Portable panels wire size

[AStinker-]

Hey guys, I need some help. Would you consider giving me some advice please?

I’m putting together a portable solar panel setup. The panel’s will be the following.

Max Power: 140Wp
Open Circuit Voltage (Voc): 21.0V
Short Circuit Current (Isc): 8.40A
Maximum Power Voltage (Vmp): 17.0V
Maximum Power Current (Imp): 8.00A

Originally I was planning on paralleling 2 of them on the same wires and just jumpering directly to my batteries when I need to use them but several said don’t do it that way so I got scared and decided I will probably get a controller. It will probably be PWM Charge Controller big enough so I can use it when I do a permanent roof install.

I would like to be able to set the panels up at least 100 ft from my batteries / (PWM Controller) if necessary to find sunlight. I was planning on using a 2 conductor 10 ga stranded wire to accomplish this. But after reading all the discussion on wire size from solar panels to controller on the thread mena661 started “Panel to Solar Controller Wire Size” for his new solar build I’m wondering if I will have too much of a voltage drop at that distance. So I would appreciate your advice if you would consider it. Will 100 ft of 10 ga wire from 2 of these panels work or not?

It would not be a deal breaker if I need to add a 2nd cable or even a 3rd if I needed to as I can buy 100 ft of stranded 2 conductor 10 ga wire for $35.00. The problem is I don’t know what I need.

I’m not sure I’m using the calculator that was posted on mena661’s thread correctly but what I am entering is Wire Size: 10 AWG Voltage: 21.8 Phase: DC Number of conductors: Single set of conductors Distance: 100 Ft Load Current : 8.95 Amp. The RESULTS: Voltage drop: 1.79 Voltage drop percentage: 8.21% Voltage at the end: 20.01. Will it work but not efficiently??? I don’t know. Then when I increase Load Current for 2 panels to 17.9 Amps it doubles the losses which drops the Voltage at the end to 18.22 volts. But also I don’t know what the minimum voltage into the controller should be for it to achieve reasonable bulk & absorption charge when batteries are at 50 to 80% SOC.

My short term reason for solar is to supplement my generator when off grid where there is limited generator run time. I want to get the most out of the panels that I can with existing conditions but right now I do not need the panels for topping off or equalizing my batteries. I will have a 485 Amp battery bank, will be off grid no more than 4 days at a time & will be on back on grid afterwards long enough to top off & equalize batteries as necessary. My problem is I will be running a 5 cu ft residential freezer with an inverter. It pulls 1.4 amp AC & I anticipate it will run at a min 50% to a max 75% of the time. If 50% of the time it will take 180 of my 240 usable amps if 75% of the time it will use over my 240 usable amps. I have a 55 amp Iota converter with Q4 controller plus a portable 40 amp charger I will be running with a generator but concerned if I can only get 2 or 3 hours of gen run time. My goal is to get as much as possible but hopefully at least supplement enough amp hrs from the portable to keep from killing my batteries. I think I will need to have the controller set so it never has over a 14.9 or 15 volt temperature compensated output but I don’t know what the voltage from the solar panels need to be to accomplish this when my batteries are at say a 60% SOC. If you could advice me it would be greatly appreciated.

Thanks,
Alan

[red31]

AStinker- wrote:

In my case it doesn't matter which calculator I use because I don't know for sure which Voltage or Load Current from the panel specs I should be using.

That's why I used the table in a morningstar manual! 🙂

[westend]

I think the use of the AC freezer is going to be dictating at how you approach the solar use. You should definitely use a Solar calculator to add up all the devices you want to power but the freezer is going to rule all.

If the freezer averages a 1.3 A draw and say, a duty cycle of 1/3, your total draw would be around 100 AH/day at 12V. Two 24V modules and a good MPPT controller would probably be a good fit. If you have batteries that need the 15V, the better controllers have a PC interface where it can be set, easily.

The downside of the bigger modules is that they are bulky to cart around. I wouldn't want to use one or more as portables because I'm old and lazy. You may feel different.

Oh, BTW, if you can, put some foam sheet insulation around the box. It really helped the duty cycle on my fridge.

[pianotuna]

Hi,

You have two choices for getting batteries truly full. One is to use shore power and the other is to use solar. No one in their right mind tries to get to 100% state of charge running a generator.

You ave over thinking this whole issue. Calculate how much solar you need, go buy a quality charge controller and the necessary number of watts of panel. Use pwm or mppt. If the system is sized properly to the type of controller chosen it will be a successful installation.

If the inverter shuts down at 15 then 14.9 should be the max charge voltage under all circumstances.

Trojan wants batteries to be exposed to 15.8 volts. It would behoove you to have dual banks so that one can be charged while the other is in use, with easily adjusted charge maximums and appropriate disconnect switches.

[AStinker-]

Almot wrote:

AStinker- wrote:
But isn't the input voltage still important somehow? If not why are we worried about voltage drop from the panels to the controller?

With MPPT the drop is important because this converts into lost amps and amp-hours.

With PWM - people more familiar with these low-tech devices 😉 will correct me if I'm wrong - the voltage drop is not important as long as it is not lower than the highest Absorption voltage that you might need, i.e. Absorption on a cold day. As noted, and I agree, that it should also not be higher than 15V on a cold day.

So 15V is your lower limit of input after voltage drop at 77F. If it's lower, the controller with temp-comp won't be able to raise it as needed on a cold day. Controller with temp-comp will be able to lower it down from 15 on a hot day.

OTH, your controller should have means to limit maximum output to 15V - maybe it already has it as factory-preset. My controller has adjustable upper voltage.

Are all you PWM guys going to let him get away with that?? :E

Yes sir, it will have to have a means to limit maximum output to 14.8 or 15V because over 15 volts will shut the Inverter down. And because I'm going to be a power hog with the freezer it appears to me I will want the absorption voltage as close to 15 volts as possible also. Right?? So is 15 volts the the magic number?? The controller has to limit output to 15 volts or slightly less therefore the "voltage at the end" of the cable from the solar panels into the controller need to be or "should be" 15 volts or above?? Yes?? No?? I'm going to sleep on it now and wait for all you guys to make my day or burst mt bubble. :B Why do I feel like I'm missing something??

[AStinker-]

Almot wrote:

westend wrote:
Most of those wire size calculators should be used with two conductors since you have both a (+) and (-) phase. Don't know if the one you used calculates like that or not.

He didn't tell which one he used. Assuming it's Southwire - most people use it - you enter "single conductor" because it's obviously not a 3-phase, but you enter a total (+) and (-) run of your wire. Very often (though more often in roof installs than in portables) the + and - leads are different length, but it is the total + and - that counts.

In my case it doesn't matter which calculator I use because I don't know for sure which Voltage or Load Current from the panel specs I should be using. You know that old saying "garbage in garbage out." But the inputs pianotuna said he would use makes the most sense to me after doing a bunch more reading, studying & playing with them for awhile. I'm about ready to say to hell with solar & buy a big enough Charger to keep from killing the batteries even if I only get 2 of 3 hours of generator run time a day!!!

[Almot]

AStinker- wrote:
But isn't the input voltage still important somehow? If not why are we worried about voltage drop from the panels to the controller?

With MPPT the drop is important because this converts into lost amps and amp-hours.

With PWM - people more familiar with these low-tech devices 😉 will correct me if I'm wrong - the voltage drop is not important as long as it is not lower than the highest Absorption voltage that you might need, i.e. Absorption on a cold day. As noted, and I agree, that it should also not be higher than 15V on a cold day.

So 15V is your lower limit of input after voltage drop at 77F. If it's lower, the controller with temp-comp won't be able to raise it as needed on a cold day. Controller with temp-comp will be able to lower it down from 15 on a hot day.

OTH, your controller should have means to limit maximum output to 15V - maybe it already has it as factory-preset. My controller has adjustable upper voltage.

[Almot]

westend wrote:
Most of those wire size calculators should be used with two conductors since you have both a (+) and (-) phase. Don't know if the one you used calculates like that or not.

He didn't tell which one he used. Assuming it's Southwire - most people use it - you enter "single conductor" because it's obviously not a 3-phase, but you enter a total (+) and (-) run of your wire. Very often (though more often in roof installs than in portables) the + and - leads are different length, but it is the total + and - that counts.

[red31]

MEXICOWANDERER wrote:

A thought that comes to mind is 100' 8 gauge twin cable is heavy, bulky and a real snake to wrestle with.

2 x 25' of 10 ga 'solar' wire is a snake that can kink easily!

I couldn't find consistency in calculators.
So I used a morning star chart


Is 12% @ 92' too much, ie 17 * .88 = 14.96 :h

What's the big deal with the panel operating on the 'right' side of the IV curve peak power occasionally vs always on the left side , yes the current drops off fast beyond Vmp.

[MEXICOWANDERER]

Over one hundred feet. You are faced with a choice. Spending a lot of money because of the distance, a) or b)

Buy an inexpensive charge controller and use low voltage panels and blow a huge wad of money on gigantic wire to run to the panels.

Use the panels in series, get an MPPT controller and save a lot of money on wire.

Another option. Use interie panels and a high voltage MPPT controller.

A thought that comes to mind is 100' 8 gauge twin cable is heavy, bulky and a real snake to wrestle with. A hundred feet of 8 gauge duplex on genuinedealz.com costs 248 dollars.

[BFL13]

AStinker- wrote:

BFL13 wrote:
This calculator us useful in that

A. it counts going both ways so you don't have to wonder if you need to double the distance.

B. You can try different gauges and see what you get for voltage drop.

What it does not do is tell you what amps you will get with what voltage drop. We had a thread about all that where that got answered using watts (so of course I got confused 🙂 )

Just fill in your own numbers instead of whatever is there now

http://www.calculator.net/voltage-drop-calculator.html?material=copper&wiresize=2.061&voltage=12&pha...

In my case it revealed how with 24v panels and 12v battery, you need to do the panel to controller at 24 and the controller to battery at 12. With 12v panels you do both at 12

Ok thanks again BFL
I hope you haven't already answered this but in the calculator do I enter the Voc or Vmp of my panels?

And for Load current do I use Isc or Imp of my panels?

Musn't say "load" here. Might get mixed up with controller "load" terminals.

Use the total Isc of the panels for expected current from the array. Use battery voltage for what is on the wires when using 12v panels. On solar that will be about 13 in the morning to about 14.5 later on.

If using 24v panels, then use Vmp for the voltage on the wires from panel to MPPT controller and Imp for the amps. Then from the MPPT controller to battery use battery voltage but now "expected amps" is way higher than panel Isc (not used by MPPT) However as a rough guide, IMO just use double the panel Isc as what you might see. Gives you a good enough idea for picking the wire gauge. What amps you actually get will depend on all that MPPT magic stuff to do with watts and AFAIK, your horoscope for that day. 🙂

[pianotuna]

vmp and the higher amperage is what I would use.

[AStinker-]

BFL13 wrote:
This calculator us useful in that

A. it counts going both ways so you don't have to wonder if you need to double the distance.

B. You can try different gauges and see what you get for voltage drop.

What it does not do is tell you what amps you will get with what voltage drop. We had a thread about all that where that got answered using watts (so of course I got confused 🙂 )

Just fill in your own numbers instead of whatever is there now

http://www.calculator.net/voltage-drop-calculator.html?material=copper&wiresize=2.061&voltage=12&pha...

In my case it revealed how with 24v panels and 12v battery, you need to do the panel to controller at 24 and the controller to battery at 12. With 12v panels you do both at 12

Ok thanks again BFL
I hope you haven't already answered this but in the calculator do I enter the Voc or Vmp of my panels?

And for Load current do I use Isc or Imp of my panels?

[BFL13]

The usual rule for car batteries is 18" limit from battery, but also you want the unprotected wire to be visible, not going into a wall where you can't see it getting that burnt look before it does catch fire.

I use ANL fuses for the big amps (over 50a) carrying wires such as for the inverters. They are inexpensive and found at car audio stores. Under 50a you see a lot of those glass fuses in twist apart holders, which are convenient at times.

I hook the ANL right onto the battery post (no holder needed for "strain" in my set-up) so no wire there at all.

[pianotuna]

Hi,

The fuse should not be farther than 12 inches from the battery. Closer is better. I use a T class fuse.

[AStinker-]

pianotuna wrote:
Hi,

Fusing is done to protect the wire. It is also done to prevent catastrophic failure. It has little to do with protecting the charge controller.

A lead acid battery is capable of doing welding--that is why you need to fuse.

Ahhhhh soooo said the blind man to his deaf brother over the telephone, now I see!

Thanks for the clarification. I should have known that. I once saw a friend weld the clasp of his watch band with 24v DC by touching the ground bus bare and the bare end of about a 24 ga alarm wire with his metal watch band. Just a momentary arc but my oh my what it did to his wrist. So in that case I guuess the closer the fuse is to the battery the better?

What type of fuse or overload mechanism do you use with your hi current DC cables?