Correct Wire Size and Type for 100w Solar Panel

T

thesaintlyone

Well-Known Member
Joined
12 Sep 2015
Messages
902
Visit site
If just using one panel prob best to use 6mm^2 cable. Connectors are standard MC-4 plugs
 
thesaintlyone said:
Planning to purchase a 100w Semi Flex Solar Panel for the Longbow tomorrow which will sit on the coachroof and connect to a 30amp Solar charge controller which is already fitted in place ready

http://www.ebay.co.uk/itm/100W-18v-Semi-Flexible-Energy-Solar-Panel-For-Battery-Charger-Boat-Caravan-HOT-/262615059324?hash=item3d2513cb7c:g:p60AAOSwvzRX0VS6

Any idea what is the ideal wire size and type for this panel???

Regards
Click to expand...

You need to know the cable run length from panel through controller to battery to determine the minimum cable size but assuming no more than about 20ft AWG 12.
If significantly less AWG 14 will be adequate but use AWG 10 if any more than 20ft

AWG 10 is 5.3 mm²
AWG 12 is 3.3 mm²
AWG 14 is 2.1mm²

i doubt if the distance is short enough to use AWG 14.
based on my knowledge of the standard la out of the W31s id expect AWG 12 be be adequate unless you run the cable via some tortuous route
 
Last edited:
A major factor in choosing wire size, especially in low voltage applications like 12V DC is the length of the run, in addition to the max current the wire might carry.

What will be the one way distance from the panel to the controller and the controller to the batteries? What is the Voc of the panel?

Here is a link to a handy, online calculator for choosing the proper wire.

http://www.genuinedealz.com/voltage-drop-calculator
 
VicS said:
You need to know the cable run length from panel through controller to battery to determine the minimum cable size but assuming no more than about 20ft AWG 12.
If significantly less AWG 14 will be adequate but use AWG 10 if any more than 20ft

AWG 10 is 5.3 mm²
AWG 12 is 3.3 mm²
AWG 14 is 2.1mm²
Click to expand...

Assuming 12v, about 8 amps over a 20' run that would drop about 0.5V which to me would be more than I would want to lose in a charging circuit. That much voltage drop would dramatically reduce the charge effciency to the battery.
 
Another thing to consider is the distance panel to controller and controller to battery. A controller can deal with voltage drop from the panels, assuming they panel's Voc is adequate. The more critical is controller to battery. In that part of the wiring you want to minimize voltage drop so the batteries get the most from the system.
 
skipmac said:
Assuming 12v, about 8 amps over a 20' run that would drop about 0.5V which to me would be more than I would want to lose in a charging circuit. That much voltage drop would dramatically reduce the charge effciency to the battery.
Click to expand...

But the panel the Op is planning to buy will not deliver 8 amps! Read the tech spec!

The current at MPP is 5.5 amps and the short circuit current is 5.9 amps ... and those will have been quoted for standard conditions.

The regulated volage will be 14.4 volts or thereabouts so even with a MPPt controller the current will be no more than 7amps in the controller to battery wiring
 
Last edited:
skipmac said:
Another thing to consider is the distance panel to controller and controller to battery. A controller can deal with voltage drop from the panels, assuming they panel's Voc is adequate. The more critical is controller to battery. In that part of the wiring you want to minimize voltage drop so the batteries get the most from the system.
Click to expand...

20.5 volts. Read the tech spec
 
Keep the cable run to the controller short, using 2.5mm cable. From the controller to the battery use 6mm cable.


Edit : I'd use 6mm from the controller to the batteries in case more panels might be added later. If there is no chance of this 2.5mm cable would be OK inside and out.

There is no chance the panel will output the voltage and amperage stated in the Ebay listing, particularly in the UK.
 
Last edited:
I have 100w in 2x50w which I am planning to put in series. The run to my batteries is 6m so I have ordered 15m of this :
http://www.ebay.co.uk/itm/192084795...49&var=491977128980&ssPageName=STRK:MEBIDX:IT
6mm2 with MC4 connectors pre-crimped on. Plan A is to cut it in half and use the MC4 ends outside.
To get the controller as close as possible I won't be able to see the leds blinking.
Mounting the controller where I can see it would mean a 2m run to the batteries, will that matter with the 6mm cable?
It's a Victron MPPT 75/10 controller.
 
aquaplane said:
I have 100w in 2x50w which I am planning to put in series. The run to my batteries is 6m so I have ordered 15m of this :
http://www.ebay.co.uk/itm/192084795...49&var=491977128980&ssPageName=STRK:MEBIDX:IT
6mm2 with MC4 connectors pre-crimped on. Plan A is to cut it in half and use the MC4 ends outside.
To get the controller as close as possible I won't be able to see the leds blinking.
Mounting the controller where I can see it would mean a 2m run to the batteries, will that matter with the 6mm cable?
It's a Victron MPPT 75/10 controller.
Click to expand...

The cable and the plan is workable, but massively over spec. You could have used 2.5mm all the way with a 6m run.

It is not a problem not being able to see the LEDs on the controller, they are a distraction anyway. My Victron controller is mounted in an overhead locker.
 
PaulRainbow said:
The cable and the plan is workable, but massively over spec. You could have used 2.5mm all the way with a 6m run.

It is not a problem not being able to see the LEDs on the controller, they are a distraction anyway. My Victron controller is mounted in an overhead locker.
Click to expand...

The obvious place to mount the controller in a Longbow if the panel is on the coach roof is in the heads compartment but closer to the battery would be better. That probably means down in the battery compartment

I am surprised that the OP has room on the coach roof for the 100watt panel he is intending to buy!
 
PaulRainbow said:
Keep the cable run to the controller short, using 2.5mm cable. From the controller to the battery use 6mm cable.
Click to expand...

Personally I'd say that's too small as the cost/hassle/benefit calc seems wrong. An MPPT controller will typically require VBat + 5V to start. Toss in a couple of connections and a small gauge wire and one risks starting the controller a little later in the morning and shutting down a little earlier in the evening. Dull UK days as well as days when the panel gets very hot will be especially problematic depending on the panel's temperature coefficient. Not to mention that if the panel is roving it is nice to have a bit of length in the cable, or the possibility of adding an extension.

Finally if OP ever wishes to add another panel in parallel he'll have to redo the entire shebang with thicker cable.
 
VicS said:
The obvious place to mount the controller in a Longbow if the panel is on the coach roof is in the heads compartment but closer to the battery would be better. That probably means down in the battery compartment
Click to expand...

I've not mounted my new controller yet ..... but the Victron manual says "never mount directly above the battery in order to prevent damage due to gassing of the battery."

That has put me off mounting it in the battery compartment even though I have sealed batteries and would have thought that it would be OK.

Has anyone else successfully mounted the controller there?

Richard
 
dom said:
Personally I'd say that's too small as the cost/hassle/benefit calc seems wrong. An MPPT controller will typically require VBat + 5V to start. Toss in a couple of connections and a small gauge wire and one risks starting the controller a little later in the morning and shutting down a little earlier in the evening. Dull UK days as well as days when the panel gets very hot will be especially problematic depending on the panel's temperature coefficient. Not to mention that if the panel is roving it is nice to have a bit of length in the cable, or the possibility of adding an extension.

Finally if OP ever wishes to add another panel in parallel he'll have to redo the entire shebang with thicker cable.
Click to expand...

The controller will start however thin the cable is, because until the controller starts, there is no current, so there fore no ohmic drop.

The cable to the panel must be rated to carry whatever the panel can throw at it. So Isc plus a margin. That is a safety limit.
If the cable is long and there is a significant ohmic drop, that affects efficiency. If you lose 5% of the volts, that's 5% of the power lost, but only in peak sunlight, when your batteries might not want the charge anyway. In reduced light the % power loss is less.

Voltage drop from reg to battery is more serious, because the reg senses the volts at its output, and therefore reduces the current to the battery, Dropping 0.5V in this run will make the reg think the battery is fully charged when it's far from it, unless the reg has remote sensing (on both + and -). So this run of cable will be far thicker than its safety limit, and hopefully not very long.

Not sure the bog is the 'obvious' place to mount any high cost electronics.
 
I read all these threads avidly because I'm also doing all this stuff at the moment. FWIW using 2.5mm oceanflex for a 4m run with a 50W panel, & as I have some left over was going to use the same to the battery, about 750mm of cable run from the battery.

But the reason for the post is that this made me laugh. Out loud.

lw395 said:
Not sure the bog is the 'obvious' place to mount any high cost electronics.
Click to expand...
 
dom said:
Personally I'd say that's too small as the cost/hassle/benefit calc seems wrong. An MPPT controller will typically require VBat + 5V to start. Toss in a couple of connections and a small gauge wire and one risks starting the controller a little later in the morning and shutting down a little earlier in the evening.
Click to expand...

An MPPT controller is not the best choice with a single panel. Not all controllers require a 5v differential. 2.5mm cable, up to about 5m to the controller is correct for a 100w panel.

Dull UK days as well as days when the panel gets very hot will be especially problematic depending on the panel's temperature coefficient. Not to mention that if the panel is roving it is nice to have a bit of length in the cable, or the possibility of adding an extension.
Click to expand...
Dull days yield less power, so i can't see your point here. He is mounting the panel to the coachroof, not roving.

Finally if OP ever wishes to add another panel in parallel he'll have to redo the entire shebang with thicker cable.
Click to expand...

Of course he won't. I advised 6mm from controller to the batteries, if he fits enough panels so that 6mm is under spec the controller will be overloaded. If he fits another panel in parallel that panel will be wired to the controller directly, it will not require the existing wiring to be altered in any way. If, on the other hand, he fits a second panel and wires it in series with the existing panel the 2.5mm cable will still be as adequate as it is now.

In short, 2.5mm cable from the panel to the controller, max length 5m. 6mm cable from controller to batteries. Job done.
 
Iminei said:
I read all these threads avidly because I'm also doing all this stuff at the moment. FWIW using 2.5mm oceanflex for a 4m run with a 50W panel, & as I have some left over was going to use the same to the battery, about 750mm of cable run from the battery.

But the reason for the post is that this made me laugh. Out loud.
Click to expand...

What you propose with your 50w panel is perfectly fine :encouragement:
 
lw395 said:
The controller will start however thin the cable is, because until the controller starts, there is no current, so there fore no ohmic drop.

The cable to the panel must be rated to carry whatever the panel can throw at it. So Isc plus a margin. That is a safety limit.
If the cable is long and there is a significant ohmic drop, that affects efficiency. If you lose 5% of the volts, that's 5% of the power lost, but only in peak sunlight, when your batteries might not want the charge anyway. In reduced light the % power loss is less.

Voltage drop from reg to battery is more serious, because the reg senses the volts at its output, and therefore reduces the current to the battery, Dropping 0.5V in this run will make the reg think the battery is fully charged when it's far from it, unless the reg has remote sensing (on both + and -). So this run of cable will be far thicker than its safety limit, and hopefully not very long.

Not sure the bog is the 'obvious' place to mount any high cost electronics.
Click to expand...

True re initial startup, but once the controller activates it will have another algorithm requiring panel voltage to exceed VBatt by 1 or 2 volts. Excessive losses in the cabling can risk the MPPT measuring an unexpected drop in voltage upon start-up to below its limit thereby triggering a shutdown for a while. That's why with a single 12V panel I would personally try and minimise voltage drops. At even 6A over say 6m with a couple of connections one wld be looking at a c.0.75V drop with 2.5mm^2 which one could halve by going to 4mm. Guess I'm a bit lost as to the preference for smaller size,

As for the bog comment; not sure I get that?
 
You must log in or register to reply here.

Share this page

Top