wire size for run from solar panels.... crazy!

[centerline]

im want to install the correct wiring for 2 120w panels.... 20amps theoretical maximum?... 12v run of 30ft...
so i go to the internet to find some information on how to figure the size for the run to the controller and battery.....
and the answers are all over the place, from 0 to 0/0 and every gauge in between to about 10 gauge.... and different percentage loss's on the same size wire for the run...
ive used different formulas and different automatic online calculators and there doesnt seem to be any standard or reasoning behind it, so all I can say is WTF..... after checking 7-8 sites without any matching answers, I gave up:cussing:

how does one really know? who sets the standard and why isnt it followed by ALL the "expert" wiring companies? and what makes a company reputable if they are giving out false information?:Liar:

I dont need the answers to all these questions, but i would like to know what size wire is acceptable to make my connection,... and what or who's formula did you use to come to that conclusion
Thanks

 

[brazenarticle]

With the panels wired in parallel isn't that a theoretical max of 10 amps ( 120 watts/12 volts? Probably more like 5 or 6 amps on a good day...?
The cables that came with the panels (2x50w wired parallel = 100 watts/8 amp max) I bought look like 14 gauge and I have a 7.5 amp fuse at the battery terminal. There is a sunsaver duo controller in between.
You might check the Sunsaver website, lots of info there.

 

[Johnb]

Without going into the characteristics of the panels etc lets just assume that you want to carry 20 amps for 30 paired feet (total of 60 feet of wire) then

#10 wire would give a volt drop of 1.2 volts and waste 24 watts
#8 wire would give a volt drop of 0.76 volts and waste 15 watts
#6 wire would give a volt drop of 0.48 volts and waste 9.5 watts
#4 wire would give a volt drop of 0.3 volts and waste 6 watts
#2 wire would give a volt drop of 0.19 volts and waste 3.78 watts
#0 wire would give a volt drop of 0.12 volts and waste 2.4 watts

You could use the above for 10 amps - then since the voltage drop and the current are both halved so the loss is divided by 4.

For 5 amps divide the loss by 16.

Hope this helps you in making your choice of wire.

 

[LakeOntario270]

20 gauge is plenty.

 

[Deadhead]

Wire Size Calculations

1. Decide an acceptable voltage loss (3% or 10%).
2. Determine total length of run (from panels to controller & back to panels).
3. Determine maximum current.
3. Plug the numbers into the formula or Table given in Calder's Boat Electrical & Mechanical (from ABYC) or use this calculator:

http://www.stealth316.com/2-wire-resistance.htm

FWIW - Last year I installed 2 @ 130 watt panels with total cable length (there & back again) of just under 60'. I used Calder and calculated I was on the cusp between #6 and #8 for a 3% voltage loss. I went with #6.

Pulling, crimping & heat shrinking #6 is a ***** but there really isn't any option. (I got to buy a MANLY crimping tool for the job!)

Don't skimp on the wire! Electrical fires are a very real possibility when undersized wire is installed.

 

[centerline]

1. Decide an acceptable voltage loss (3% or 10%).
2. Determine total length of run (from panels to controller & back to panels).
3. Determine maximum current.
3. Plug the numbers into the formula or Table given in Calder's Boat Electrical & Mechanical (from ABYC) or use this calculator:

http://www.stealth316.com/2-wire-resistance.htm

FWIW - Last year I installed 2 @ 130 watt panels with total cable length (there & back again) of just under 60'. I used Calder and calculated I was on the cusp between #6 and #8 for a 3% voltage loss. I went with #6.

Pulling, crimping & heat shrinking #6 is a ***** but there really isn't any option. (I got to buy a MANLY crimping tool for the job!)

Don't skimp on the wire! Electrical fires are a very real possibility when undersized wire is installed.

I dont/wont skimp on anything... and I have the tools it takes for whatever the wire size will be

so i need help from someone with real life numbers as I dont have any experience with solar panels.... and a couple of the posts are confusing, in relation to the information given in my original post...
if one 12v, 120w panel puts out a theoretical maximum of 10 amps, wouldnt 2 of them put out a theoretical maximum of 20amps?....

if the one way run is 30 feet, wouldnt the formulas and calculators take into account that there will be another parallel wire for the return path of the electricity for that circuit?... or do I take the length of the measured run and times it by two to get the real length when calculating for the correct wire size?....

I pulled a wire thru to get a true measurement of the run... 32 feet one way...
2 panels at 120watts each, 12v.

hell, I thought I knew all this stuff and/or how to find it until I saw so many formulas and calculators giving so many different numbers..... and i find out that neither myself or the onlines sites, know squat about it....

 

[JohnShannon]

I have difficulty imagining the wire routing on a Mac25 that could result in 60 ft of cable.
Will these panels be in a towed dingy?

You need at a minimum to use thick enough wire to insure the wire doesn't overheat.

http://www.powerstream.com/Wire_Size.htm

You could use as little as 16AWG based on that.

But the loss in say 20 ft of wire (there and back) 80 millohms 20A is 32W out of 240 not so good.

Beneath that you are playing the game of how thick a wire vs. power loss. 10 AWG will give you 1/4 the loss of 16AWG 20milliohms (0.4V drop) 8W of loss seems like a good place to be.

In the end you got to figure I suppose the cost of the wire vs the cost of a bigger panel.
If you are looking to get to some economic optimum.

What size batteries, hope you have a MPPT charge controller.

 

[DanM]

Centerline,

I'm a big fan of Nigel Calder's "Boat Owners Mechanical and Electrical Manual".

His table says to use #2 SAE wire for 70 feet. He then mentions in his notes that SAE wire is 10-12% smaller than AWG wire so if you use #2 AWG it will be oversized which is (his words) "all to the good". Using his chart you are right at the break over point of going smaller though, 60' would put you at #4 SAE. You could go #8 SAE with a 10% voltage drop versus the 3% voltage drop with the larger wire as well.

My experience with all things electrical is always to go bigger on the wire and try to limit voltage drop. There is very little down side to this idea other than cost of the wire.

All that being said, (and given that I've never installed solar panels!) the guidance I've given you is probably overkill for real world application unless your boat is on the equator and you expect max performance from the panels.

DanM.

 

[tkanzler]

I would think voltage drop would govern here, as you only have a small voltage difference between the panel and the battery(ies) to start with, and the wire run (out and back) is rather long.

 

[Sumner]

..if one 12v, 120w panel puts out a theoretical maximum of 10 amps, wouldnt 2 of them put out a theoretical maximum of 20amps?....

The panels are probably putting out a maximum of 7 amps...

http://www.solarblvd.com/Solar-Pane...lar-Panels-with-Y-Connector/product_info.html

.... as they rate the watts as amps X working voltage which is about 17 volts (7 * 17 = 119). So maximum amperage which you will maybe only get if the sun is ever directly overhead for the two panels is 14 amps.

..i....I pulled a wire thru to get a true measurement of the run... 32 feet one way......

I like this calculator (you just have to pick one)...

https://www.colemanair.us/vp_asp/Scripts/Articles/TheBasicsAndBeyond5_WireSize.htm

.... and putting in your numbers you come up with #4 wire for a 2% drop over 32 feet (it uses a oneway distance, but calculates for the return run or two ways).

Also don't forget if some of that run is say from a common post...

... like above (top left arrow) and then on to the battery on larger cable you don't need to figure all the way from the panels to the battery. In both of our installs we wire from individual panels to a common post and then to the controller and on to the post shown above. In this case I size the intial wire size from the panel to the first common post accordingly for the current it is carrying and the distance. Then go to the larger wire from the common post at the panels on to the controller and to the other post with the larger battery cables on it that take the current to the battery (like I showed above). If you did that and say had 6 feet from each panel to a common post and 6 feet at the other end to the battery with the large cable you would have 14 amps for 20 feet and I'd feel comfortable with a #6 wire for that part of the run and #14 for the first 6 feet from the panel to a common post.

I'd put in a 20 amp fuse in the #4 wire between the post near the battery (not at the battery) for the wire going to the controller and maybe a 10 amp fuse on the smaller wires going to the panels.

I did something like that above where I have 6 80 watt panels and a smaller fused wire to each and then a larger cable from the box to the controller and then a larger single fuse between the controller and the battery.

You asked in the PM about panel sizes being consistent. I made a mount for 3 panels and then on a trip broke one panel and had to order a replacement. The ones they then had for the same wattage where a totally different length and width. When we got the 6 panels for the Endeavour later I worried about the same situation so ordered a spare and it was about 1/2 inch different than the other 6. It will fit as I.....

...left some wiggle room on the mount ....

...for the Endeavour on the sides and ends. Still panels are constantly changing. I'd try and get 2 the same to begin with if you can before you build the mount if it is going to be a very complicated mount,

Sum

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[centerline]

I have difficulty imagining the wire routing on a Mac25 that could result in 60 ft of cable.
Will these panels be in a towed dingy?


In the end you got to figure I suppose the cost of the wire vs the cost of a bigger panel.
If you are looking to get to some economic optimum.

What size batteries, hope you have a MPPT charge controller.

a 30ft run isnt too unreasonable in a 25ft boat... the panels will be mounted 7' above the hull of the boat at the stern.... the run to the bulkhead where the controller is mounted is 15ft forward.... the controller is mounted 3ft up the bulkhead from the hull.... the run to the battery from the controller is 4 ft.... i added a foot extra cuz the heavy wire may not lay as nice around the corners or for other slack if ever needed.... so a 60ft round trip is not unreasonable, but it adds up quickly...

the cost of the wire is not so much of an issue, but there is an optimum size somewhere that would fit my needs perfectly, and that is what ive been searching for.

and yes, an MPPT controller....

im adding one panel this year and plan to add another next year, or sooner if the mood strikes me, but i dont want to have to do the wiring twice....

 

[JohnShannon]

There is no optimum, bigger wire will always give lower power loss.
There is however a point of diminishing returns.
Using JohnB's numbers going from 8 to 4 gives you 9watts extra power going from 4 to 0 gives only 3.6W extra so the cost, weight and hassle went up a lot for only 3.6W gain.

 

[centerline]

The panels are probably putting out a maximum of 7 amps...

http://www.solarblvd.com/Solar-Pane...lar-Panels-with-Y-Connector/product_info.html

.... as they rate the watts as amps X working voltage which is about 17 volts (7 * 17 = 119). So maximum amperage which you will maybe only get if the sun is ever directly overhead for the two panels is 14 amps.



I like this calculator (you just have to pick one)...

https://www.colemanair.us/vp_asp/Scripts/Articles/TheBasicsAndBeyond5_WireSize.htm

.... and putting in your numbers you come up with #4 wire for a 2% drop over 32 feet (it uses a oneway distance, but calculates for the return run or two ways).

Also don't forget if some of that run is say from a common post...

you don't need to figure all the way from the panels to the battery. In both of our installs we wire from individual panels to a common post and then to the controller and on to the post shown above. In this case I size the intial wire size from the panel to the first common post accordingly for the current it is carrying and the distance. Then go to the larger wire from the common post at the panels on to the controller and to the other post with the larger battery cables on it that take the current to the battery (like I showed above). If you did that and say had 6 feet from each panel to a common post and 6 feet at the other end to the battery with the large cable you would have 14 amps for 20 feet and I'd feel comfortable with a #6 wire for that part of the run and #14 for the first 6 feet from the panel to a common post.

I'd put in a 20 amp fuse in the #4 wire between the post near the battery (not at the battery) for the wire going to the controller and maybe a 10 amp fuse on the smaller wires going to the panels.


I did something like that above where I have 6 80 watt panels and a smaller fused wire to each and then a larger cable from the box to the controller and then a larger single fuse between the controller and the battery.

Sumner, I do know solar panels are rated for more than what they can or will put out, and reading your post makes me think I could get away with sizing the wire for 15 amps...

and im understanding that the run should be measured from the junction where the wires from two panels will connect together, to the controller..... and sized accordingly for that length.....
and the run from the controller to the battery (or common lug) could be sized differently for that shorter length of run.... are these assumptions correct?....

these are BIG factors in doing a solar installation that probably get overlooked quite often...

 

[sinnettc]

Lou, I went with #10, though my run is shorter than yours.

 

[centerline]

Lou, I went with #10, though my run is shorter than yours.

Hi Chris, going with the colemanair calculator that Sumner supplied, and the understanding i have now, i should be able to use #4AWG from the panels to the controller, and #10AWG from the controller to the batteries..... and maintain less than a 3% loss.

thats awfully large sized wire for the small lugs on the solar controller.... but I will make it work somehow

 

[walt]

You are seeing why higher current applications generally dont use 12 volts. The power loss in the wire goes up with the square of current.

Here is an option. Since you are going to put up one panel first, size the wire for one panel. When you add the second panel, wire it in series with the first panel so that the current stays the same but the voltage will double and the deliivered power also doubles. Since the current remains the same, the wire parisitic power loss also remains the same as with the single panel. You now hava a "24 volt" panel with voltages up near 40 volts but I think that is still safe on a boat.

If you wire the second panel in parellel (I beleive that is what you are proposing), your wire power loss will be 4x what it was with the single panel and you will need a lot more copper with its cost and weight (weight looks to not be important).

Parcial shading may be slightly more important with the series panels but probably not much. This will be the tradeoff you make for 1/4 the power loss in the wires.

I would guess that MPPT controller will have no problem with the panels in series but you will have to check the specs.

Wire power loss anywhere in that system between the panels and the battery does matter and takes away from what gets generated to what ends up in the batteires. However, since the controller is making decisision on how to charge based on what it thinks is the battery voltage, any loss between the charger and the battery has the additional undesirable affect of fooling the charger into thinking the battery voltage is higher than it actually is. You will have to figure out the tradeoffs but I would either go larger gauge on this run or keep it as short as possible.

#4AWG from the panels to the controller, and #10AWG from the controller to the batteries..... and maintain less than a 3% loss.

I missed the logic on this?

 

[centerline]

centerline, "#4AWG from the panels to the controller, and #10AWG from the controller to the batteries..... and maintain less than a 3% loss".

I missed the logic on this?

im not really sure i fully understand it, but i think i do.... with the controller being somewhere in between the panels and the battery, the controller breaks the run/distance into 2 parts.... because its a controller.
so the run/distance from panel to controller is measured and the wire sized accordingly for it, to get the raw power to the controller....

the run/distance from the controller to the battery is carrying a controlled voltage/amperage source, different from the raw incoming power, so the wire can be sized according to the maximum current flow of that particular distance and not figured into the total distance from the panels to the battery.....
I will be doing some more research on this theory to see if it works this way...

 

[walt]

You should break that wiring into two parts because the current in each run can be different. I’m not going to calculate the power (I think that might have already been done in this thread) but as an example, if the run from the panel to the controller burns up 5 watts in the wire and the run from the controller to the battery burns up another 5 watts, you total loss due to the wire runs is 10 watts. Sizing of wire in both of those runs in important for power loss and as mentioned, it’s also important for the additional reason of the controller sensing the correct battery voltage.

I don’t know if that MPPT controller will automatically work with either a 12 volt panel or a 24 volt panel - it might. If its not automatic, it could be a jumper setting or something like that. Another thing to keep in mind (because your using an MPPT controller) is that for a 12 volt panel, the current between the controller and the battery might be up to 20% higher than the current on the panel side. And, if you were to series the panels to get a "24 volt" panel, the current on the side between the controller and the battery could be 2.4 times the current on the panel side. For example, if you had two 12 volt panels in series to make a "24 volt" panel and you had 7 amps coming from the panels, the current out of the MPPT controller into the batteries could be as high as 16.8 amps.

 

[centerline]

You should break that wiring into two parts because the current in each run can be different. I’m not going to calculate the power (I think that might have already been done in this thread) but as an example, if the run from the panel to the controller burns up 5 watts in the wire and the run from the controller to the battery burns up another 5 watts, you total loss due to the wire runs is 10 watts. Sizing of wire in both of those runs in important for power loss and as mentioned, it’s also important for the additional reason of the controller sensing the correct battery voltage.

I don’t know if that MPPT controller will automatically work with either a 12 volt panel or a 24 volt panel - it might. If its not automatic, it could be a jumper setting or something like that. Another thing to keep in mind (because your using an MPPT controller) is that for a 12 volt panel, the current between the controller and the battery might be up to 20% higher than the current on the panel side. And, if you were to series the panels to get a "24 volt" panel, the current on the side between the controller and the battery could be 2.4 times the current on the panel side. For example, if you had two 12 volt panels in series to make a "24 volt" panel and you had 7 amps coming from the panels, the current out of the MPPT controller into the batteries could be as high as 16.8 amps.

the controller is a blue sky SB2512i..... I figured the amps at 16 in a 25ft run, and a 6ft run from the controller to the batteries.... with the size of wire I thought would work, it should be fairly efficient for the most part, except if I use too much power in the evening.... then, at maximum sun the following day, the efficiency percentage may drop a little, but I think it should be well within safe limits....
so if im wrong, set me straight so I can get it right the first time

 

[Sumner]

...I think it should be well within safe limits....
so if im wrong, set me straight so I can get it right the first time

You are well within the safe limits of what those wires can carry amperage wise and I think you will be very efficient with what you are doing.

Walt is right that there could be more amps after the controller as that is the benefit of an MPPT controller. Still you are well within the current carrying capabilities of the wire you chose and I also believe that it will be very efficient for that part of the run. In theory a MPPT can increase the output over a PWM by 30%. I think in reality 10% to 15% is more like it so your wire size is still fine as I doubt there will be very many times that the panels are at 100% efficiency and you figured the wire sizes as if they will be.

The series connection on the panels is a consideration and I might try running some of the 80 watt panels on the Endeavour like that at some point (3 pairs of 80 watts in parallel with each pair in series). I wouldn't do this if you are going to get any shading, but your location is good. Some people wire the panels so that they can switch between series and parallel and have tested that way with mixed results. More on that here...

http://www.cruisersforum.com/forums/f14/pwm-vs-mppt-tropics-97172.html

... a few pages into that thread.

I see one potential problem using the controller you have in series and that is that it is rated to 35 volts max. with a 28 volt recommendation. The panels you have are rated at 17 volts with a 21 volt open circuit rating. The 17 twice would be 34 in series so over the recommended voltage for that controller. If you bought one 24 volt panel instead of the two 12 volt panels that would be an option as they operate at 28 volts (most do), but I think you already have the panel.

We have that controller on the Mac, but the Blue Sky 3024 on the Endeavour and it is rated to 57 volts so 2 panels in series would be fine with that controller. I think some MPPT controllers are good to over 100 volts so one has to check the specs on what they are using.

I'd stay with two 12 volt panels and put them in parallel with just 2 myself and that is what we have done on the Mac and will start with on the Endeavour. We have multiple panels on both boats to take care of maybe loosing a panel while out (which did happen) and having them all in parallel simplifies the situation if that happens,

Sum

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