simple and fairly low powered 12 v DC circuits like lighting I use
    http://www.homedepot.com/p/Cerrowir...ow-Voltage-Landscape-Wire-241-1602C/202206449
    but don't use those vampire tap connectors. You can find low voltage wiring all the way to at least 8-gauge online.
     
    Last edited: Jul 23, 2015
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  13. Upex

    Thread Starter Member

    Nov 10, 2013
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    Ernie, not sure what gague 2.5mm is, think closest is 13, but a norm for house wire here, we use 1mm, 2.5mm and 4mm generally for lights and sockets and such, but not too much 4mm, as its expensive and often not needed unless a long run and whatnot. Ie 4mm is £1.50+ for a meter (£0.50+ per foot), whereas 2.5mm is 1/3rd the price.

    Nsaspook, ill see what drop it comes up with. The c.c and battery will be as close as possible, ie < 1 meter, its just the length to space out the lights a bit that's driving the length.

    The home depot wire looks good, ill see if I can find anything similar here. Wish we had your stores, ours are pants and have to visit many different places to pick up different bits!

    This is the cable I'm thinking of:
    http://www.screwfix.com/p/prysmian-6242y-twin-earth-cable-2-5mm-x-100m-grey/20967

    Voltage drop calculator says I need 10mm (awg 7) for a 6amp 10m led and 16mm (awg 5) for the 15m leg, both a max 2% voltage drop.

    That's massive and won't fit in the terminals from switch and whatnot. Seems low voltage doesn't like traveling far, as that much much smaller 2.5mm can be used indoors on 230v for 100m+ and 30amps!
     
  14. nsaspook

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    Aug 27, 2009
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    The tight voltage requirements are mainly for the panel -> CC -> battery connections but the wiring drop is a good reason not to have one big string but to star pattern the lamp connections from a switch/common point if possible with low voltage to reduce copper costs. (now you know why utility voltages are high) Your cable should work OK as it's close to 12 AWG. The voltage drop to the lamps is not that critical because you won't have high surge loads.

    The US NEC recommends 5% as the max feeder and branch circuit drop with utility voltages and with (electronic) LED lights I don't think you will see any difference with a little more.
     
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  15. Upex

    Thread Starter Member

    Nov 10, 2013
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    Thanks nsaspook,

    If I interpret you correctly, the star would be more legs from switch to few lights each?

    I'd rather than a leg of 3 lights and a leg of 4, have 7 legs each with just one light - more cable, but the current would be much less this lower drop over the distance.

    That sounds doable, ill math it out.

    Cheers for the heads up on panel cc battery, didn't realise that was so critical, so will get them real close and use some hefty cable to help to.

    Thanks for the assistance, feel like I'm getting there now lol
     
  16. Upex

    Thread Starter Member

    Nov 10, 2013
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    3
    So, assuming 1 light per leg, drawing 1 amp, then the maximum length leg of 15m, provides a voltage drop of 2.83% (0.34v) using 1.5mm cable, or using 2.5mm its 1.75% (0.21v).

    The above it based on a one way length of 15m to, which is more than ill need to be honest, more like 10 each way, which lowers it to 1.92% (1.5mm) or 1.17% (2.5mm).

    So from the above, it would appear to me that if I go with 1 light on the longest runs, then even 1.5mm would be fine to use. Would anyone agree, disagree?

    For the shorter runs, I assume it'd be ok to use 2 or 3 lights? So long as they meet the max draw on wire and stick within the aforementioned allowable voltage drops?

    Not done the calc yet, but think 2 or 3 close lamps on one leg, then 2 legs with 1 lamp each type scenario would be ok? I guess I'm asking if the draw of the load on each leg has to be equal, or if it can be different on different legs?

    Thanks, Upex
     
  17. Upex

    Thread Starter Member

    Nov 10, 2013
    67
    3
    Ok, so planned out where the lights are going, and if I partly drop the internal external split, I think that I can do 2 lights per leg, with the biggest voltage drop being 3.08% on the longest leg: the one that goes to e5, but that assumes a 2amp load, which is bit more that it calcs at, so should be a bit less in reality.

    (Crude set up pic attached). This is using 1.5'mm wire though, so if that's too much, then could use 2.5mm.
    P_20150724_114532.jpg
    If that one leg drop of 3% is ok, then what would be the best way of pairing up the e2, e3, e4 and e5 lights, or does it not matter? Ie pairing shortest (e2) with longest (e5).

    Thanks all for your help, after this is nailed, only the switching to figure out.

    Cheers, Upex
     
    Last edited: Jul 24, 2015
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  18. nsaspook

    AAC Fanatic!

    Aug 27, 2009
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    I think you have that part nailed. Just use a good connection method for all the wiring junctions. I prefer solder and shrink instead of wire-nuts or other friction type connectors for low voltage branch circuits. A proper air tight crimp connection will work to attach wires to lugs but you need the proper tools and connectors so I usually solder those too instead of using a crimp. (Don't solder the ends and then crimp)

    I also have a power pigtail fused at the battery to connect various types of power adapters using MC4 or anderson power connectors.

    If you want to see a bad voltage drop in 12 volt solar wiring look at this while running an AC unit. :(
    mbmc1.jpg

    The red outline is the voltage drop for a ~35 foot loop of 10 AWG wire from the remote solar array to another array near the house near the charge controller. I have some scrap 3/0 3 phase cable to fix the drop but the ground is too hard to manually shallow trench another conduit this time of year.
    4479619961_b25729a1f3_z.jpg
    wire.jpg
     
    Last edited: Jul 24, 2015
  19. Upex

    Thread Starter Member

    Nov 10, 2013
    67
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    Wow, that's a heck of a drop, and yet still not far in real terms.

    (Another thing I'm having to look at soon is sending power to my garage, circa 120m +/- 10m for twist and turns, but that'll be 230v so much easier, baring the cost of the blooming armoured cable).

    So regard the choosing of the outside pairs, is it better to pair up the furthest light with the closest (e5 and e2) or doesn't it make any difference? Not clued up anywhere near enough to try to know if having one lamp connected at 1m and the other at 8m is a better pairing or even any different to using a pair with 1 at c. 6m and 1 at 8m (e4 and e5). My head says its better to have shortest and longest together and then middles together, average the pairs type approach, but I don't understand this wild world of electrons at all!

    I'll definitely be soldering all connections, my much preferred route, except for large cables where the old hydraulic 'cold weld' seems to work well enough (like on big Andersons - use a 175amp on my motorbike that crimped very well with a 8ton press).

    Cheers, Upex
     
  20. nsaspook

    AAC Fanatic!

    Aug 27, 2009
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    Don't over think it. Lay it out so it's easy to install without excessive voltage drops. It might not be the best configuration electrically but running wiring in some hot dusty space will make you wish for easy runs.

    And don't forget about the safety earth ground rod if the DC system is isolated from the normal utility earth ground. (Not sure what the UK earthing system is) You don't need it for everything to work but it's a really good idea to have so any static or induced charge from storms and lightning strikes has a safe place to go as all that wiring is just a big antenna.
    ground.jpg

    I ran a wire to the main solar panel (green) and bonded (black) the rod to my existing ground system at the utility meter. (I pulled out the slack later to make the wiring a straight shot to the rod with nice slow curves for any bends)

    ground_today.jpg
     
    Last edited: Jul 24, 2015
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