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Old 03-09-2016, 08:39 PM   #1
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Next solar questions - voltage drop

Now that I have my 5th, I got up on the roof. Found the mc4 connector and getting the lay of the land where some panels may fit. I wanted to install up front, close to the connector and close to the batteries, but there isn't the big free open space like I have in back. 12volt panels would fit better along the sides up front.

If I go with big 24volt panels (like 285W), I'll likely have to put them in back. That makes the wiring quite a bit longer. I'm thinking that while 24 volt has less drop, the increased length isn't doing me much of a favor.

In trying to calculate the voltage drop, should I add the drops from the individual parts? For instance, if I used 12 v panels along the side, some panels may be 8' from the connector. Would I take that 8' length and cable size, find drop, and add to the drop from roof top connector to controller?

I'm starting to see why everyone wants to series connect higher voltage panels....
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Old 03-09-2016, 09:35 PM   #2
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With the correct gauge wire, the voltage drop will be minimal.

On my install, I placed (1) panel up ft, and one at the back, to maxiximize my ability to take on solar power when camping in partially shady areas.

( two 18v , 100 w panels in parrellel)

Poor placement of a panel, where shadows from other roof top devices cast on only one or two of the cells on the panel will have a bigger impact than the loss from an 8 ft cable run if sized properly.

There is a solar group here on JOF with some good materials as well
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Old 03-09-2016, 09:46 PM   #3
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Although it's not the right idea after buying a rig with prewired solar. If you're going with high wattage, maybe you're better off using the right sized wire and running it down your fridge chase and over to the battery area where you can mount your controller.
Then you're closer to the backside of the roof too.
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Old 03-10-2016, 07:50 AM   #4
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I do NOT plan on running down the fridge vent. If I need to upsize the pre-wired solar, I just plan to pull up the roof top connector and use the Pre-wired wire, to pull the correct sized wire down to the basement where I'll install the charge controller.

I am trying to figure out the best way to look at voltage drop across the wiring, or to find out if I am over thinking it. I'll try to describe a little better:

I have panels that go on the roof. Each panel will have a pigtail on it that is say 3 or 4 feet long. These pigtails will combine to a different wire (call it an extension cord) that runs from the pig tails to the roof top connector. Then there is the wire from the roof top connector to the charge controller location. With that, I have the potential for 3 different wire gauges over the entire length of the run. Do I need to sum up voltage drop over each segment with a different gauge wire?

I can change the wire gauge from the end of the pigtails to the charge controller. The section from the pig tails to the roof top connector would have to be built up with MC4 ends since i haven't seen any available for purchase in anything larger than 8 gauge.

Or just give up on the MC4 entirely and use/make a combiner box at the location of the current roof top connector....
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Old 03-10-2016, 04:33 PM   #5
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Yes, I think you do have to consider each point of voltage loss in aggregate , but I'm think how you calculate that wil be different if you are in series versus parrellel

I also believe both the positive and negative sides will induce loss along the way. ( return trip of negative side must be included in total length for loss calculation )

Perhaps Mustang will jump in here... He is the resident guru on that type of stuff
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Old 03-10-2016, 04:49 PM   #6
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I used two Renogy 100w, 19.8 v panels, in parrellel

One at the back feeds the one in front, then down to the controller.

I'm taking in 17+ volts, at almost 200 watts at the controller , who in turn is charging the battery array at a full 14.6 volts / around 12 amps
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Old 03-11-2016, 10:07 AM   #7
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I would calculate each leg (gauge) of the journey separately and add up the voltage drop to get your total. What you described above sounds correct.

Voltage Drop Calculator
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Old 03-11-2016, 11:57 AM   #8
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A typical 120WATT panel will only produce around 5-6AMPS DC current. Its where you combine the addt panels where the larger size cables will be needed to run to the battery terminals...
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Old 03-11-2016, 12:17 PM   #9
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Quote:
Originally Posted by Subaru297 View Post
I would calculate each leg (gauge) of the journey separately and add up the voltage drop to get your total. What you described above sounds correct.

Voltage Drop Calculator
You can't use the normal wire calculator programs for figuring out a system with 2 legs leading to one drop. This type of circuit is considered a series/parallel circuit and requires a whole different formula.

I personally think that it is overkill to worry about voltage loss if you are using the proper size cable for the job. The loss will be minimal and the easiest method for compensating for voltage drop is increase the voltage (if possible). The entire circuit may be .1 or max .2 volts....

Don

I can give you the formula if you want to be exact, but it really is not worth it.
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Old 03-11-2016, 12:28 PM   #10
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I'd be interested in the correct formula.

Thanks
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