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Old 08-22-2012, 02:48 PM   #15
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Originally Posted by jmpratt View Post
Conductor size is a function current and wire length. For a given conductor size you will have a voltage drop per foot per amp. Figure out what your max current will be then look online at an ampacity chart. Then buy some 10 or 12 gauge wire. 4/0 welding cable is extreme overkill.
Somewhat correct, but look at the installation instructions for any Magnum inverter and you will see that for any for any run from batteries over 400 amp/hours, 4/0 is the minimum wire gauge recommended. Nothing under 2/0 is ever recommended for even very short runs.

Plus, 12 AWG is too small for anything other than perhaps a 100 watt solar panel (we actually use a minimum of 10AWG for panel runs to the roof top combiner box) and a minimum 8AWG up to 2AWG for the run from the box to the controller, and then to the batteries. But perhaps you have other proprietary knowledge that AM Solar (in the RV solar business for over 25 years) does not.
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Old 08-22-2012, 02:54 PM   #16
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Innovative as long as the segment protectors don't grow legs and walk away. We have a goal zero "briefcase" panel for emergency charging.
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Old 08-23-2012, 12:33 AM   #17
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Quote:
Originally Posted by lewster

Somewhat correct, but look at the installation instructions for any Magnum inverter and you will see that for any for any run from batteries over 400 amp/hours, 4/0 is the minimum wire gauge recommended. Nothing under 2/0 is ever recommended for even very short runs.

Plus, 12 AWG is too small for anything other than perhaps a 100 watt solar panel (we actually use a minimum of 10AWG for panel runs to the roof top combiner box) and a minimum 8AWG up to 2AWG for the run from the box to the controller, and then to the batteries. But perhaps you have other proprietary knowledge that AM Solar (in the RV solar business for over 25 years) does not.
Yup, battery to inverter is high current (roughly 10 times inverter output current) so thick cables are required. However, solar panel to battery will not have high current. A 120 watt solar panel will put out about 8.5 amps at 14 volts. 4/0 cables will be overkill for solar panel to battery connections unless you plan on having 3600w solar panels.
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Old 08-23-2012, 01:33 AM   #18
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TouringDan
I bought a single 85 watt solar panel and controller. I built a cover for the panel that is hinged. The cover serves two purposes. One to protect the face of the panel when stored. The other to provide a stand for the panel.
When the cover is folded open to expose the panel to the sun, it looks like a small pup tent. When fully opened the cover holds the panel at about a 55 degree angle. I can adjust the cover from the 55 degree angle down to the point where the panel will lay flat on the ground.
The controller is mounted on the cover and wired to a 15' cord (3 conductor #10 SO type) with a 7 pin plug on the end of the cord. I mounted a 7 pin receptacle in the side of the trailer adjacent to the battery compartment. It is the same 7 pin connector used on the UCord. But only 2 pins are wired. The common/ground pin and the pin for the charge line. I wired the receptacle to the battery and have a 30 amp fuse in the positive wire.
So it is a simple matter of setting up the panel at the right angle and plugging it in to the receptacle on the trailer. When I set the panel up for the morning sun, it is at it's steepest angle, 55 degrees. The sun does not have to be much above the horizon to hit the panel at a straight on shot. As the day progresses I adjust the angle to where by noon or the panel is lying flat on the ground. When the sun has reached it's peak and starts to get lower in the sky I rotate the panel and adjust the angle up as the sun gets lower. I don't use any fancy protractors or device to get the angle perfect. I just stand in front of the panel and use my shadow to adjust the panel left or right and up and down.
At the same time that I wired up the solar panel I wired in an 1100 watt inverter. The meter on the inverter gives me an indication of what the charge level is on the battery. While the inverter is not on continuously I can turn it on momentarily to check the battery voltage.
I do carry a group 27 battery in the back of my truck, just in case there is a need for more ampere hours, such as running the furnace for an extended period. The battery has a male and female 7 pin connector wired to it. With the battery in the truck I can charge it while driving and not towing by plugging it into the receptacle for a 5th wheel trailer inside the box. I can also place the battery next to the trailer and plug it into the same receptacle that the solar panel plugs in using the male connector wired to the battery. At the same time if I choose to use the solar panel, I can plug it in to the female receptacle wired to the battery. Thus charging both batteries.
At this time I carry the panel on one of the twin beds when traveling, I am considering raising the bed platform and making a drawer just below the mattress level to store the panel. But so far the bed storage has worked.
Hope this helps.
Could you just plug the panel into the umbilical receptacle at the front of the trailer where the TV plugs in?
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Old 08-23-2012, 07:39 AM   #19
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Twinkie

Thanks for the details and I really like your creative thinking. Sounds like it all works well for you and is all very flexible. Some photos would be helpful too, to kind of fill in the blanks.

Thanks, Dan
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Old 08-23-2012, 08:08 AM   #20
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Originally Posted by jmpratt View Post
Yup, battery to inverter is high current (roughly 10 times inverter output current) so thick cables are required. However, solar panel to battery will not have high current. A 120 watt solar panel will put out about 8.5 amps at 14 volts. 4/0 cables will be overkill for solar panel to battery connections unless you plan on having 3600w solar panels.
If you actually read my post, you would see that the maximum for solar panel to controller to battery runs is 2AWG. 4/0AWG is only mentioned for battery to inverter connections.

"Somewhat correct, but look at the installation instructions for any Magnum inverter and you will see that for any for any run from batteries over 400 amp/hours, 4/0 is the minimum wire gauge recommended. Nothing under 2/0 is ever recommended for even very short runs.

Plus, 12 AWG is too small for anything other than perhaps a 100 watt solar panel (we actually use a minimum of 10AWG for panel runs to the roof top combiner box) and a minimum 8AWG up to 2AWG for the run from the box to the controller, and then to the batteries. But perhaps you have other proprietary knowledge that AM Solar (in the RV solar business for over 25 years) does not."
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Old 08-23-2012, 08:16 AM   #21
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Uzzah;
I don't have a receptacle on the front of the trailer. The battery is located near the bath in the back of the trailer. That would mean an additional 26' of wire in the circuit between the panel and battery. Contributing to the voltage drop.
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Old 08-23-2012, 08:18 AM   #22
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I will post pics as soon as I can.
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Old 08-23-2012, 11:42 AM   #23
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I see a brewing debate here about wiring sizing.

There are some physics involved. Copper isn't a perfect conductor - it has some resistance - and the higher gauge (thinner) the wire the more resistance you'll see for a given amount of current.

You can think of the copper wire like a big heating element - no matter what, some of the power you are putting into it is going to get dissipated - in the form of heat. You turn on the power, the wire heats up a little bit, and you can actually measure that the voltage drops a little bit between one end of the wire and the other.

For copper cable, the three major variables that matter to calculate voltage drop are the length of the cable, the diameter / gauge of the cable and the number of amps you intend to run through it. There is more to it than this - ambient temperature, whether you are running in conduit, solid vs. stranded, AC vs DC etc - but these first three factors are the most important for this discussion.

There are many calculators available on the internet that will allow you to precisely calculate the amount of voltage you will lose for a given choice of length / gauge / amps.

So here is where the confusion probably comes in. When wiring up normal 120V electrical systems, the typical standard is to size your wiring for 5% or less voltage drop.

The normal rules of thumb, which just about everyone who has done a lot of residential electrical work are used to following, are 14 gauge is adequate for 15 amps, 12 gauge is adequate for 20 amps, 10 gauge for 30 amps, 8 gauge for 40 amps, 6 gauge for 55 amps, etc. There are plenty of calculators on the Internet that will tell you what sized conductors you need to deploy to stay within the 5% voltage drop limit.

So why are the guys here talking about what seems to folks used to working with 120V systems like crazy thick and expensive cables? Because voltage drop does not vary with input voltage.

Here's an example -

For a 25 foot long, 50 amp cable at 120 volts (6,000 watts) - plugging this into a calculator you find that if you use a 6 gauge cable, you should expect to see a voltage drop of about 1.2 volts - for a 1% transmission loss. You cable will be radiating about 60 watts of energy over it's length - think about the heat of a 60 watt lightbulb spread out over 25 feet - no big deal.

But for the very same setup running at 50 amps at 12 volts (600 watts) - the voltage drop is still 1.2 volts! - which is a transmission loss of 10.2%. So you are wasting a lot of your precious solar power. Note that this is not dangerous - you are still radiating the exact same 60 watts of energy over 25 feet - it's just a much higher percentage of the power you started with.

If you step up to 2 gauge cable - which is about as thick around as your index finger - you will get down to 4% transmission loss - 24 watts. And so on. But is this worth it do to? It depends on the price of thicker copper wire vs. the price of more solar panels.

I am sure that 10 years ago when copper cost less than half of what it does today, and solar panels cost 4X what they do today, it was a no brainer to always deploy very heavy gauge cables.

Note this same thinking should also make it obvious why larger solar systems don't run at 12 volts - you want to run at the highest voltage you can between the panels and your charge controller, so you will have the least percentage transmission loss (and so you don't really need crazy thick cables) - and you want your charge controller very close to your batteries, so when you are forced to step down to 12 volts, you lose the least amount of power possible.

I hope this helps.
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Old 08-23-2012, 04:54 PM   #24
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Portable Solar Panel

Here are photos. Panel in travel mode: Panel at different angle for optimum exposure: Connector next to battery compartment: Spare battery with 7 pin connectors:

Solar Panel pictures by dougbowman88 - Photobucket
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Old 08-23-2012, 06:55 PM   #25
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Thanks Ddruker for that explaination. I'm not an engineer but I play one on TV. Seriously, I do have some background in electronics and physics but not enough to make my opinions carry much weight. I do tend to do a lot of research before I jump into an install such as this and I knew that the reasoning behind the larger cables involved minimising the voltage loss but I could not have explained it as elegantly. I was kinda thinking I was mistaken from the way I was jumped but I guess I wasn't completely wrong. This is why we all enjoy the forums for discussions like this.
Learn something new everyday and you dont have to eat that danged apple.
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Old 08-24-2012, 06:58 PM   #26
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Thanks everyone for such a lively and informative discussion. I'm in the middle of a patio project that can double as an outdoor rv pad and replacing the chrome strip between the banana wrap and side segments so the rock guard solar panels are going to have to wait a couple more days before I can get to really sink my teeth into this project.

So far from a wee bit of surfing on the web I'm still not sure if I can get the maximum surface area used due to the odd shape of the rock guards.

Here's today's picture proof of the patio;-) Thats the better part of 35ton of rock/cement and gravel....my butt hurts.
Cheers
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Old 08-25-2012, 01:00 PM   #27
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Quote:
Originally Posted by lewster View Post
If you actually read my post, you would see that the maximum for solar panel to controller to battery runs is 2AWG. 4/0AWG is only mentioned for battery to inverter connections.

"Somewhat correct, but look at the installation instructions for any Magnum inverter and you will see that for any for any run from batteries over 400 amp/hours, 4/0 is the minimum wire gauge recommended. Nothing under 2/0 is ever recommended for even very short runs.

Plus, 12 AWG is too small for anything other than perhaps a 100 watt solar panel (we actually use a minimum of 10AWG for panel runs to the roof top combiner box) and a minimum 8AWG up to 2AWG for the run from the box to the controller, and then to the batteries. But perhaps you have other proprietary knowledge that AM Solar (in the RV solar business for over 25 years) does not."

Lewster, I have no idea what you're talking about. I replied to Rumrunner about not needing to use 4/0 cables for panel to battery connections. Apparently, you agree with that. What's the issue?
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