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Subject Topic: Solar cable sizing
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Personoldskoolskater
11/5/2015 at 1:22pm
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Hi All,

I'll be buying 2 x 100watt flexible solar panels for fitment to the 'van roof and will be connecting in parallel, but am wondering what size cable to use from the panels to the MPPT controller next to the 110ah battery (a run of approx 4 metres)?
Given that I'm connecting in parallel will 4 sq mm cable the way forward or would 2.5mm suffice?

thanks


Personfreeatlast
11/5/2015 at 6:06pm
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200 watts at 12v = 17 amps. Therefore 2.5mm multistrand cable will be fine (22 amps capacity). You could get away with 2mm cable (18 amps).

It is immaterial what you connect in parallel to it.


Personnavver
11/5/2015 at 10:26pm
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I would go at least 6mm or preferably 10mm to limit the volt drop.
6mm 4.1%
10mm 2.49%
16mm 1.59%

Could try 6mm for each panel with 2 cables.

We try to keep it below 1% on commercial installations.
This is assuming 12 volts. Do you know the panel voltage before it reaches the controller as it may be more.


Personoldskoolskater
11/5/2015 at 11:00pm
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Thanks for your answers folks, the panel specs are as follows:

Maximum Power: 100 Watts
Maximum Voltage: 17.8V
Maximum Current: 5.62A
Open circuit voltage: 21.6V
Short circuit current: 5.97A
Conversion efficiency 22%

Hope this helps - looks like 4mm should be the minimum I should be looking at rather than the maximum!

cheers


Personfreeatlast
12/5/2015 at 12:37pm
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I'm sorry but navver is completely OTT.

Do the maths.

2.5mm cable has a resistance of 2.75 ohms per 1000 metres. On 4 metres that is a resistance of 0.011 ohms. That equates to a voltage drop of 0.06volts.

A 6mm cable would have a voltage drop of 0.009 volts.

Frankly on 17.8v, a voltage drop of 0.06v is negligible.


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Personnavver
12/5/2015 at 11:06pm
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100watts is 17.8 volts at the max current of 5.62amps. So 11.24amps for 2 panels.

Volt drop for 4mm is 11mV/A/m from table 4D2B of BS7671 2core cable DC.

VD = 11 x 11.24 x 4 = 494.56mV = 2.8% Volts drop which is too much.

Volt drop for 6mm is 7.3mV/A/m from table 4D2B of BS7671 2core cable DC.

VD = 11 x 7.3 x 4 = 321.2mV = 1.8% Volts drop which is still too much if you want to stay below 1%.

2.5mm has a volt drop of 18mV/A/m.

I'll write to the IEE now IET to tell them they've got their tables wrong.


PersonOnewheelonmywagon
13/5/2015 at 12:06am
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So, for 2.5mm cable
VD = 18 x 11.24 x 4 = 809mV, less than a volt.

Not much when the panels are supplying 17.8 volts, and the OP has an MPPT controller.
I would concentrate more on the cable from controller to the battery which will be operating at 13 to 14 volts.


Personnavver
13/5/2015 at 12:28am
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But still 4.5%


PersonOnewheelonmywagon
13/5/2015 at 12:34am
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So why should we aim for less than 1% ?


Personoldskoolskater
13/5/2015 at 5:20pm
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Thanks All for your opinions, much appreciated - so what cable from controller to the battery then?


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Personnavver
13/5/2015 at 8:13pm
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We go for 1% on commercial installations with 3phase invertors located as close as possible to the panels. With 400volts on the ac side 1% is 4 volts. If the invertor is some way from the supply authorities intake the invertors protection must trip the installation if voltage goes above 440 (400+10%). with 4 volts drop in the cable we get early tripping when the supply authorities network is lightly loaded and hence near its 440 volt maximum.

Your situation is different in that the controller is near the battery and the dc side is the longest run. I don't think there is any inherent problem with excessive volt drop in the dc other than loss of efficiency.

Commercial installations sell their power via the feed in tariff etc so power is money. They run all year round is it is worth investing in bigger cables to reduce losses. You have no such financial loss to worry about.

So if you go with 2.5mm you will lose 4.5% of your free electricity, but who cares when it's free and installation will be easier.

Of course, you can reduce this loss to a quarter by connecting the panels in series but your controller will need to be able to handle the higher voltage.


Personoldskoolskater
13/5/2015 at 10:05pm
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That is interesting indeed as there's no reason why I can't connect in series rather than parallel. The controller is a Western WRM-15 so presume the higher voltage wouldn't be a problem but will check.

If i can quarter the losses by doing this then the 4 sq mm cable that comes with the panels should do the trick nicely when handling volt drop.


Personoldskoolskater
13/5/2015 at 10:17pm
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The specs show I should be ok


PersonOnewheelonmywagon
13/5/2015 at 11:16pm
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I believe MPPT controllers work more efficiently if you connect the panels in series.


Personoldskoolskater
13/5/2015 at 11:20pm
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Looks like that will be the way forward for sure - series it is.

Just need to look at the bonding of the flexible panellist the roof now - think it will be Sikaflex 252 around the perimeter and a few blogs in the middle


Personoldskoolskater
13/5/2015 at 11:21pm
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Blobs not blogs - bloomin autocomplete!



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