Different Pos & Neg PV cable length requirement

[Jez O'Hare]

Dear Forum, I am finally installing my 24v house system using 3x DSSR50 with 6x 72cell Longi 550w panels. On the DSSR50 guide layout image the red positive PV cables are separate and therefore a much longer length is required than the black neg cables which are all joined up into one cable. Please may I just check that this is correct? I plan to use 6x +/-15m red cables from each panel to A & B on DSSR50 and maybe 3x +/-15m black cables paralled at the panels, or perhaps I dont even need so much black cable as the diagram shows. Many thanks.

[Dacian Todea (electrodacus)]

The cable requirement will be the same for positive and negative.

If your panel is 15m from battery then you need 15m red cable for positive going to DSSR50 (DSSR50 will be maybe 1m from battery) and then from DSSR50 positive output 1m of cable to battery probably thicker.

Then you also need about 15m of black PV cable from PV panel negative all the way to battery negative terminal.

It is not practical and very expensive to use a much thicker negative cable from all panels common.

You will get around 14 to 15A from those panels so at most you can connect 3 panels to a single DSSR50.  It seems you will be connecting two panels only so not a problem.

Since panels are just 15m away from battery and you have 72 cell panel you can even use 12AWG (4mm^2) PV cable if you can find it lower cost than more common 10AWG (6mm^2). There will be zero disadvantage in using 4mm^2 PV cable for just around 15 ton 20m.

Of course you could replace the 6 black cables with a single thicker cable capable of handling 100A but that single thicker cable will be more expensive and you need some sort of junction box and connectors to parallel first all the negatives for the six panels so it will sure be more expensive than just 6 black cables with MC4 connectors directly connected to PV panels and have the 6 cables then connected to battery negative. 

You should see a typical charge current of around 90A with peak around 110A in some weather conditions.