Next solar questions - voltage drop

Cdash · 03-09-2016, 08:39 PM

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....

Jmooney · 03-09-2016, 09:35 PM

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

klassic · 03-09-2016, 09:46 PM

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.

Cdash · 03-10-2016, 07:50 AM

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....

Jmooney · 03-10-2016, 04:33 PM

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

Jmooney · 03-10-2016, 04:49 PM

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

Subaru297 · 03-11-2016, 10:07 AM

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

RoyBraddy · 03-11-2016, 11:57 AM

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...

**Mustang65** · 03-11-2016, 12:17 PM

Quote:

Originally Posted by Subaru297

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.

Subaru297 · 03-11-2016, 12:28 PM

I'd be interested in the correct formula.

Thanks

Cdash · 03-11-2016, 01:37 PM

The more I measure, think and price out options, the more I think I am going with 24 volt panels and MPPT.

Check my logic here:

I haven't done a proper power audit, but figure I am going to run 2 Golf Cart batteries and upgrade to 4 if 2 don't cut it, although 2 should based on what I have looked at. With that in mind, I am sizing my Solar (at least upgradable) to enough to run 4 batteries. To me, with what I have read, 4 batteries is around 400A-hrs, and solar, by rule of thumb would be around 400W to maintain the 400A-hrs. This puts me into a Tristar 45 (if I go Morningstar), which has a top capacity of 600W. If I got to 600 W, I'd be great with 4 batteries and it would be ok on less sunny days.

Looking at costs for 600 watts of system (I'll call this "nominal" since I can't get to 600 watts exactly with 24 volt panels) I find the costs come out nearly equal. The savings in panels for a 24 volt panel system are offsetting the cost of the controller. This was just looking at panels, controller, monitor and temperature compensation. The were within 10% of each other and the higher voltage seems to have a few benefits that would be good. Primarily, not much worry about wire size upgrades.

So I am really heavily considering buying a couple 260W or 285W panels (Solar Blvd seems to have good prices if I can't find locally) and a Tristar 45 MPPT. Really thinking about mounting one panel on the roof and keeping the other stored to be used as portable for the heavily shaded sites. This will give me good flexibility. I want to mount the panel towards the front of the roof since that end of the trailer is least likely to be shaded.

The other thought with buying so much wattage right now, even when I don't need it, is that the panels will be matched and I won't have to worry about it. I'll have it all now.

Am I crazy in my line of thinking?

Cdash · 03-11-2016, 01:40 PM

Quote:

Originally Posted by Mustang65

I personally think that it is overkill to worry about voltage loss if you are using the proper size cable for the job.

Isn't calculating the voltage drop the way to properly size the cable?

**Mustang65** · 03-11-2016, 01:49 PM

Quote:

Originally Posted by Subaru297

I'd be interested in the correct formula.

Thanks

Ok, here you go....

To calculate it out, you need to know a few numbers (resistance). You can not just add up all the lengths of your 3 cables and run it against an online cable calculator because your circuit is a series-parallel circuit. You have 3 legs 1(leg from controller to junction box) 1 (leg from panel #1 to junction box), 1 (leg from panel #2 to junction box). Look up the resistance for that cable size (the numbers are stated in resistance for each foot). Multiply the length of each leg * the resistance per foot number and write it down. Once you have the resistance for each leg, the formula will be a series parallel calculation.

Calculate Resistance in parallel (Leg#2, Leg#3) first:

1
_______
1 1 (this is 1/R1 + 1/R2.... it will not allow me to put spaces)
__ + ___
R1 R2

Now calculate TOTAL Resistance of the circuit in series (leg #1 + Combined parallel total from above calculation of (Leg#2, Leg#3):

Rt (Total):

Resistance of Leg#1 + Calculated Leg (Leg#2, Leg#3) = Total cable resistance

You now have the Total Cable Resistance Rt

V (volts)/ rt = Voltage drop

8AWG:
1000' of cable = 0.6282 Ohms
1' of cable = .0006282 Ohms

Or once you cut the cables to their required lengths you can take an Ohm meter and measure the resistance of each of the cables.

Have fun, just use 8AWG for the two panel legs and a 6AWG down, this should allow for future panels

Don
Really had to dust off some of those unused brain cells...

Jmooney · 03-11-2016, 02:40 PM

Like I said, Mustang would be the best bet to answer

Subaru297 · 03-11-2016, 04:06 PM

I take it back. I don't want to know

klassic · 03-11-2016, 06:46 PM

Cdash... How would you plug in your portable panel?

Cdash · 03-11-2016, 07:51 PM

Quote:

Originally Posted by klassic

Cdash... How would you plug in your portable panel?

Get or make some cables with MC4 on one end and terminal rings on the other end and connect to a lug or terminal block near the controller. Could use a wing nut on the lug to put it on and take it off, or just leave it attached and coil up the cable when the portable isn't in use. I'd connect the roof top solar to this same lug to combine them and then run to the controller.

Seems like an easy way to do it.

Other thought is to use the "solar on the side" connector and just plug it in there and then I wouldn't have to remove anything from the lug.

klassic · 03-11-2016, 08:04 PM

Side solar goes direct to the battery
You would be at 24v.
And from what I am led to believe the side solar portable panel has a small controller of some sort??
Onefastdaddy has a schematic from Jayco of the size side solar posted.

**Mustang65** · 03-11-2016, 08:15 PM

Quote:

Originally Posted by klassic

Side solar goes direct to the battery
You would be at 24v.
And from what I am led to believe the side solar portable panel has a small controller of some sort??
Onefastdaddy has a schematic from Jayco of the size side solar posted.

I found a MC4 cable connector that converted from a MC4 connector to a pigtail and I connected my cable run to the pigtail end. If you can't locate the MC4 to pigtail converter by a short MC4 to MC4 extension cable and cut it in the middle and wire it to your connecting cables.

Don

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