Correct cable sizing for solar panels.

[VicS]

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

Ive been over it again and still come up with between 4 AWG and 6 AWG ( 21.2 mm² and 13.3 mm² respectively) but that is for the entire 8m distance (16 metres total wire length) and taking Maplin's I_max of 9,72 amps per panel

Also calculated using some basic physics and a dollop of arithmetic, rather than using tables and online calculators, and arrived at 17.3 mm ² for a 3% volts drop

In either case rounding up comes to 20mm², while rounding down comes to 15mm²

 

[RichardS]

Ive been over it again and still come up with between 4 AWG and 6 AWG ( 21.2 mm² and 13.3 mm² respectively) but that is for the entire 8m distance (16 metres total wire length) and taking Maplin's I_max of 9,72 amps per panel

Also calculated using some basic physics and a dollop of arithmetic, rather than using tables and online calculators, and arrived at 17.3 mm ² for a 3% volts drop

In either case rounding up comes to 20mm², while rounding down comes to 15mm²

Is the OP's controller really that far from the batteries?

Richard

 

[VicS]

Is the OP's controller really that far from the batteries?

Richard

Hopefully not but ll he has told us is that its 8m from panels to battery.

"Just hoping to add a trifle to the sum total of human knowledge" or some such BS that I read somewhere recently.

 

[RichardS]

Hopefully not but ll he has told us is that its 8m from panels to battery.

"Just hoping to add a trifle to the sum total of human knowledge" or some such BS that I read somewhere recently.

What? I ask a genuine question to try to help us all understand why you're recommending twice as heavy gauge cables for a 300W system than you helpfully did for my 400W system and I get a sarky comment.

Have you even read post #15?

Richard

 

[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 about right, 1kWh is about 50% discharge of 140Ah 12V batteries at 100% efficiency.

 

[PaulRainbow]

Hopefully not but ll he has told us is that its 8m from panels to battery.

Not sure what maths you are using, but 15/20mm cable is absolutely nuts.

From the panels to the controller the voltage will be 20v or so, 6m of 10mm cable will have about 1% volt drop at 10a, still less than 1.5% at 15a (which he'll be lucky to see in the UK). 2m of 6mm from the controller to the batteries is also 1% at 10a and about 1.5% at 15a.

Again, i've never, ever, seen a 15/20a controller that can take anything like 20mm cable. Neither have i seen a solar kit that comes supplied with such ridiculous cable. Victron, for instance, recommend 6mm cable, up to 5m, for their 15a MPPT controllers

 

[PaulRainbow]

That's about right, 1kWh is about 50% discharge of 140Ah 12V batteries at 100% efficiency.

Not quite sure what your point is, but, 1kwh is about 83ah, i don't use that in a 24hr period. So, during the Summer the panel yield during the day is substantially greater than my consumption, therefore, at the end of the day (when the panels stop charging) my batteries are fully charged. During the evening/night my use might be 20ah/30ah or so, Worse case scenario then, by morning my batteries (390ah) will be circa 90% charged. As i said, between Feb (when i fitted the new solar system) to Sept, i ran the boat exclusively on solar power, apart from some minor engine charging.

 

[PaulRainbow]

What? I ask a genuine question to try to help us all understand why you're recommending twice as heavy gauge cables for a 300W system than you helpfully did for my 400W system and I get a sarky comment.

Have you even read post #15?

Richard

Imagine how thick the cables are on a BMW 5 series estate, fitted with a 170a alternator and the battery in the boot. :hororr::hororr:

 

[RichardS]

Imagine how thick the cables are on a BMW 5 series estate, fitted with a 170a alternator and the battery in the boot. :hororr::hororr:

Indeed. My Jag has a 200A alternator in the front and the 100Ah battery in the boot. I don't know what size the cables are but they are gigantic compared to Vic's 10sqmm ones on my boat. However, I expect the Jag cables are sized for starter motor current draw rather than alternator output.

Richard

 

[PaulRainbow]

Indeed. My Jag has a 200A alternator in the front and the 100Ah battery in the boot. I don't know what size the cables are but they are gigantic compared to Vic's 10sqmm ones on my boat. However, I expect the Jag cables are sized for starter motor current draw rather than alternator output.

Richard

If you need 20mm cables for 15a of charging you must have something resembling the national grid under the Jag

 

[gregcope]

Something to consider is to wire the controller into the back of a off-1-2-both switch and the negative bus bar ~ this not only reduces the amount of cable required in many cases as you are likely to put your controller near the other lecy gubbins, but also benefits from cables already run to the batteries and these are usually very large.

 

[lw395]

Indeed. My Jag has a 200A alternator in the front and the 100Ah battery in the boot. I don't know what size the cables are but they are gigantic compared to Vic's 10sqmm ones on my boat. However, I expect the Jag cables are sized for starter motor current draw rather than alternator output.

Richard

The current from the alternator is mostly going to loads like the electric power steering, lights, heaters and so forth.
One side of the circuit is the body shell.
The other side will be a fat wire.
A bit of voltage drop will not matter, the car battery will get charged adequately if the cable limits the charge to say 20A.

On the starter motor side of the equation, I'd not be surprised if the cabling is not rated for running the starter more than bursts of 20 seconds or something?
It's not a comparable problem to solar which may run at full power for hours.
And the boot of a beemer is probably only 2.5m from the starter?

 

[RichardS]

The current from the alternator is mostly going to loads like the electric power steering, lights, heaters and so forth.
One side of the circuit is the body shell.
The other side will be a fat wire.
A bit of voltage drop will not matter, the car battery will get charged adequately if the cable limits the charge to say 20A.

On the starter motor side of the equation, I'd not be surprised if the cabling is not rated for running the starter more than bursts of 20 seconds or something?
It's not a comparable problem to solar which may run at full power for hours.
And the boot of a beemer is probably only 2.5m from the starter?

You're deffo on the wrong track there I'm afraid LW. The positive cable will indeed run the full length of the car but even the negative cable, although only 20 or 40 cms long still have to be very chunky 'cos for a high compression 4 litre V8 on a winter's morning you would be looking at 500+ Amps.

Even though this might only be flowing for 5 or 10 seconds there is no comparison with any solar array on any boat I have ever seen or will ever see. Now, if we're talking about the National Grid solar arrays, you have a point.

Richard