difference between solar and automotive cable, voltage drop silliness and advice!

System
System Posts: 2,511 admin
in Solar Beginners Corner #1
Hello,

I am upgrading the camper van solar system I installed a year ago. Currently - a 80w panel wired to the regulator and the leisure battery in 4mm2 (12awg) solar cable. After a little research on how to add 2 120w panels and a mppt regulator. I've found it is not as simple as I first thought. I also now realise a 4m cable run in 4mm2 connecting an 80w module with the regulator is overkill.

Most of my answers have been found whilst I lurked on this superb forum, I still have a few - so I know you will be able to tell me the main difference between solar cable and 12v automotive cable?

I can't see the difference for this project, I have noticed the solar cable is tinned – cable life is not a massive issue to me.
The voltages are low: I've decided to wire the 3 modules in parallel. A Vpmax of 17v for the 80w and 16v for the 120w will mean the voltage is between 16 and 17V?

I have also been thinking about voltage drop. The cable runs are short: 4.5m is the maximum distance from a module on the roof to the regulator. So I can ignore voltage drop.

I didn't though and may be getting silly with my next thoughts.

I had decided on 4mm2 cable - as it seemed daft to have a 80w module on higher rated cable than the 120w modules.

I did some calculations and found voltage drop to be around 1% on 4mm2 (12awg) cable: if I run the 3 strings to join at busbars near my regulator and battery. I know this will be 6 cables, but its convenient to join them in the cupboard containing my 12v fusebox etc.

Would I be getting silly if I used 6mm2 (14awg) to join the cables when they enter the van and head for the bus bars. They have to be joined here any way. Using 6mm2 from here on would half an already low voltage drop.

I am also thinking each “string” will have a fuse, I have no idea if a fuse is necessary to protect the panels, maybe the 80w one? It is not to protect the cable as 2.5mm2 (14awg) could have coped with being the cable size on the strings?

I should put the fuses on the cables where they enter the van: close to the modules as possible and the cables need connected here anyway. I am thinking again with the distances involved the fuses can go in the 12v cupboard for convenience. Although, this means an extra connection – increasing voltage drop!

I am 99% sure I have it figured out – however being a novice in this area. I am bound to be wrong somewhere or missed something - so any comments would be appreciated.
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Comments

  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #2
    Re: difference between solar and automotive cable, voltage drop silliness and advice!

    Some rather generalized wiring instructions.

    Wire is wire, for the most part. It doesn't care if it's in a car or in a house. The main differences between one and another are the gauge, the # of strands, the material (some is aluminium - best to avoid it), and the insulation. What you may see marketed as "solar panel wire" probably has UV-rated insulation. Most wire does not, and if exposed will deteriorate.

    Panels in parallel: more than two and each should be fused. You are about to add two 120W panels to an existing 80W. Be sure and check that the Vmp on the new panels is not too different from that of the old panel. Chances are that they're all "12 Volt" panels with a Vmp around 17-18 Volts. But it doesn't hurt to check. Fuses should be according to wire size, which should be large enough to both handle the current and keep the Voltage drop low. You seem to have grasped this already. :D

    I think you made a tiny error on your conversion, as 14 AWG is smaller than 12 AWG. To my knowledge, 14 is 1.6 mm and 12 is 2.0 mm. 6 mm is around 4 AWG.

    There's no reason why you can't run separate wires from each panel and then combine them inside. The fuses can be any place within the circuit that's convenient to access; they'll function the same (NEC regs notwithstanding).
    If you had an MPPT type controller it would be better to run them in series, but that's redesigning the whole system so let's not go there.
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  • System2
    System2 Posts: 6,290 admin
    #3
    Re: difference between solar and automotive cable, voltage drop silliness and advice!

    Thanks caldicoot thanks for the relpy and reasurance that I am thinking along the right lines.

    Your statement
    If you had an MPPT type controller it would be better to run them in series

    Helped me make sense of my mppt instructions I have a german charge controller, much like chinese instructions they just don't seem to sense in places when translated into english.

    I know you know your solar, I've learnt quite a bit from your posts whilst reading other threads. So having you say that made me go back and try to make sense of the finer points in the abismal instruction booklet.

    I had figured out with a peak voltages of 17V for the 80W and 16V for the 120W modules. In series they would produce 49V
    I now realise 49V would increase the charging current. The controller charging a 12V battery with 49V would increase the maximum charging current by a factor of around 4?

    I carried out my calculations again – the 120W panels produce 7.4A at peak performance. However in series this would be restricted to 4.7A by the 80W panel? If so, in series the maximum charging current would be 19.19A Imp (4.7 * 49/12). I had foolishly assumed the maximum charging current would only be 4.7A if they were in series.

    This figure is comparable to the peak current of the modules in parralel 7.4 + 7.4 + 4.7 = 19.5A. 49V is good news for my obsession with voltage drop.

    System redesign is not a problem as I have not bought the bits to join everything up yet.

    However would the same principle not apply with the modules in paralel? – 16V increasing the maximum charge current by a factor of 1.333333.....? Making max Imp 26A when they are paralleled, or is the voltage difference not significant enough to count?


    You are also correct in stating
    I think you made a tiny error on your conversion, as 14 AWG is smaller than 12 AWG. To my knowledge, 14 is 1.6 mm and 12 is 2.0 mm. 6 mm is around 4 AWG.

    When I first read this I thought I shouldn't try and be clever and convert cross sectional area to AWG – I am usually wrong when I get try to get smart......And Yep wrong again I thought 12AWG is roughly 4 mm2 and foolishly went higher to get the gauge for the 6mm2.

    However............ I should have been clearer that the wire measurments are cross sectional area. Being in the uk and brought up on metric - I find square mm easier to use. I also know when wire dimensions go up the gauge number goes down I have a AWG chart bookmarked that i've had to constantly refer to, so I can understand AWG sizes when reading posts.

    The chart I have is also a bit rubbish and only shows even wire sizes. I found a better chart and
    perhaps 11AWG is a better anolouge for 4mm2 being 2.3mm in diameter and 4.17mm2
    and 9 AWG for 6mm2 being 2.9mm and 6.6mm2
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #4
    Re: difference between solar and automotive cable, voltage drop silliness and advice!

    "Brought up on metric". In the UK. *sigh* So if I ask you how many ounces in a pint ... :p
    I was brought up using SAE, Imperial, and metric ... and frankly the metric system isn't that great. :roll:

    Okay, back to panels.
    Your panels are mis-matched. The Vmp's are within 1 Volt, but the Imp? That's a problem for serial wiring. If you could utilize these panels with an MPPT controller (I'll guess you have a Steca of some sort?) The charge current would be the available Watts divided by the charging Voltage. For instance:
    320 Watts (less derating) / 14.2 charging Volts = 22.5 Amps. The Imp of the panels doesn't really enter in to it.

    It does, however, affect the array's ability to produce. It will "throttle" the current and limit the available Amps. The larger panels can even burn out the smaller one. Frankly, 7.4 Imp and 4.7 Imp (odd coincidence?) are too far apart to be successfully used in series in my opinion. No telling what the controller will make of it. In all likelihood it will cost you 80 Watts of potential power. In parallel the loss would be minimal. Panels are a current source, not a Voltage source. That's hard to wrap your mind around. It's like saying "birds fly through water" which doesn't make any sense - until you meet a penguin.

    A perfectly good wire size chart: http://www.powerstream.com/Wire_Size.htm
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  • System2
    System2 Posts: 6,290 admin
    #5
    Re: difference between solar and automotive cable, voltage drop silliness and advice!

    Ohi, cheeky. - When companies in the uk start selling cable by the flagon, I will buy it that way.
    Until then....... its metric. Although some sleekit companies have stared to sell liquid (i.e. milk) by the half litre instead of 568ml

    Thanks again for the advice. – my old pwm controller was a steca. The mppt one is an ivt. I have no idea of the quality of this equipment. All I know is: it is german on the outside. The instructions seem to suggest otherwise. I don't know, I just hope it does what its supposed to.

    Pengins flying through water? Thats what they do? Is a better analogy of solar not - Solar energy hitting a panel releases electrons to flow round the circuit. Which = current. More sun = more flow. Go down town on a sunny day: the amount of folks flowing in and out of your personal space can be unbearable? (even worse when a sale is on?)

    Which is one of the main reasons you wont get me down the shops on a sunny day/saturday :p
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