Correct cable sizing for solar panels.

[AndrewB]

I have two solar panels each rated at 150w (21v open circuit) connected in parallel, to charge my two 105Ah 12v batteries via a regulator and an ammeter. The distance between the panels and batteries is 8 metres. The 2.5 mm² cable suggested by the retailer - Maplins - burnt out!

When I use an online DC cable sizing estimator (http://www.solar-wind.co.uk/cable-sizing-DC-cables.html), with a loss of 3%, it gives a 50 mm² cable. That's massive! - not to mention expensive.

Can this really be right? What do you use?

 

[NormanS]

Just for interest, I tried that cable size calculator for my modest 100w panel. I said 5amp, and it came up with 16sq mm, which is crazy, but it also said, 99amp max, which is also crazy for a 100w panel.

 

[geem]

I have two solar panels each rated at 150w (21v open circuit) connected in parallel, to charge my two 105Ah 12v batteries via a regulator and an ammeter. The distance between the panels and batteries is 8 metres. The 2.5 mm² cable suggested by the retailer - Maplins - burnt out!

When I use an online DC cable sizing estimator (http://www.solar-wind.co.uk/cable-sizing-DC-cables.html), with a loss of 3%, it gives a 50 mm² cable. That's massive! - not to mention expensive.

Can this really be right? What do you use?

Do you really mean burnt out? Was the cable tinned or plain copper? I run my panels on 2.5mm2 also 150w but at 24v. Not had a problem but my cable run is shorter than yours and my amps are half yours? A neat solution would be to use an MPPT controller and wire the panels in series to drop the current by half

 

[lw395]

You need to size for two purposes.
1) safety. How much current can a cable take without overheating
2) acceptable losses.

300W of panels is a serious business. You need to design for at least 30A, plus preferably some margin.

2.5mm cable is domestic ring main cable. In a spur situation it is typically rated 20A
Several things come into that, including ambient temperature, whether the cable is thermally insulated, bundled with other cables, the max temp of the insulation etc.

Acceptable losses are less of a driver in solar panels than other 12V systems. If you look at the V/I curve of a typical panel, you will see that a volt of loss does not affect the current anywhere near as badly as say having a volt of loss between your alternator and your battery. Plus at lower light when the power is most valuable, the losses drop as a percentage anyway.
So a finger in the air minimum will be something like 6 sqmm, more is better obviously.

A bad connection can be just as bad as cable that's too thin. At 30A, connectors need to be 'proper'.

 

[RichardS]

I have two solar panels each rated at 150w (21v open circuit) connected in parallel, to charge my two 105Ah 12v batteries via a regulator and an ammeter. The distance between the panels and batteries is 8 metres. The 2.5 mm² cable suggested by the retailer - Maplins - burnt out!

When I use an online DC cable sizing estimator (http://www.solar-wind.co.uk/cable-sizing-DC-cables.html), with a loss of 3%, it gives a 50 mm² cable. That's massive! - not to mention expensive.

Can this really be right? What do you use?

I'm got a similar distance to run and I'm using 4 mm sq / 11 AWG. The panels are 4 x 100W wired two in parallel by two in series so the cables carry 45V max and around 15A max without any problems. They are probably a bit on the thin side and 5 mm sq would be better but they're fine. Are you an order of magnitude out with your calc?

Richard

 

[PaulRainbow]

I have two solar panels each rated at 150w (21v open circuit) connected in parallel, to charge my two 105Ah 12v batteries via a regulator and an ammeter. The distance between the panels and batteries is 8 metres. The 2.5 mm² cable suggested by the retailer - Maplins - burnt out!

When I use an online DC cable sizing estimator (http://www.solar-wind.co.uk/cable-sizing-DC-cables.html), with a loss of 3%, it gives a 50 mm² cable. That's massive! - not to mention expensive.

Can this really be right? What do you use?

Depends how you have them connected. If you have joined hem together close to the panels and run a single pair of cables to the controller (assuming it's close to the batteries) you need 10mm sq and a 30a fuse. If you ran a pair of cables from each panel to the controller and connected them there, then 4mm sq and 15a fuses. Use 6mm cable from the controller to the batteries.

However, your system could be better designed, IMO. I would connect the panels in series, as soon as the cables are inside the boat then use 4mmsq cables and a 20a fuse. Connect to a MPPT controller and use 6mm cable to the batteries.

Your panel output is a poor match to your battery capacity. On sunny days you panels will quickly get the bulk of the charge required into the batteries, then spend the rest of the day doing very little. At the end of the day, when the panels stop charging, you will have limited power available. I would be looking to add another battery, or two if you have space.

 

[lw395]

....

However, your system could be better designed, IMO. I would connect the panels in series, as soon as the cables are inside the boat then use 4mmsq cables and a 20a fuse. Connect to a MPPT controller and use 6mm cable to the batteries.

Your panel output is a poor match to your battery capacity. On sunny days you panels will quickly get the bulk of the charge required into the batteries, then spend the rest of the day doing very little. At the end of the day, when the panels stop charging, you will have limited power available. I would be looking to add another battery, or two if you have space.

6sqmm from the controller to the batteries could be a bit light, as any drop here directly reduces the charging voltage to the batteries. It depends on the length, ideally keep it short and do the sums.
The controller may have remote voltage sensing, but possibly only on the +ve side?
The OP is in Dover, so the panel to battery ratio might be justifiable, particularly this time of year?

 

[Richard10002]

Connect in series, 42V, (300W/42V) = 7.25A, Cable run = 16m, cable size =4mm2

Are you connecting them directly to the battery, or have you got a controller?? PWM or MPPT? make, model, capacity?
Cable run from controller and back, via the panels? Cable run from controller and back, via the batteries?

 

[PaulRainbow]

6sqmm from the controller to the batteries could be a bit light, as any drop here directly reduces the charging voltage to the batteries. It depends on the length, ideally keep it short and do the sums.

I'd fit the controller as close to the batteries as possible. 6mm sq will be fine unless he fits the controller a long way from the batteries, which makes no sense.

The controller may have remote voltage sensing, but possibly only on the +ve side?

Not sure what controller he has, but i'd fit a decent MPPT such as a Vicron. The Victron has battery voltage sensing, but i would always disable that, too many cases of mis-sensing.

The OP is in Dover, so the panel to battery ratio might be justifiable, particularly this time of year?

It won't be an issue this time of year, 300w won't keep his current batteries fully charged whilst he's onboard, unless he has ridiculously low consumption. But in the Summer he'll be wasting a lot of his yield, then in the evening/through the night, he'll deplete his batteries more than he needs to.

 

[geem]

Depends how you have them connected. If you have joined hem together close to the panels and run a single pair of cables to the controller (assuming it's close to the batteries) you need 10mm sq and a 30a fuse. If you ran a pair of cables from each panel to the controller and connected them there, then 4mm sq and 15a fuses. Use 6mm cable from the controller to the batteries.

However, your system could be better designed, IMO. I would connect the panels in series, as soon as the cables are inside the boat then use 4mmsq cables and a 20a fuse. Connect to a MPPT controller and use 6mm cable to the batteries.

Your panel output is a poor match to your battery capacity. On sunny days you panels will quickly get the bulk of the charge required into the batteries, then spend the rest of the day doing very little. At the end of the day, when the panels stop charging, you will have limited power available. I would be looking to add another battery, or two if you have space.

I said much the same in my post but would agree that large panels/small batteries is not much use. We have similar sized panels and 1000 amp hr of battery capacity. The batteries go to float about 1330 most days. The large battery bank gives the benefit that you dont cycle them as much as a small bank so they should last longer. We use four 8D batteries that weigh 65kg each. Not all boats have the space for this but it does work well

 

[AndrewB]

Thanks guys, this is most helpful. As I just bought a new PWM 12v/25A controller with the second solar panel, and an MPPT controller for >15A (which I might achieve at peak times) is rather expensive, I'll go with changing the cable, at least for the time being. Your suggestions make it sound like 8mm² cable will be adequate.

I take the point about increasing the battery bank size, and plan to do that as well.

 

[VicS]

Thanks guys, this is most helpful. As I just bought a new PWM 12v/25A controller with the second solar panel, and an MPPT controller for >15A (which I might achieve at peak times) is rather expensive, I'll go with changing the cable, at least for the time being. Your suggestions make it sound like 8mm² cable will be adequate.

I take the point about increasing the battery bank size, and plan to do that as well.

8 mm² will be Ok for the connections between the individual panels and the controller. I'd estimate their max currents will be about 10 amps , but the specs for your panels should tell you exactly what it is .

However for the combined output from the controller to the battery, which is more critical from the volts drop point of view, 20mm² would be preferable unless the cables between the controller and the battery are relatively short.

I am surprised your 2.5mm² cable actually burnt out. It should be able to safely handle the combined output of your two panels! Are you sure you do no/ did not have a short circuit allowing a massive current from the battery to cause it?
The final connection to the battery must be made via a fuse, rated a bit above the max current you might expect from the solar system but well inside the max current rating for the wiring. 30 amps perhaps ?? This will protect the wiring from high currents delivered by the battery in the event of a short occurring.

 

[PaulRainbow]

However for the combined output from the controller to the battery, which is more critical from the volts drop point of view, 20mm² would be preferable unless the cables between the controller and the battery are relatively short.

That's just insane, 20mm is totally wrong unless he's fitting the controller on the boat three berths down.

Max output for his panels will be circa 8a each, it''l be rare that he sees a combined total of 15a. Keeping the controller reasonably close to the batteries, 6mm is correct. I've yet to see a 15/20a controller that can take 20mm cable.

 

[PaulRainbow]

Thanks guys, this is most helpful. As I just bought a new PWM 12v/25A controller with the second solar panel, and an MPPT controller for >15A (which I might achieve at peak times) is rather expensive, I'll go with changing the cable, at least for the time being. Your suggestions make it sound like 8mm² cable will be adequate.

I take the point about increasing the battery bank size, and plan to do that as well.

If you join them together close to the panels and run a single pair of cables to the controller (assuming it's close to the batteries) use 10mm sq. Use 6mm cable from the controller to the batteries, keep the controller as close to the batteries as you can and use a 30a fuse (as close as possible to the batteries).

 

[RichardS]

That's just insane, 20mm is totally wrong unless he's fitting the controller on the boat three berths down.

Max output for his panels will be circa 8a each, it''l be rare that he sees a combined total of 15a. Keeping the controller reasonably close to the batteries, 6mm is correct. I've yet to see a 15/20a controller that can take 20mm cable.

That is a bit surprising as Vic recommended that I buy 10 mm sq cables from Furneax Riddell for the 1 metre-odd run from my Victron to my batteries and that was for 400W of panel. The cables are great and the service from F-R was great so I'd pass on Vic's exact recommendation to the OP.

Richard

 

[lw395]

...
It won't be an issue this time of year, 300w won't keep his current batteries fully charged whilst he's onboard, unless he has ridiculously low consumption. But in the Summer he'll be wasting a lot of his yield, then in the evening/through the night, he'll deplete his batteries more than he needs to.

Using some actual data for today, a 3.6kW system on a roof has delivered 4.7kwh. A fair to middling day compared to the rest of the month.
Peak power today was over 3100W
Scaling this for 300W panels suggests about 32Ah for a day like today.
Average data for the summer suggests about 4x the yield in the best months, dropping to 3x for the ends of the summer season.
assuming that the panels are unlikely to be better positioned than on a pitched roof, it looks like the batteries are unlikely to get more than 50% charge in a day, apart from the very best days in August.
I'd suggest its quite likely the excess charge on nice days in August could be used to chill some beers while the sun shines.

 

[PaulRainbow]

Using some actual data for today, a 3.6kW system on a roof has delivered 4.7kwh. A fair to middling day compared to the rest of the month.
Peak power today was over 3100W
Scaling this for 300W panels suggests about 32Ah for a day like today.
Average data for the summer suggests about 4x the yield in the best months, dropping to 3x for the ends of the summer season.
assuming that the panels are unlikely to be better positioned than on a pitched roof, it looks like the batteries are unlikely to get more than 50% charge in a day, apart from the very best days in August.
I'd suggest its quite likely the excess charge on nice days in August could be used to chill some beers while the sun shines.

My 260w panels yielded 130wh today. During the height of Summer i was getting up 1kwh or so per day. Batteries were going into float around midday. I didn't use the mains charger until early September.

 

[PaulRainbow]

That is a bit surprising as Vic recommended that I buy 10 mm sq cables from Furneax Riddell for the 1 metre-odd run from my Victron to my batteries and that was for 400W of panel. The cables are great and the service from F-R was great so I'd pass on Vic's exact recommendation to the OP.

Richard

Perhaps 20mm was a typo, not like Vics to be so far out.

 

[lw395]

My 260w panels yielded 130wh today. During the height of Summer i was getting up 1kwh or so per day. Batteries were going into float around midday. I didn't use the mains charger until early September.

That's in the med?

 

[PaulRainbow]

That's in the med?

Nope.

Yields from my current log :

[TD="width: 64, align: right"]130[/TD]

[TD="align: right"]100[/TD]

[TD="align: right"]110[/TD]

[TD="align: right"]130[/TD]

[TD="align: right"]90[/TD]

[TD="align: right"]50[/TD]

[TD="align: right"]190[/TD]

[TD="align: right"]130[/TD]

[TD="align: right"]200[/TD]

[TD="align: right"]130[/TD]

[TD="align: right"]110[/TD]

[TD="align: right"]130[/TD]

[TD="align: right"]210[/TD]

[TD="align: right"]180[/TD]

[TD="align: right"]160[/TD]

[TD="align: right"]190[/TD]

[TD="align: right"]100[/TD]

[TD="align: right"]70[/TD]

[TD="align: right"]140[/TD]

[TD="align: right"]230[/TD]

[TD="align: right"]220[/TD]

[TD="align: right"]70[/TD]

[TD="align: right"]230[/TD]

[TD="align: right"]160[/TD]

[TD="align: right"]220[/TD]

[TD="align: right"]160[/TD]

[TD="align: right"]300[/TD]

[TD="align: right"]210[/TD]

[TD="align: right"][/TD]

[TD="align: right"][/TD]

[TD="align: right"][/TD]