Can I use Multiple Grounds in my RE System

[wooferhound]

I have everything now to put up my Renewable Energy System and use some power.

 200 watt wind genny

 4 solar panels equaling 55 watts

 New Car Battery (I know, wrong battery)

 enough wire to get the power where I need it

 No Charge controller yet (Hey Ghurd)

I will be using about 5 amps or so in my computer area to run the modem, a small TV and a 12 volt CFL, probably more later. So I'm about to run the wire from the battery to my computer area which is a little less than 100 feet. The battery will be in a shed about 20 feet from the house. I already have a 12ga wire with ground buried going to the house. I plan on running the same 12ga wire around the house to my computer area like in this diagram . . .

The 12ga wire that I'm using is 3 conductors: 1 black, 1 white, one bare copper ground. I want to use the Bare Copper Ground wire as my Negative wire which is represented as the green conductor in the above diagram. I want to use Both of the other conductors Black & White combined as my positive conductors represented as the red conductor in the diagram. This would make a 12ga ground and an 8ga Positive wire.

Now I'm getting to my question. I want to use multiple Grounds on the Negative wire to help make up for the smaller gauge wire that I'm using for the ground. This would be using the grounds to help carry some of the current. But I've heard several times from the users of this forum that multiple grounds in RE systems are a bad idea for some reason like voltage differentials or ground loops or something like that. So is it OK to wire my system as described and shown in the Drawing ?

[terry5732]

Grounds on a DC circuit will do nothing. Check between your distant ground rods with an ohmmeter. But if you have differing materials for ground rods you might get an Earth battery!

[stephent]

I would use at least one ground at the wind genny tower for sure...

And the tower IS required by US code to have at least one too.

US code requires one for solar if mounted on rooftop. (grid tie or not)

But the multiple ground rods don't have to be differing material to make a weak battery.

And the small weak currant (it's noisy) induced between the rods can make some electronic equipment have conniption fits.

[wpowokal]

My system has multipul grounds, although not to skimp on cable, there are several articals here....

http://www.sandia.gov/pv/docs/John_Wiles_Code_Corner.htm

I suspect it would cost you more for decent ground rods than some cable.

allan down under

[juddley]

As stated earlier.... you get no effective ground return path for current at 12vdc between the ground rods.

Your Bare copper Ground wire would be carrying the entire load as single #12 conductor. (5amps @ 12v = 60watts) The Ground resistance between the two ground rods would be approx. 1Kohm per meter or 30Kohms... effectively carrying no load. I had this exact same setup to a remote 110v generator shed but there was a separate bare copper line laid in the ditch with the romex cable that carried part of the load.

My calcs show a loss of 8 watts with a 60 watt load.

12ga = .0016 ohms per foot 100ft = .16 ohms

load 5amps 12 volts = 60 watts

12. volts / 5amps = 2.4 ohms "effective load"
    
200. ft (both directions) X .0016 ohms per foot = .32 ohms
     
     

.32 ohms X 5amps= 1.6volts dropped = (1.6V X 5A) = 8Watts.

So it wouldn't get hot and you have 255 watt source capacity... my only concern would be the voltage drop. Not sure how the TV, Modem, etc would operate at lower voltage.

juddley

[RP]

Hey Woof, be sure to measure the diameter of that bare ground wire.  It's not uncommon for the ground wire to be smaller than the main conductors since it's not supposed to carry a continuous current.

It may only be 14 gauge...

[wooferhound]

Two of the ground rods are already in place on each side of the house. one is for the main breaker panel, the other is for the cable TV connection. I would need to add one ground rod at the Shed/Battery location.

[Volvo farmer]

I got taken to school on grounds when I had to get my RE system by the local electrical inspector. Here's the way it's supposed to be wired: Everything is supposed to be grounded, solar panels, shed, tower, and house. The grounds are supposed to be tied to DC- and 110V neutral, but only in ONE location.

 I don't know what you're trying gain having the positive conductor larger than the negative, the current will still be limited to a 12ga wire, DC needs to flow equally between the conductors and as others have mentioned, you can't use the ground as a return path.

[wooferhound]

Yes my house is wired with 12ga wire that has 14ga grounds in it. The wire I'm using for my RE System is Outdoor rated wire that states that it has a 12ga ground printed right on th wire.

[wooferhound]

The Positive conductor (8ga) will have half the resistance of the Negative conductor (12ga). The user juddley states above that 100ft of 12ga wire has .16 ohms resistance. so that would mean that 2 12ga wires in parallel would be .08 ohms. That would be a total of .24 ohms using 100ft of 12ga with 100 feet of 8ga for the return path. If I use only 12ga to go both directions I would have resistance totaling .32 ohms.

There are 3 conductors in the wire I have. I might as well use all 3 wires to carry the power as I don't need an actual ground at the Point Of Use for the Modem/TV/Light.

[finnsawyer]

I assume you are still connected to the grid.  Well, sixty cycle can do some strange things.  Diary farmers have fought some tough battles trying to stop leakage currents from affecting their milk production (cows don't like their feet tickled by stray currents).  If you don't connect that bare ground wire to the ground rods at both ends as intended you leave your system open for stray sixty cycle voltages to appear at the tower.  Such voltages are detectable both by bio systems (you) and by using a voltmeter.  On one occasion I did measure such a voltage at a garage I was building before I connected to the ground rod.  The bottom line is that currents can pass through the Earth and cause voltages to appear in the darndest places.  So the advice is to connect the bare ground wire to the Earth grounds and use the other two wires for the positive and negative.

[Ungrounded Lightning Rod]

The ground between the ground rods will have too high a resistance to carry any significant power in your low-voltage DC system.

However, if (WHEN) lightning strikes ground nearby (unless you REALLY luck out and it strikes right on the electrical midline between your ground rods - every time) your negative DC conductor WILL carry a LOT of lightning current.

This will probably melt the wire, as well as destroying all your electronics.

If you have any sharp bends in the path, the lightning will jump out of the wire just short of the bend and back in just past it.  This will drop enough energy into the wire where the ends of the arc land that it will explode into a ball of copper vapor and plasma, which will then condense as molten copper on nearby flammible surfaces, igniting them.

This stuff about tying neutral to ground at only ONE point is very important.  Especially when you have such a nice lightning rod as a windmill tower carefully situated with the high end above all surrounding obstructions - and thus far above their "cone of protection", providing a nice roost for the thunderbird right at one end of your wiring.  B-)

[dpshort218]

I guess you have figured out by now that the earth is not a good conductor. (Thank God)

First, some National Electrical Code terms. - "Equipment Grounding Conductor". This is the Bare wire and should be tied to every exposed metal part of the system, tower, solar panel frames, etc. Normally, it never carries any current.

You didn't show any fuses or disconnecting switches. I would suggest a fuse and optionally a switch each for the batteries, wind generator and solar panels.  I would then bring them together in your shed as your positive bus in some small metal box. This will protect your system if there is a short in one of the dc sources. Another fuse or fuses can run from this to the load or loads you serve.

Next is the "Grounded Conductor". This is the negative wire of the 2 insulated wires (white). It is a current carrying conductor. It becomes grounded by a single point bond somewhere between the source of generation and the first main disconnect. The most logical place is in the shed in this "panel". This panel can be a neat box with switches and fuses or inline fuses and wirenuts. The switches are just a convenient optional disconnect method. You need to run a wire from your negative line to the ground of the panel. It can be as simple as a wirenut connection or a grounding terminal strip. of This bus is then tied to a ground rod (or rods) by the "Grounding Electrode Conductor". This too can be bare. Iy is also tied to the bare "Equipment Grounding Conductor". The negative (white wire) leaving this box is the grounded conductor. It is never to be tied to ground beyond this point. You can add ground rods to the equipment grounding conductor if you wish. You might put one at the wind tower or house to help against lightning.

[getterdone]

david, thanks for this drawing. it will be useful, cause i'm fixing to wire up my system in the next couple weeks.

[laskey]

I've got a problem with people saying the Earth isn't a good ground.  It's an excellent ground, that's why they call it "ground" or "earth".  There are a lot of high voltage DC systems that use earth return, so they only need one wire to transmit over long distances.  The problem is that no one wants to establish a good enough ground to do the job.  Heck , it's probably cheaper to run a  100 feet of 2ga wire for negative, than it is to drive enough stakes to get a good ground return at 12 volts for the same distance.

as for wooferhound's setup, you should really use the ground wire as a ground wire and hook it to your house's ground (the neutral line), and if you want to double you wire up, run a second line of 12ga.  if you only have a 20 foot run (it thought that's what you said), then buy a 100 foot roll, and bury 4 more runs.  NM-W-12 Isn't that expensive,and you gotta re-dig your trench anyway.  Then you get even less cable loss. In for a penny, in for a pound, I always say.

Hope that helps.

Zir

[finnsawyer]

It can be a problem to get a good enough ground.  The Mining Engineer that I work for occasionally has had trouble getting a decent ground at his mine site.  There is a method for testing the effectiveness of a ground rod.  I don't remember the details, but basically you run wire out a certain distance.  Two pins are placed in the ground, one at the end and another some distance toward the ground rod.  A battery is then used to introduce a current between the ground rod and the terminal pin.  You measure the voltage drop between the interim pin and the ground rod.  I also believe you measure the current.  Certain conditions must apply for the ground to be considered effective.

This shows that one can pass current through the ground, but it is unpredictable in general.  Of more concern, I think, is the possibility of stray currents, both ac and dc, existing in the ground.  The windmills that M. Jacobs developed were used to counteract dc ground currents that ate away at the steel in oil pipelines.  They introduced an opposing current.  The problem, as I see it, is that if you leave the tower ground float relative to the rest of the system grounds, you could get a shock at the tower from the bare wire when you shut down the system at the shack or house (leaving the bare wire connected to the ground rod there).  A stray current could be passing through the Earth from the shack ground to the tower ground resulting in a voltage difference between the bare wire and metal parts of the tower.  So, the bare wires (static grounds) should be connected to the ground rods at all points of the electrical system for safety.  

[wooferhound]

It seems to me that if you are going to have a single point Earthground in an RE System, it could Only Be at the Tower. And as soon as you add 1 more Ground Rod ,  well then you might as well ground it all over the place.

[dpshort218]

Wooferhound,

The point of a single earth ground is to ground one of the current carrying conductors at only one point. It doesn't mean you can't have more than one ground rod. There are 2 distinct grounding systems. One is the "grounding electrode" system. This is the single point bonded system of one of the current carrying conductors (negative wire) to the grounding electrode. This grounding electrode can be a system of interconnected ground rods, water pipes, buried wire, etc. It also can be as simple as a single round rod. The point is to only have one tie-in point to the grounded negative conductor. The other system is the "equipment grounding". This is usually the bare wire. It is attached to the grounding electrode system only at the same point as the grounded negative conductor. This equipment ground should also be tied to every metal part of electrical equipment. This includes towers, computer frames, metal buildings, etc. In addition, supplemental ground rods can be driven for a bit of added protection from lightning and short circuits. The tower would be a likely place. This is often done in tall parking lot lighting systems. Since this is only a 12vdc system, there won't be the dangers involved if this was a 120 vac or higher dc system. Good grounding can still help protect equipment and prevent a lot of strange electrical behaviors, especially involving electronics.

David

[bahnfeldt]

why get wound up about all this grounding you have a islated supply (dc) to house

there is no potential between live and earth this way

the tower  i suppose the steel work to keep at o potential , it always worries me that you create discharge from batts when they are grounded although in perfect world this should not happern

[finnsawyer]

It' not the potential between live and Earth that is the problem.  It's the potential between the 'dead' side (white) and/or static ground (bare) and the Earth ground at the tower that is the problem.  In house wiring the dead side (neutral for 220) and static ground are both connected to the ground rod.  The danger is that he may not have the wiring from the alternator totally isolated from the house ground at the shop.  This could result in a voltage difference between the bare wire and the tower ground due to unknown currents flowing in the ground.  That is, there is no guarantee that the tower will be at 0 potential.  Zero potential relative to what, anyway?

[dpshort218]

Here is a link to a pretty clear explanation of N.E.C. grounding requirements and the whys and whens of it.  
Shocking Story of Grounding

[wooferhound]

I finally built this system

you can see it and the results here . . .

http://www.fieldlines.com/story/2008/3/4/6303/63451