Wire loss too much ?

[PaulM2]

I'm connecting a solar array of about 200w. It makes me around 10a into my 12v battery bank. My array will be 75 feet away from the batteries. I'm planning on using two runs of 14 gauge for the positive and two 14 gauge wires for the negative, this will half the resistance. Do you think this will still be too small? I'm using this wire because I got it cheap.What's concidered an "ok" loss.

[Norm]

 Not saying it won't work but and I'm not

positive...but I don't think it works that way...

I believe resistance goes by cross section of

the area of the wire.

        Wiser ones may quickly disagree.

                 ( :>) Norm.

[SuperTech]

I am assuming that you measured the 10 amps with full sun and battery bank right next to solar array. With the insertion of the 75 ft. of cable you are going to addabout .194 ohms of resistance to the circuit. Current is your enemy! The higher the current your loss is going to increase by the square of the current-That means at 10 amps you will have a voltage loss of 1.94 volts and a powerloss of 19.4 watts in the wires. At 1 amp you will have only a .194 volt loss and .2 watts of power loss. Using your wire arrangement you are equivalent to an 11ga. wire close to 10 ga.

 Number 8 ga. wire will give you 1/2 the loss. You will lose about 10% of your power at full load with your wiring set up. Other factors need to be considered-power usage vs. total power available on a typical day to recharge your batteries. You need to consider this. If you need more current you are going to have to go to a smaller ga. wire size. I obtained the resistance figures from a copper wire table and by using ohms law to calculate the voltage and subsequently power loss. Hope this helps.

[witapple]

I may be just a dumb farmer but I get a little different figure than you do on how much power will be lost. The total resistance of the 2 runs of #14 together should be .097 ohms (it is the same as 2 resistors in parallel) so that would transalate to about 1 volt being lost at 10 amps, and bout 10 watts lost as heat in the wire. That would still be marginal in my book but if it provides enough power to do what you want to do it certainly isnt a problem.

[wpowokal]

http://www.windsun.com/Hardware/Wire_Table.htm

[ghurd]

My favorite chart!

Looks like #6 is the way to go.

Doubled #14 is going to have some heavy loss, too much to bother even trying.

G-

[searat]

Well, I sent a very similar question in a month ago.  I have (2) 94 W panels, have seperate runs using 12 gauge wire, seperate runs for each panel, at around 40 feet. But I run them at 24v and use a MPPT charge controller to resolve everything down to 12v.  When I calculated my losses it was less than 3 percent, if I remember correctly.  I used 12 ga for the same reason (I had 250 feet of it and therefore didn't have to buy anything).  I would calcuate your losses, and use heavier wire.

[antw]

I may be wrong here but if you are not using a MPPT controller and are only getting 10a into your 12v battery bank from about 200w of solar then at battery voltage 14v you are collecting 140w, which means you are loosing 60w because the solar panels are not running at their optimum voltage (normally around 17v).

Surely loosing a volt or two in the cable run will not affect the power that is getting to the batteries, as it will let the panels run at a higher voltage, hence producing more power to cover the losses in the cable?  Battery voltage of 14v plus 2v cable loss is 16v total, which the panels should be quite happy to generate?

I would love to hear other opinions on this as I will be sizing the cables for my panels soon.

Regards

Antony

[ghurd]

PVs should be thought of as Current Sources more than Watt Sources.

I would figure a 200W PV at about 11A peak at '12V'.

That is under perfect conditions (but often I see more amps than this).

So 10A would be a realistic expectation for a 12V 200W system,

under less than ideal conditions like on the Earth,

or someone not baby sitting the PVs for a many days around noon.

PVs have a built in margin of losses for wire, controllers, etc.

Except for so-called 'self-regulating' panels. Don't get me started!

The 5% figures are good. Most of the time the amps are less than the max,

so the real total loss is a far less than the 5%.

More than half of the charging time the amps, and therefore the losses,

are less than half of the charts losses.

This can really become a factor with smaller PV systems.

Like $1000 for wire, would be better spent on $400 for wire,

and $600 for more PVs. The total output could be much more.

It is all a balancing act. Like magnets, coils, and blades.

G-

[Zix]

PaulM2,

Your plan should work ok there will be a small loss but unless your using the full load all the time you probably won't notice it.

Good Luck Zix------------------------

[scottsAI]

Antw,

You make a very good point.

Answer will depend on the output curve of the panel.

Like you say at 17v, loosing 2 volts in the wiring may not make any difference.

The solar panel output can only be equal to the voltage of the battery without MPPT plus what ever voltage is dropped in the wiring + diode. If this voltage is lower than the output capable voltage of the panel, then there is no Real losses due to the wiring.

With a 200w panel, the output is 200w at 17v, 11.7amps. Loading the panel by directly connecting it to a battery the current will still be 11.7amps. (may go up just a bit). The loss of 60w is in the panel. Adding some resistance will allow the panels output voltage to rise the same, until 17 volts is reached, the current will remain the same. The end user power will remain the same. Therefore using a heaver gage wire will not benefit you one tinny bit.

When the battery is fully charged the battery voltage will go up to 14 or so volts. At this point the panels curve may effect how much current you will get. But, since it's charged it does not matter.

So to determine, you need to look at the power curve of your panel.

PaulM2,

Suggest to use a charging controller to prevent overcharging the battery. Make sure the voltage set point matches your battery type. Ebay they seem to go for <$50.

Sounds like you have the wire. Since you have measured it already, connect the wire runs right there and test it out!!!!

Make sure the test is with everything in the circuit. Controller, or diodes etc. for both.

Have fun,

Scott.

[Dreadstar]

1 panel 2, 3, or 4 panels?

As you will see here from comments and suggestions from learned people on this board bigger is better bumping your run to 8ga will drop your losses alot.

Home depot has 3/8 cable it has 3 8ga insulated wires with a solid copper ground 4 conductors total. 125' was under $100 in Cali. I used this on my array and got another 16 watts down. With a run of only 8 feet from disconnect to panels. I had 12ga on it prior with a little extra at the end As you will see on this board. Keep your wire runs as short as possible. Wire resistance adds up to watts lost.

And when you have a setup like yours and mine (4 x 55 watt panels) every little bit of loss equates to that much less you can get out of the small array.

You could use the solid copper ground as a leg if you wish it will carry power.

A MPPT controller would help as well because of the loss from 17-21volt open circuit to the closed circuit if you have a controller like mine or direct battery connection. (not recommended) .

If you do not plan on increasing the size of your array this will do and it is much cheaper than the Outback.

BZ MPPT-250: 12/24 Volt, 250 Watt MPPT Charge Controller

Only $99

they have them here www.discountpv.com/chargecontrollers/

I love the feedback that i can get from the people that use this board. But not everyone has the same needs or ability to add more panels. If you can only mount 200 watts of panels then you need to get them as efficient as possible.

Sure another 80 watt panel will offset the wire loss and it's cheaper than an outback.

But i have one coming and will then rewire the panels for 48vdc use instead and drop my amps output down 1/4 with a corrosponding drop in wire loss over the same distance/ga of wire. But still run my battery bank at 12vdc.

www.thesolar.biz has the outback mx-60 shipped for around $500

watts is watts i prefer putting them into my batteries and not using them prior. As heat in wire going from the panels to the charging station.

And since eventually (once i move) i do plan on increasing my array hence the outback is the best choice for both short term (since i can not increase the current size of the solar panel array) and long term Once i am able to get more panels in use.

[GeeWiz]

As an alternate, there are a number of charge controllers that can allow you to run 48v (or 24) to the battery, and then step it down to 12v.   I used RV products sb3048, but it was customized by them to take 48v in and put 12v out. Had it now 4 years without a problem.

With my 360watts I regullarly see 21 amps.  This also made it possible for me to run 100' from my solar panels to the battery bank.  True this controller was like $500, but still well worth it for what I needed to do.

Gw

[ghurd]

Paul,

I can't find the other...

so here is a water heater.





G-