10 mm solar cable advice needed

[incredibalex]

I'm planning on adding 3x165w (Imax: 4.72A ISC: 5.18A) solar panels to my 12v system and mounting them at the front of my boat. This will put them 11m from where the MPPT controller will be mounted. Not ideal I know... I've been told that a 6mm solar cable will be sufficient but as I have found some 10mm at £1.35/m I am considering going for the thicker stuff.

 

Can anyone give me any reason why I shouldn't go for the 10mm cable? Or maybe more specifically why I shouldn't go for this stuff in particular? http://quickbit.co.uk/10mm-solar-cable

 

Thanks in advance.

 

Alex

[by'eck]

10mm² as a minimum - thicker would be better.

[jodansgang]

10mm will make the connections onto the panels themselves quite difficult. Make sure the "lugs" are suited for both the cable and the terminals on the panels.

I am speaking from experience as my system is similar.

[John V]

Thicker the better....within reason of course....I've got some 16mm 3 core flex......proper bxxxard to get a 13A plug on

[jim and pat dalton]

We bought our solar PV package from Bimble solar (3 x 165w panels) and their cable supplied to us was 6mm.

10mm would not have worked with any of the plugs or the Mppt fittings. I'm sure, well I know the rule that thicker the cable you have the less voltage drop.

But that fact is we are getting plenty of power back from the panels and are going to increase our battery bank to accommodate more. Our panels are mounted on the front of the boat and the Mppt controller in the engine ole.

[Roger Crown]

If you are using a MPPT controller you could consider wiring the panels in series (nominal 36volt system), which would reduce the volt drop considerably.

As Jim and Pat above comment, 10mm2 cable does not readily fit with standard solar panel waterproof plug and socket connectors (Mp4)

 

If wiring in series you may want to consider by-pass diodes some info here -- http://www.solar-facts.com/panels/panel-diodes.php

 

Regards

Roger

Edited May 22, 2014 by Crown Narrowboats Ltd

[Grace and Favour]

OP would need to source specialist connectors I fear - the normal MC3 / MC4 ones will never ever fit, surely?

 

 

Makes much more practical sense to fit panels in series

Edited May 22, 2014 by Grace & Favour

[Clifford]

Interesting to look at the power dissipated in the wiring..

 

For 480 watts of 12v panels wired in parallel wth 10m of twin 6sqmm cable.

 

 

 

Resistance of 6mm single wire = .003 ohm per metre

Resistance of twin wire = .006 ohm per metre

Resistance of 10m of twin wire = .06 ohm

 

Current in wire to controller = power / voltage = 480/12 = 40 amps.

Volt drop along wire (both ways) = current * resistance = 40*.06 = 2.4v

Power wasted in heating wires = voltage drop * current = 2.4*40 = 96W

 

So in the worst case you would lose about 20% of the power you are collecting, by heating up your wiring.

 

Putting panels in series would reduce this. EG 4*120w in series would raise the voltage at the controller to to 48v, but cut the current to 10 amps.

 

The volt drop in the wires is reduced (in proportion to the current) to a quarter (0.6v). But the power dissipated is reduced BOTH by the reduced current AND the reduced volt drop, so it is reduced by a SQUARED factor (1/4 * 1/4 = 1/16), so power wasted = 96/16 = 6W.

 

(ie Power wasted = 0.6 volts * 10 amps = 6W)

 

So by going to series wiring, you've gone from wasting 96 w to wasting 6W.

 

In reality, you'd have to look at the real lengths of cable involved, but if the furthest panel was 10m away from the controller, the above is about right. Series probably reduces it a bit.

 

10sqmm cable would reduce the above losses to 6/10. saving about 40W for parallel (about 2.5W for series). That is, gaining you an extra 3.5A (.2A in series). So possibly worth it for parallel, probably not for series.

Edited May 22, 2014 by Clifford

[Ex- Member]

Standard 6mm solar cable is designed for the job and works as designed with MC4 connectors.

 

We have had no issues at all and the furthest 240w panel is a 11m away from the controller. Our main 500w array is much closer though around 5m away. we are also wired in series but were initially wired in parallel which again caused no issues or cable problems.

[Stubaby]

The internet is littered with online calculators that will take the witchcraft out of the voltage drop worries.

 

Here are a few ...

 

http://www.calculator.net/voltage-drop-calculator.html

 

http://www.solar-wind.co.uk/cable-sizing-DC-cables.html

 

http://photovoltaic-software.com/DC_AC_drop_voltage_energy_losses_calculator.php

[smileypete]

The OP has quoted the Imp (current at peak power, not a mythical dwarf like creature ) as 4.72A

 

So with panels in parallel the max current to MPPT is 14.16A. With the panels in series, the max current to MPPT is 4.72A.

 

20m of 6mm² is 0.056 ohms. So, with power loss P=I²R that gives 11.4W loss with panels in parallel or 1.26W loss with panels in series.

 

Really, 6mm² solar cable is plenty adequate, even 4mm² would do OK, but sometimes it doesn't work out all that much cheaper anyway.

 

The cable between MPPT and batts is a different matter, 10mm² or 16mm² is better as any voltage drop will reduce the batt charge rate disproportionally at times.

 

cheers, Pete.

~smpt~

Edited May 22, 2014 by smileypete

[incredibalex]

This is all great info. I have gone for the 6mm as I'm pretty sure I can deal with the 1.26W drop and I know the MC2 connectors are definitely going to be compatible.

 

Thanks all for your advice.