Confusion over solar cables

[shinobidef]

I have a 640w solar array (2 x 320W panels in series) - each panel has a VoC of 40.9, so that's a total VoC of 82 and a amperage of about 53A. Looking at the solar panels, they are 4mm with MC4s.

The solar panels need to be fed into a 50A MPPT controller which says in the instructions that a minimum of 10mm cabling should be used (but to be honest, it doesn't even look like the terminals would take anything larger). My battery bank has a voltage of 12 (2 x 6V).

Anyway, I've run my data into a few solar calculators to work out the cable I need and all of them are coming out with 4-25mm for a 3 metre distance between solar panel and MPPT with 3% voltage drop.

E.G. wire-sizing-calculator (distance doubled as per instructions - so 6 metres) - 15mm
Wire sizing calculator for Solar Panel Arrays - 4mm
http://www.solar-wind.co.uk/cable-sizing-DC-cables.html - 25mm

The confusion has arisen because:
1) A solar company has advised me to use the same cable as is currently on the panels (4mm) - quite a difference between what the calculators are coming out with - however, 4mm has only a 36A rating whereas my output into the MPPT will be 53A at most.
2) If I use anything larger than 4mm (as per the calculators), I won't be able to connect the cable to the male/female MC4 connectors currently on the panels, nor will I be able to fit anything larger than 10mm into the MPPT
3) I'm getting different outputs from the calculators which makes me wonder if I'm using them properly!

Can anyone help? I'm utterly bewildered!

 

[Admin1]

The amps of your solar is watts divided by voltage. So 630 divided by 82 = 7.7A max. So 4mm CSA (11 AWG) is fine for the input to the solar controller. You could have a 10 metre run and still have lower than 1% voltage drop.

630 watts divided by your charge voltage 12v - 14.4v (may vary) = 52.5A - 43.75A. So you need to cable for a minimum of 52.5A at 12v, 10mm cable is fine depending on the length of run. If you kept the cables to 2 metres then 10mm would give you only a 3% max voltage drop.

The best thing to do is to get the solar controller as close to the batteries as possible, to keep the battery cables short. The voltage drop on the solar side will have no real effect due to the higher voltage.

 

[st3v3]

These allow big things in small holes...


Oooh er....

Pin crimps.

 

[shinobidef]

Thank you. I realise I had the amperage listed incorrectly in my first message as the amps wouldn't be doubled if in series.

Just to check I've understood this, are you suggesting a 4mm cable or a 10mm cable from MPPT to solar? Looking at the below, the cables seem to be based on two different calculations (one with wattage/VoC and one with wattage/system voltage, and then I've seen another calculation online that says to work out amps with wattage/Vmpp) - the different formula and calculator outputs have made my head spin.

If I use a 10mm cable, how will I connect this to an MC4 cable on the solars (they're currently using a 4mm cable)? The MPPT controller instructions do suggest using a cable of no less than 10mm but I'm not sure if it's referring to the panels, or to the battery - or whether this is dependent on whether the panels are in series or parallel.

Should I be connecting the panels in parallel or series? I'd prefer series as it's just more simple, but will that be ok with the MPPT?

Finally, if I connect in series, am I ok to use the existing 4mm solar cable to connect the +ve to the -ve on the panels?

 

[listerdiesel]

Spreading your queries over a few forums makes it difficult to keep up with you!

I think you need to slow down and look over your work plan and try to get details sorted one at a time, you are getting ahead of yourself and losing detail in the process.

If you open the connection boxes on the back of the panels, you will see the terminals.

We fitted 6mm cable to all of ours, 6mm HO7 TRS cable, which was easier to gland through the roof etc than two small singles.

Later panel from the Mercedes

Peter

 

[shinobidef]

Spreading your queries over a few forums makes it difficult to keep up with you!

Lol - I only have two "go-to" forums

Thanks for everyone's responses (and for photos also). I'm hoping to order everything I need in one fell swoop as so frustrating to get stuck into something, only to find out your're missing bits and pieces, or have ordered the wrong stuff. Still finalising the wiring diagram but feel like I'm almost there now, except for the solar cable.

 

[Admin1]

Thank you. I realise I had the amperage listed incorrectly in my first message as the amps wouldn't be doubled if in series.

Just to check I've understood this, are you suggesting a 4mm cable or a 10mm cable from MPPT to solar? Looking at the below, the cables seem to be based on two different calculations (one with wattage/VoC and one with wattage/system voltage, and then I've seen another calculation online that says to work out amps with wattage/Vmpp) - the different formula and calculator outputs have made my head spin.

If I use a 10mm cable, how will I connect this to an MC4 cable on the solars (they're currently using a 4mm cable)? The MPPT controller instructions do suggest using a cable of no less than 10mm but I'm not sure if it's referring to the panels, or to the battery - or whether this is dependent on whether the panels are in series or parallel.

Should I be connecting the panels in parallel or series? I'd prefer series as it's just more simple, but will that be ok with the MPPT?

Finally, if I connect in series, am I ok to use the existing 4mm solar cable to connect the +ve to the -ve on the panels?

4mm CSA cable from solar panels to solar controller (make sure the controller will work at 82v).
10mm CSA from solar controller to batteries (keep this cable as short as possible).

Basically the higher the voltage, the lower the amps for the same wattage. The lower the amps, the thinner the cable.