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Old 12-26-2010, 07:02 AM   #1
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Getting it from the solar panel into the trailer

I need help in finding a watertight from the solar panel into the teardrop trailer connection. What I have is a Perko watertight deck plug sitting on top PERKO Inc. - Lighting Fixtures - Watertight Deck Connection
The 180W panel when not attached to the teardrop sits on top of the garage attached to a grid tie inverter, (a watt generated is a watt saved).
I don't know if I am being overly concerned about voltage drop it is a high voltage panel.
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Old 12-26-2010, 11:11 AM   #2
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It looks like that should work just fine. Use the heaviest gauge wire that that plug and receptacle accept.
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Old 12-28-2010, 04:26 PM   #3
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What exactly is the voltage of your panel? At 12 Volts it produces 15 amps. A #10 wire would do the job. If the voltage is higher then a smaller wire will work fine.

Go to the "Solar That Really Works" thread and use the wire sizing guide on the site listed there.
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Old 12-28-2010, 05:22 PM   #4
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Under load it appears to be 40+V, I guess I am getting paranoid because of the voltage drop dialogue.
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Old 12-29-2010, 09:27 PM   #5
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Under load it appears to be 40+V, I guess I am getting paranoid because of the voltage drop dialogue.
What manufacturer/model is it?
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Old 12-30-2010, 08:39 PM   #6
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It is a Suntech STP185S open circuit voltage is 45V (I have seen it higher?)
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Old 12-31-2010, 01:42 PM   #7
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Do the Math

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It is a Suntech STP185S open circuit voltage is 45V (I have seen it higher?)
First, get the electrical specs for your panel:
http://www.energymatters.com.au/imag...5S24adplus.pdf

Look for these values (names in parenthesis are the terms Suntech uses) in the panel's specs:

Vnominal = the "nominal" panel voltage = 24 V. This is the value people typically use when referring to a panel. However, output voltage is typically higher -- as you saw with a 45V output. That's the Vopen-circuit voltage (see below).
Vmax voltage (or "Optimum Operating Voltage") = 36.4 V
Imax current (or "Optimum Operating Current") = 5.09 A
Vopen-circuit voltage (or "Open - Circuit Voltage" = 45.0 V
Ishort-circuit current (or "Short-Circuit current") = 5.43 A
Pmax = Vmax voltage x Imax current = 36.4 x 5.09 = 185.3 W


Second, use these values to compute fuse size. This is the fuse that should be between a PV array and the controller (and the batteries).

Fuse size = 1.56 x Ishort-circuit current = 1.56 x 5.43 = 8.5A (a 10 or 15A fuse will do).

If you based wire size on just fuse size amperage (15A), you'd only need 14 gauge wire.

However, you need to figure line loss for a low voltage system like this.


Third, to figure line loss, I use this online "VDI" calculator because it lets you pick the percentage of line loss:
How to Size Wiring and Cabling for Your System - AltE

This forumula is used to compute a VDI value which is then used to select the wire size:
VDI = (AMPS x FEET) / (% VOLTAGE DROP x VOLTAGE)

For your panel, figuring a 1% voltage drop at 12 feet:

VDI = (5.43 x 12) / (1 x 24) = 2.7 = 3 (rounded up for greater wire size)

For a VDI = 3, a 12 gauge wire is recommended for a 24V PV with a 1% voltage drop over 12 feet.
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Old 12-31-2010, 06:41 PM   #8
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Thanks
That will save me a good bit in not buying more wire gauge than I need.
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Old 01-02-2011, 11:52 AM   #9
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Wait a minute here.... The obvious question to me is how are you hooking up to the battery? Are you using an MPPT controller that can transform that high voltage down to what the battery needs? The cheapest controller I know of that would do what you need is a the Morningstar Sunsaver MPPT. It is an ugly little black box, but is three stage, has built in temperature compensation and will accept up to 75V input. It is a great tool, but still around $300.

Why would anybody want to do all of that calculating when a simple chart can tell you all you need to know? Even though I could get technical like that, I refuse to do so because it just makes most people's eyes glaze over. I thought these forums were for helping people, not for impressing them. KISS

All of this to save $3 or $4 on wire? Consider using #12 landscape lighting wire from Home Depot or Lowes.
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Old 01-11-2011, 04:27 PM   #10
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Yep I have a Morningstar MPPT and HB quite frankly you are to blame for being a bit paranoid about wire size, so see some one did listen. I have a bunch of 10 Ga so will use that.
This is a Grasshopper (more square) design teardrop 6X11 set up for boondocking with water filters and pump that can safely pull from lakes and streams, variable height suspension for mild off road, Bullfinch exterior shower point, 6 gal gas electric water heater, Eberspacher diesel heater, Satellite radio and TV etc.
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Old 01-11-2011, 06:10 PM   #11
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When I was a kid we used to talk about sports. Then we talked about cars. After that and for many,many years we talked about girls. Now you guys are talking about volts, ohm, amps and mppts and I don't understand them. I wish I was young enough to go back to talking about girls again even though I didn't understand them either.

Joe from Ct.
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Old 01-12-2011, 09:39 AM   #12
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Wire size

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Originally Posted by Shadowcatche View Post
HB quite frankly you are to blame for being a bit paranoid about wire size, so see some one did listen.
So, there is good news & bad news.

The good news is that a lot of people are listening and they are becoming successful with solar power. My wife keeps telling me that I should be compiling a record of the letters and emails, but this is not what is important to me. I must tell you though, that my favorite was from a guy in Alaska with no solar panels who told me that by reading my info he was able to substantially cut the gas bill for his generator by just turning the charge voltage up. He figured this out by installing a Trimetric meter at my suggestion and that told him that his charging was not working. Again, you don't need to understand all of the physics. You just need to find a voltage drop chart or Google "voltage drop calculator". I would rather that people overreact than to listen to the wrong advise and end up running that #@%! generator.

The bad news is that most of the solar dealers will STILL tell you #8 is big enough for four panels or more. It definitely is not, unless you are using high voltage in the run down from the roof and you cannot do that unless you spend more money for the big MPPT controller. Even then, the bigger wire is a good buy because they get more volts to the controller, which can then turn them into more amps going into the battery. If you are on a tight budget, the MPPT is not a good bargain on small systems unless like Shadowcatche here, it is being used so that a high voltage panel which is basically free can be used. (Smart idea)

I am talking to yet another person now who was sold a kit with 30ft of #10 wire to go with a 130 watt panel and a crappy controller that will not work. You don't need to be an electronics genius to look that up in a chart and figure out that it won't work, even if he had been sold a good controller. You just need to listen to the right advise instead of the advise supplied by somebody whose profit motive is to sell you another panel after what he sold you doesn't work.
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Old 01-13-2011, 10:49 AM   #13
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I'm running into a bit of the situation currently arguing with a number of people who are using the vehicle charging system to charge the trailer battery. Most of these are home built teardrop trailers or tiny travel trailers and the level of sophistication goes from absolutely drop dead gorgeous woody's to a 4 x 8 built on a Harbor Freight trailer. Ours will be the most sophisticated one out there that I am aware of.

My contention is that in all likelihood the tow vehicle charging system is not doing an adequate job of charging the trailer battery. They are in essence in parallel and the mismatch in batteries is likely to be very great. I suggested that a Balmar programmable dual voltage regulator might be a wise investment and I am getting the well this is the way they've been doing it for years argument and KISS.
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Old 01-14-2011, 10:21 AM   #14
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HandyBob
I suggested that a Balmar programmable dual voltage regulator might be a wise investment and I am getting the well this is the way they've been doing it for years argument and KISS.
I am not familiar with those things, but I "suspect" that it would be a waste of money. I doubt that it could raise the voltage to a higher level than it receives from the already regulated alternator output. If using big booster cables directly from the tow vehicle batteries to the trailer you can do a decent job of charging and can eventually get the trailer battery up to within a couple of tenths of a volt of where the vehicle voltage regulator is set. However, it does not really charge all the way to full unless you are willing to idle the engine for hours on end. So, the battery dies an early death from undercharging.

If they are using the trailer connector that also runs the tail lights, that is another matter. It does not work because of the voltage drop in the wire. You just need to measure the volts at the trailer battery & compare it to the vehicle's battery to see the problem. If you had an expensive clamp on DC ammeter, you could see how the amps drop off way before the battery in the trailer is full because there is not enough voltage (pressure) pushing them in. A hydrometer would supply the real proof.

Of course I cover this in my RV Battery Charging Puzzle, if only people would be willing to read it.
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