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Old 03-02-2015, 19:18   #1
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Solar panel Installation and wire sizing from the solar panels to the mppt charger

Iím installing my solar panels on top of the new hardtop I built over the cockpit. The hardtop measures 88 inches wide by 96 inches long and the two solar panels each measure 41 inches wide and 81 inches long.

Each solar panel is rated at 435 watts with a rated Voltage of 72.9 volts. The rated current is 5.97 amps and the panel efficiency is 20.1%. The open-circuit voltage is 85.6 volts and the short circuit current is 6.43 amps. The output cables are 700 mm with MC4 connectors. The total output of both panels should be 870 watts.

The 75 amp mppt charger has a max input voltage of 80 volts with a max output current of 78 amps.

The 12 volt 696 amp house battery bank is located 6ft from the mppt charger. The run from the panels to the mppt charger is about 30ft. I expect to wire the panels in parallel ie: Pos to Pos & Neg to Neg.

What amperage can be expected from the mppt charger to the batteries?

What is the size of the wire that should be used for the run from the solar panels to the mppt charger?

Thanks for your input,
Jim
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Old 03-02-2015, 19:32   #2
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Re: Solar panel Installation and wire sizing from the solar panels to the mppt charge

Is this on a 12v system?
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Old 03-02-2015, 19:57   #3
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Re: Solar panel Installation and wire sizing from the solar panels to the mppt charge

Wire run for 30 ft at 72 volts , 14g would give you about 1% drop. I'd use 12 to gave a margin for error.

Assuming 90% efficiency in the charger looking at around 56 amps at 13.7 v to battery.

Numbers are ball park values.



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Old 03-02-2015, 20:48   #4
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Re: Solar panel Installation and wire sizing from the solar panels to the mppt charge

wow... I can only dream of 56 amps going into my batteries from solar!
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Old 03-02-2015, 21:40   #5
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Thumbs up Re: Solar panel Installation and wire sizing from the solar panels to the mppt charge

Thanks for the response.

I'm planning a trip to Canada and Alaska this spring leaving from Deer Harbor, Orcas Island.

I expect this setup to take care of all my refrigeration, 12v water-maker, LED Lights, Radar, and AC on board.

Fair Winds,
Jim
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Old 03-02-2015, 21:48   #6
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Thumbs up Re: Solar panel Installation and wire sizing from the solar panels to the mppt charge

Thanks for the conformation.

Your recommendation of #12 stranded wire was what I thought would do the job but for good measure I bought 30ft of stranded #10 wire.

Fair winds,
Jim
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Old 03-02-2015, 22:25   #7
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Re: Solar panel Installation and wire sizing from the solar panels to the mppt charge

I noticed you say controller max voltage is 80 and your panels have an open circuit voltage of 85 volts. Sounds to me like you might fry your charge controller. No?
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Old 03-02-2015, 22:50   #8
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Re: Solar panel Installation and wire sizing from the solar panels to the mppt charge

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Originally Posted by rcmpegasus View Post
Wire run for 30 ft at 72 volts , 14g would give you about 1% drop. I'd use 12 to gave a margin for error.

Assuming 90% efficiency in the charger looking at around 56 amps at 13.7 v to battery.

Numbers are ball park values.



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12amps at 60 feet is 10awg for 3%. that is 32v in blue sea cacl. can't go any higher. but probably not much different at 70.

you probably used 6a at 30 feet. but he has 2 panels and use round trip


I guess it depends if he makes his series connection at panels or at mppt. and runs 2 or 4 wires down.


I would go 6 or 4 awg battery to mppt
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Old 04-02-2015, 01:25   #9
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Re: Solar panel Installation and wire sizing from the solar panels to the mppt charge

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I noticed you say controller max voltage is 80 and your panels have an open circuit voltage of 85 volts. Sounds to me like you might fry your charge controller. No?
Irwin has a good point, you will fry your charger.

Besides, when its cold the voltage will be even higher.
I am guessing it can get quite cold in Alaska. Then your looking at 90V+

I have the same Sunpower 435W panels on my house here in Norway, works great.

Would love to see some pictures on how you installed then on your boat.
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Old 04-02-2015, 02:07   #10
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Re: Solar panel Installation and wire sizing from the solar panels to the mppt charge

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Originally Posted by irwin37 View Post
I noticed you say controller max voltage is 80 and your panels have an open circuit voltage of 85 volts. Sounds to me like you might fry your charge controller. No?
we started out with 2 solar panels at 260v and a blue sky 2512 mppt controller. we added a 3rd panel and were in the southern caribbean, colombia, and getting good sun and seeing 27-29 amps, in excess of the 25 the mppt was designed for. however before i added the 3rd panel i called both the panel mfg and blue sky and my son who is an ee and all confirmed that the controller would be ok as long as i did not exceed where it was now. ie do not add a 4th panel.

as for your going north, not sure how efficient your panels will be at that latitude. as you will get sun but the angle will be less than what it would be farther south and thus may affect the amount of energy you get.

i know there are a few really smart ee's on this board that can answer that.
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Old 04-02-2015, 04:07   #11
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Re: Solar panel Installation and wire sizing from the solar panels to the mppt charge

Most controllers will limit the current to self protect themselves. They cannot do this for voltage and will usually be damaged.

As the solar panel voltage can occasionally exceed Voc the controler needs to have a maximium voltage that comfortably above Voc.

I am afraid Jim, as others have said, you need a different controller for those panels.

Be careful with the wiring, anything above 50v is potentially lethal.
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Old 04-02-2015, 04:46   #12
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Re: Solar panel Installation and wire sizing from the solar panels to the mppt charge

rate the cables for the solar panel to the MPPT controller on short circuit panel amps ( assuming all panels are in parallel it will therefore be "no of panel x short circuit amps")

Rate the mppt cable to the battery on the max MPPT controller output amps.


simples

dave

PS: agree re panel voltage , you'll fry controller
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Old 04-02-2015, 07:41   #13
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Re: Solar panel Installation and wire sizing from the solar panels to the mppt charge

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Originally Posted by goboatingnow View Post
rate the cables for the solar panel to the MPPT controller on short circuit panel amps ( assuming all panels are in parallel it will therefore be "no of panel x short circuit amps")

Rate the mppt cable to the battery on the max MPPT controller output amps.


simples

Nothing is ever simple on CF Dave .

From the panels to the controller I would use Imp not Ioc. The maximium power current can occasionally be exceeded, but this is very rare and the excess will only be very slight. The short circuit current will never be reached. This is for a single panel, or panels wired in series. If wired in parallel you need to to add the Imp of each panel.

I am not sure what you mean by "max MPPT controller output amps". If you mean the maximium rating of the controller, this is not relevant. The simplest way is to covert the total solar panel wattage to current. The battery will be charging so the voltage is unlikely be below 13v, but you can use 12.5v or even 12.0v if you want to cover all possibilities. Solar panels can very occasionally exceed their specified watts but once again this is very rare and the excess will only be slight.

So for this example:

Panel to controller 5.97x 2 = 11.94A
Controller to batteries 435 x 2 / 13 = 66.9A

If we work out the voltage drop with various wires:

Panel to controller
8AWG = 0.90 v
6AWG = 0.57 v
4AWG = 0.36 v

Controller to batteries
4AWG = 0.40 v
3AWG = 0.32 v
2AWG = 0.25 v

So what voltage drop do we accept? All the above results are calculated on the maximium panel output so the voltage drop will normally be less. There is no perfect answer. Some common solutions are a total voltage drop of 10%, others like 5%, others 3%. The voltage drop from the controller to the batteries is more critical as this will effect the voltage set points.

I would suggest 4 AWG between the panel and the controller and 2 AWG between the battery and the controller would be a reasonable compromise. One caution is to actually measure the wire distance once it passes around bulkheads etc. The actual distance is often greater than the estimation.

There, I made it complicated .
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Old 04-02-2015, 07:56   #14
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Re: Solar panel Installation and wire sizing from the solar panels to the mppt charge

Many thanks for all of the input.

Iíll be making the parallel connection at the panels and the run to the MPPT controller will be a little less than 20ft.

Havenít purchased the controller yet and will check with the manufacturer re max open circuit volts of the panels vs the max volts of their controller. Their controller pdf states that it will limit the max amps to 78 amps output, but doesnít mention limiting the volts and states 94% to 97% efficiency.

Fair winds,
Jim
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Old 04-02-2015, 09:20   #15
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Re: Solar panel Installation and wire sizing from the solar panels to the mppt charge

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Originally Posted by boatrips View Post
Iím installing my solar panels on top of the new hardtop I built over the cockpit. The hardtop measures 88 inches wide by 96 inches long and the two solar panels each measure 41 inches wide and 81 inches long.

Each solar panel is rated at 435 watts with a rated Voltage of 72.9 volts. The rated current is 5.97 amps and the panel efficiency is 20.1%. The open-circuit voltage is 85.6 volts and the short circuit current is 6.43 amps. The output cables are 700 mm with MC4 connectors. The total output of both panels should be 870 watts.

The 75 amp mppt charger has a max input voltage of 80 volts with a max output current of 78 amps.

The 12 volt 696 amp house battery bank is located 6ft from the mppt charger. The run from the panels to the mppt charger is about 30ft. I expect to wire the panels in parallel ie: Pos to Pos & Neg to Neg.

What amperage can be expected from the mppt charger to the batteries?

What is the size of the wire that should be used for the run from the solar panels to the mppt charger?

Thanks for your input,
Jim


I don't believe you should use those panels with that controller. They are rated at a higher Voc than the max. V of the controller. The panels can possibly hit those voltages under certain conditions (so it's been posted on here) and controllers cannot take any over voltage.

I'd suggest a different controller, there are a number of them that are rated for 150v input.
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