Solar Panel wiring help / guides / feedback

[gregcope]

Hi All,

I am planning on mounting;

2x36W panels on either side of the coach roof - wiring to go to the bridge deck instrument panel. These will always be connected. The first 50cm of cable will need to be 2.5mm2 or smaller due to having to run them in halyard runs under the coachroof deck back to the Bridge deck. These are going to be mounted on the liftable panels on a Starlight 35.

Mock up here;



Cables exit underneath, on the edge nearest the Sprayhood, and I propose to drill a hole and run them inside of the Halyard run (ie underneath the liftable pannel - attached in the outside corner of the panel, back long the inside edge of the liftable panel to the bridge deck spary bar(?) that the Spray hood is attached to - then inside that towards the centre, where the instrument binnacle is.

Also plan on adding at a later date - 2x32W flexible panels - again wiring to connectors on the bridge deck instrument panel These may or may not be connected - logic being we would deploy them when needed (e.g. Anchorage fridge running/wine cooling!!!). Probably on the spray hood/deck deployable.

I plan on using a Victron Energy MPPT Blue Solar 75/15 controller. This takes 6mm2 cables (solar/load/batteries). It will be mounted in a locker near the batteries.

I have read that the panels should be connected in parallel.

Question;

Where should the parallel join be made?

I have a few options;

1. Run individual 2.5mm2 cables from each panel/connector to the controller. Cheap, easy, easy to debug issues. Worst voltage drop.

2. Run individual 2.5mm2 cables from each panel/connector to the bridge deck and then connect them in parallel here, running a 6mm2 cables from here to the controller. Nasty connection needs to be made in the bridge deck area. Cheapish. Medium voltage drop.

3. Run 6mm2 cables everywhere. Most expensive. Best voltage drop. Nasty connection somewhere inaccessible.

Thoughts/comments?

 

[chrishscorp]

What voltage will the panels be kicking out?
and do the manufacturers state in the destructions a minimum cable size?

 

[gregcope]

Vmpp 19.1 V
Voc 22.6 V

No manufactures do not state a min cable size.

 

[VicS]

I have read that the panels should be connected in parallel.

Assuming you are talking about 12volt solar panels I think you will find if you read the Victron controller instructions they recommend that two 12 volt panels should be connected in series:

12V battery and mono- or polycristalline panels
● Minimum number of cells in series: 36 (12V panel).
● Recommended number of cells for highest controller efficiency: 72
(2x 12V panel in series or 1x 24V panel).
● Maximum: 108 cells (3x 12V panel in series)​

This means that when controlled at Pmax the current will be a little less than 2.5 amps from the two 36watt panels in series 2.5mm² will be suitable for a maximum total run length ( pos, neg and interconnection) of between 50 and 60 ft.

The same will apply to the two flexible panels when you add them.

Id think this might allow you to run both pairs directly to the controller without any intermediate connections at the bridge deck.

 

[gregcope]

Thanks VicS.

I did read that, but got confused about Cells and panels.

I have read a few references that parallel, although not so good from a controller efficiency, is better from a shading POV. The other logic for parallel is that one panel could have issues, and the other would still work (same as shading really).

 

[William_H]

Regarding wiring size for solar panels this is not as critical as one might imagine when related to other electrical services. A 36 watt panel will typically deliver 1.8 amps at max 20 volts. This gives an internal resistance of the panel of about 12 ohms. This means that if you have 1.2 ohms of cable resistance you will lose something like 10% of your power. But that is the resitance of a long thin cable. So while heavy cable is far better for lower volt drop and more robust for long life a smaller cable will not be very detrimental to system performance. So to the OP just use what you can fit in. good luck olewill

 

[chrishscorp]

32watt panel

22 volts is 1.4 amps max 19 volts is 1.68 amps max
connect them in paralell to the controller (shading and bird deposits on panels)
Son says to run in 2.5mm he says 6mm is over the top. keep the joints dry and clean coating in Vaseline will help keep out corrosion
Hope that helps

 

[MikeCC]

The aim is to maximise power at the controller. Have you done a voltage drop calculation on the cable run from panel to controller? That's what should determine your cable sizing. You may find 2.5 is fine or maybe 4mm. For a short run, 6mm may be over the top. Just because the controller takes 6mm doesn't mean it's the recommended cable size.
Don't forget for calcs that the voltage and current will be different depending on whether you connect in series or parallel. Plenty of online calculators for this.
Do use tinned cable for maximum life.
Bit confusing that Victron refer to 12V panels though.

 

[William_H]

The aim is to maximise power at the controller. Have you done a voltage drop calculation on the cable run from panel to controller? That's what should determine your cable sizing. You may find 2.5 is fine or maybe 4mm. For a short run, 6mm may be over the top. Just because the controller takes 6mm doesn't mean it's the recommended cable size.
Don't forget for calcs that the voltage and current will be different depending on whether you connect in series or parallel. Plenty of online calculators for this.
Do use tinned cable for maximum life.
Bit confusing that Victron refer to 12V panels though.

"The voltage drop calculation" This is my point that volt drop is not such a great concern. It is no like a mast top nav light or worse a volt drop of a charger. The solar panels typically starts at 20v. If this were connected to a battery direct no controller then around 7 volts gets dropped in the panel internal resistance. If you have a PWM controller again the panel will drop this kind of voltage internally in "on mode". So even a 1 volt drop will make little difference.
Only with an MPPT type controller is volt drop going to be a concern then only as a small fraction of total power.

 

[gregcope]

Both the panel and regulator supplies have replied back with parallel.

Panel supplies has said 2.5mm2 is probably fine. The regulator supplier suggests bigger.

Will drop a voltage drop on the various 2.5mm2 and 6mm2 options.

Anyone know of a small (to take either 8 panel connections - 2.5mm2 or 6mm2 to 2 6mm2 back to reg) connector box?