Posted: 10/21/2013 2:04:59 PM EDT
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Been looking into getting a small off grid solar setup and I'm looking at the kits on solar blvd and some similar ones on ebay in the 240-400 watt range in 12v.
I'm not a complete idiot when it comes to electricity so when I saw that most of these kits come with a 10AWG cable to go from the panels to the controller I looked it up using an online wire size calculator and the best case scenario is that 10AWG is marginal for this connection because of the line loss. Obviously this is dependent on a lot of factors but lets assume 12v, 280 watts for 16.8 amps. Lets assume a 25ft cable run from the panels to the controller and another 10ft from the controller to the batteries and I'm looking to keep the line loss to 5% or so. 8AWG is better but still not the best but it's still relatively inexpensive. Can anyone confirm that I'm thinking correctly or should I just not worry about it and go with the 10AWG? |
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AWG 10 copper has very close to 1 OHM per thousand feet.
your 70 foot ( round trip ) lead length will have 70/1000 ohms At 17 amps, you will have 1.19 volts drop along the cable. The wire will dissipate 20 watts of your solar power. I would recommend mounting the batteries immediately adjacent the charge controller, and put the battery bank as near as possible to the Solar panels. |
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This is for a small system set up to charge batteries and run a couple lights, maximum. I want to keep it 12v for simplicity. Quoted:
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Bump up the voltage Many solar setups run 48-60VDC into the charge controller/inverter. This is for a small system set up to charge batteries and run a couple lights, maximum. I want to keep it 12v for simplicity. You could probably even bump it up to 24VDC and gain enough to reduce the loss. Some charge controllers can take different input and output voltages so you could keep the battery side as 12VDC. |
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You could probably even bump it up to 24VDC and gain enough to reduce the loss. Some charge controllers can take different input and output voltages so you could keep the battery side as 12VDC. Quoted:
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Bump up the voltage Many solar setups run 48-60VDC into the charge controller/inverter. This is for a small system set up to charge batteries and run a couple lights, maximum. I want to keep it 12v for simplicity. You could probably even bump it up to 24VDC and gain enough to reduce the loss. Some charge controllers can take different input and output voltages so you could keep the battery side as 12VDC. I bought a lightly used Xantrex C60 for a song on Craigslist. It says it has switchable 12v/24v but I'm not sure it could do like you're describing. Any clue? |
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I bought a lightly used Xantrex C60 for a song on Craigslist. It says it has switchable 12v/24v but I'm not sure it could do like you're describing. Any clue? Quoted:
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Bump up the voltage Many solar setups run 48-60VDC into the charge controller/inverter. This is for a small system set up to charge batteries and run a couple lights, maximum. I want to keep it 12v for simplicity. You could probably even bump it up to 24VDC and gain enough to reduce the loss. Some charge controllers can take different input and output voltages so you could keep the battery side as 12VDC. I bought a lightly used Xantrex C60 for a song on Craigslist. It says it has switchable 12v/24v but I'm not sure it could do like you're describing. Any clue? No, that particular Xantrex model won't work the way Desert_AIP is describing. It'll safely handle a 24 volt solar panel array, but it WON'T down-convert that voltage for use with a 12 volt battery bank. If used with a 24 volt solar panel array, it'll require a 24 volt battery bank. |
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It's an important concern, but... You're investing a few hundred bucks to learn? That's awesome! I wouldn't spend a lot of time worrying about it for an inexpensive, experimental system. Just to put your mind at ease, your 12v (nominal) panels will probably be much higher than 12V. Here's an example of what I think you're looking at from bvld: Model Name: PV-SC140J12 Number of Cells: 36 cells in series (4 x 9) 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 Dimensions (inches): 59.00 x 26.50 x 1.25 Weight: 30 pounds So at max power you're actually sending 17V, not 12. From a mathematical loss standpoint, that's a big jump -- run it thru your calculator and see how your losses change. Also, calculate your panel <-> controller run separately from your controller <-> battery run, and add up the losses in Watts to see how much power you're losing. The losses are stacked, but it's not a contiguous run of unbroken wire -- you're got a controller in the middle! One thing you absolutely do want to make sure you do is not skimp on wire size for the current you're going to run - that could cause excessive heating or even a fire. If you ever decide to build a large or even medium system, look into higher voltages. High voltage is your friend! My system has 10ga wire from the panels to controller - a run of about 225 ft. But I'm running [email protected] at max power. Loss is calculated at 3.7V, or 0.90%. It would've been impossible for me to do this if I had to work w/ 12, 24, or even 48V. Even the stuff like outback that will go up to 150V wouldn't have worked without me investing several $K's in wiring, which I wasn't willing to do. I run 4ga from the controller to the power distribution box, and from there 4/0 to the battery bank. |
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It's an important concern, but... You're investing a few hundred bucks to learn? That's awesome! I wouldn't spend a lot of time worrying about it for an inexpensive, experimental system. Just to put your mind at ease, your 12v (nominal) panels will probably be much higher than 12V. Here's an example of what I think you're looking at from bvld: Model Name: PV-SC140J12 Number of Cells: 36 cells in series (4 x 9) 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 Dimensions (inches): 59.00 x 26.50 x 1.25 Weight: 30 pounds So at max power you're actually sending 17V, not 12. From a mathematical loss standpoint, that's a big jump -- run it thru your calculator and see how your losses change. Also, calculate your panel <-> controller run separately from your controller <-> battery run, and add up the losses in Watts to see how much power you're losing. The losses are stacked, but it's not a contiguous run of unbroken wire -- you're got a controller in the middle! One thing you absolutely do want to make sure you do is not skimp on wire size for the current you're going to run - that could cause excessive heating or even a fire. If you ever decide to build a large or even medium system, look into higher voltages. High voltage is your friend! My system has 10ga wire from the panels to controller - a run of about 225 ft. But I'm running [email protected] at max power. Loss is calculated at 3.7V, or 0.90%. It would've been impossible for me to do this if I had to work w/ 12, 24, or even 48V. Even the stuff like outback that will go up to 150V wouldn't have worked without me investing several $K's in wiring, which I wasn't willing to do. I run 4ga from the controller to the power distribution box, and from there 4/0 to the battery bank. Excellent post, thanks. The panel you listed is the one with the specs I listed previously. I may look into adding more panels in the future and if so I would up the system to 24v. You are correct that this is an experiment but it would also be my SHTF power setup, in conjunction with my small Generac generator. Someday I'll play with building a gasifier for that. 
A few 12v lights or some 110VAC LED's would be nice to have when the rest of the world is dark. Plus the ability to charge power tool batteries and Eneloops would give a person an advantage. |
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No one has mentioned that if you use an MPPT controller you can run a higher voltage from the panels (series / parallel or combo of both) to the controller with smaller gauge wires and the controller will convert most of that power (95-99%) to 12V. Noted, thanks. I'm just a babe in the woods when it comes to solar power but the responses here have helped me. My initial system will tenatively look like this: 2 140 watt panels from Solar Blvd wired for 12v operation. 8AWG cable w/MC4 connectors (sourced from Ebay). The cable run will be as short as possible to the controller (can get it under 20ft most likely). Xantrex/Trace C60 charge controller with digital meter and battery temp sensor. 8AWG cable from controller to batteries (is this enough??) Undecided on the batteries. Might go with a couple Group 27 deep cycles in parallel to start with an eye on replacing with Surrette or similar with a future system upgrade. I got a 1000w no name true sine wave inverter on Ebay like this last year for cheap. We'll see if it works. I'll probably upgrade this item first. I also have a 1000w and a 1500w Xantrex Xpower MSW inverter. I'm excited to start this new adventure! |
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Quoted: 8AWG cable from controller to batteries (is this enough??) http://www.cerrowire.com/ampacity-charts Some books recommend that you de-rate by 20-25% |
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I think this has been posted on here before but this link was very helpful.
http://handybobsolar.wordpress.com/the-rv-battery-charging-puzzle-2/ |
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The best info I've found on wire sizing for off-grid solar electric systems is in Jeffrey Yago's “Achieving Energy Independence One Step at a Time.” Out of print, I think, but still highly useful. His chapter on wire sizing was really helpful for a newbie like myself. Yago wrote a lot for Backwoods Home, much of his information is available for free there.
He says a 2% or less voltage drop in an off-grid system is what you should shoot for, and this includes ALL the components, from panel to socket (charge controller, inverter, etc.). Based on his table on 2% wire loss and length of wire run, for a 15-amp 12V system he recommends 4 gauge for a 30 foot run, or 2 gauge for a 48 foot run. For a 20-amp system you'd need 2 gauge for a 36 foot run. Wow, I hadn't realized that before. My experimental system right now consists of a 158W panel mounted on top of a cargo trailer with a 150 ah battery inside. Maybe a 3 foot run. 10 gauge is fine for that, which is what I'm using. But I'm hoping to get two more panels in the near future, which would boost total short-circuit current to ~26 amps. Might need bigger wires, even for my short run.... |
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Undecided on the batteries. Might go with a couple Group 27 deep cycles in parallel to start with an eye on replacing with Surrette or similar with a future system upgrade. You might also take a serious look at 6 volt GC-2 size golf cart batteries. They're available at Sam's or Costco for around $85 apiece. Two of them connected in series will give you roughly 220 amp-hours of capacity, with excellent deep-cycle performance. One advantage of connecting a pair of batteries in series is that both batteries share the charge or discharge current equally. This tends to equalize the wear and tear on them, which helps them last longer. The same series connection scheme is also often used in much larger systems. Some examples: Four 6-volt batteries in a 24-volt system Twelve 2-volt batteries in a 24-volt system Eight 6-volt batteries in a 48 volt system Twenty four 2-volt batteries in a 48 volt system |