Twin core cable for solar panels

[PeterF]

I am considering fitting solar panels and these will have to be mounted on the front half of the boat. I expect to run the cable back along the roof before  taking it inside to avoid having to make a hidden route internally. I have been looking at installations on other boats in passing during my current journey and a lot of these had only a single cable running over the roof with what appeared to be a reinforced translucent outer sheath and obviously at least two cores. I have tried to find such cable as it will be neater than having both a positive and a negative wire, but have come up empty handed. Does anyone know of this type of cable. The other thought I had is, is it just reinforced plastic hose with the two standard cables run through together.

[Chewbacka]

A quick google shows most sell single core, but I did find this. http://www.sunstore.co.uk/product/4mm-outdoor-solar-cable-2-core-1m/

[reg]

Whatever cable you use to run along the roof it is essential to have adequate UV protection. 

 

For exposed areas, such you plan, I would use purpose made solar cable, has correct level of uv protection and is double shielded. 

The cable Chewbacka linked to appears to meet the uv requirement. Only wish I had known about when I did my setup. 

Edited June 26, 2018 by reg

[PeterF]

Thanks to both of you for that info. I have done some more searching, the stuff looked like the cable in this link, although that is not rated as outdoor cable unless protected. The clear outer sheath will not be UV resistant. However it will provide some mechanical protection. SY armoured flexible cable. I have only linked one example of this, not necessarily the exact number of cores and conductor area.

Edited June 26, 2018 by PeterF

[Tony Brooks]

If you are running the cables close to the full length of the boat even 4 sq mm CCSA cable may not be large enough if you have a larger array operating with panels in parallel. You need to work out the likely current flow and then specify the cable to minimise volt drop. Remember the cable length is out and back. If there is to be more than one panel then connecting them is series with an MPPT controller would reduce the current flow down the cable.

[matty40s]

The one you link to is going to see a lot of voltage drop over a front end solar panel to back end controller. 

Just bite the bullet and buy a reel of 6mm2 photovoltaic cable and maximise your output. If it has to run the length of the boat see if you have an internal roof conduit to take it, or just run it down the side channel like countless others.

[PeterF]

Tony, thanks for the reminders, yes I expect to need 6mm2, I had originally thought parallel, but new I have read up panels in series is best.

 

Matty, the cable I linked was an example of the type and agree it would be too small. I have no roof conduit in my boat, I could run it under gunwhale, but again not sure if there is a conduit or I would just box it in. I would still need to get it from roof to under gunwhale without looking a mess. Putting it along the roof is the simpler option, which I have seen on lots of boats.

[reg]

Just to add that I used 6mm for my setup which is similar to the one you propose. 

 

I would recommend  that it at all possible the cables should he run internally. Number of benefits

• Only need solar cable for the external short run 
• Normal internal cable can be used so saving money
• No danger of cables along length of roof comprising a potential safety hazard. 
• Internal cables not subject to UV radiation

This one Is something I've always believed but happy to be educated if I'm wrong

External cables resting on roof could heat up in hot weather which makes them less efficient? 

Edited June 26, 2018 by reg

[reg]

Out of interest how many panels do you envisage using and what size will they be? 

[PeterF]

2 x 180W, after the prompt from Tony, wired in parallel so around 40V and 9A.

 

I will have a good look at internal, remove some light fittings and see what is above the ceiling lining or consider making a small channel for the wires down to gunwhale level with some oak trim that does not protruse much and hence will not be a head or arm hazard.

[reg]

1 hour ago, PeterF said:

2 x 180W, after the prompt from Tony, wired in parallel so around 40V and 9A.

 

I will have a good look at internal, remove some light fittings and see what is above the ceiling lining or consider making a small channel for the wires down to gunwhale level with some oak trim that does not protruse much and hence will not be a head or arm hazard.

I think Tony said, and I agree, that they should be wired in series giving 80v and 4.5a. When paired with a decent mppt controller this is a better setup as you are more likely to be able to get a charging voltage during low light level, also you are only passing a 4.5a current along the cables to the controller. 

The mppt controller will boost the current input based on the total power available so an input of 9a at 40v has the same power as an input of 80v at 4.5a.

This assumes you will be using a decent mppt, which for the size of your array, you should. 

 

Series connection primary benefits

Lower current along solar to controller cable

Higher  voltage avaliable during low light conditions so more charging hours e.g mornings, evening, overcast and winter conditions. 

 

[PeterF]

Oops, should have written series, I did the volts for series and wrote the wrong word.

[WotEver]

12 hours ago, reg said:

External cables resting on roof could heat up in hot weather which makes them less efficient? 

It wouldn’t be measurable

 

[Tony Brooks]

FWIW; I have had some automotive PVC insulated  2sq mm CCSA draped from my garage, over a support on the greenhouse and down to ground level to supply a can deterrent (Ha Ha) for many years and the outer sleeve has not noticeably deteriorated.

[reg]

1 hour ago, WotEver said:

It wouldn’t be measurable

 

OK thanks for that, wasn't sure hence question mark. 

[reg]

44 minutes ago, Tony Brooks said:

FWIW; I have had some automotive PVC insulated  2sq mm CCSA draped from my garage, over a support on the greenhouse and down to ground level to supply a can deterrent (Ha Ha) for many years and the outer sleeve has not noticeably deteriorated.

Thanks for the observation. Was talking to someone recently who had had their panels installed using non solar wiring, although only a couple of years old, the cable had deteriorated and gone brittle with inner wires showing through at some points. Red had gone pinkish and had a 'bloom'. So on balance my preferance is to go for cable that is rated as providing UV protection. 

Edited June 27, 2018 by reg

[MtB]

1 hour ago, WotEver said:

It wouldn’t be measurable

 

 

Yes it would, and it would be zero! 

[Scholar Gypsy]

Resistance does increase with temperature for most conductors, by about 0.5% per degree C.   So measurable if you  have sensitive enough equipment?

[reg]

48 minutes ago, Scholar Gypsy said:

Resistance does increase with temperature for most conductors, by about 0.5% per degree C.   So measurable if you  have sensitive enough equipment?

So for an increase of say 25°c there will be a 12.5% increase in resistance? 

So during this hot weather cables laid along a roof would have an effect on current flow? 

 

 

[MtB]

55 minutes ago, reg said:

So for an increase of say 25°c there will be a 12.5% increase in resistance? 

 

That seems highly unlikely to me.

 

But then a cable rising in resistance from 0.001 Ohms to 0.00125 Ohms is hardly going to matter a damn, is it?!

[reg]

28 minutes ago, Mike the Boilerman said:

 

That seems highly unlikely to me.

 

But then a cable rising in resistance from 0.001 Ohms to 0.00125 Ohms is hardly going to matter a damn, is it?!

Good point well made, original question was purely academic. Pragmatically I accept it would make no real difference. 

[WotEver]

5 hours ago, Mike the Boilerman said:

 

Yes it would, and it would be zero! 

If there’s one thing I can’t stand it’s a pedant. Well, to be strictly correct I guess it’s not just one thing that I can’t stand, there are several things. And ‘not stand’ is probably too strong a phrase, with ‘dislike’ being more accurate...

 

Did you hear about the pedant who walked into a bar? Well, actually it was a restaurant with a bar. Technically I suppose it was a brewpub because it had an on-site micro-brewery. 

[Psycloud]

I use this  https://www.ebay.co.uk/itm/5M-Twinflex-6mm-Battery-Cable-Double-Insulated-Solar-Charge-Control-Cable/202068610908?hash=item2f0c3a4f5c:g:DyMAAOSwPh5ZEvFb