Grounding for solar application

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  #1  
Old 08-12-05, 08:23 AM
Solarfix
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Grounding for solar application

I have built a cabin at a lake where there isn't any public electricity. I have installed a solar panel to charge 2 - 6 volt batteries connected in series. The cabin is wired as any house would be for AC. My solar panel charges the batteries through a controller and the batteries supply 120 volts AC through an inverter.
I installed my 12V water pump at the cabin to pump water from the lake to the cabin, a distance of about 200 feet on the ground and about 75' in height. The batteries are grounded via two routes: 1. at the earth ground installed by the electrician at the AC panel, that is, through the inverter to the panel and 2. at another earth ground for the solar panel via the solar controller. When I ran a 4 gauge wire to the pump positive terminal from the battery and connect the pump's negative terminal to a reinforcing bar driven into the ground about 2 feet, I get a reading of 0.03 volts at the pump. The question is why can't I expect the full 12 volts to be delivered to my pump?
 
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  #2  
Old 08-12-05, 08:33 AM
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If I read correctly you are using a sinlge wire as one conductor and terra firma as the other conductor? This will not work. Earth ground is used to establish a plane of reference and so all metal objects in the home which might otherwise become energized in a fault situation will be at the same potential at the earth you are standing on.

The conductivity of the earth is not that good; or put another way the resistance is not that low. You need to run another wire.
 
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Old 08-12-05, 10:40 AM
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What you are trying to do is known as 'single wire earth return'. This won't work for your application; follow 594tough's advice and add a second wire.

The 'earth' is actually a pretty good conductor. The materials have poor conductivity as compared to copper, but there is a darn lot of them. So to a very good approximation, you can consider the earth a perfect conductor, and imagine that all of the resistance is concentrated at the grounding electrodes. Thus grounding electrodes are rated in terms of 'resistance to earth'. A really good grounding electrode will have a resistance of perhaps 5 ohms. But most home electrodes are in the 25-100 ohm range. That rebar in concrete is probably 100 ohms.

So in the circuit that you describe, you probably have 150 ohms of resistance. If you only need 10mA of current, then your circuit would probably work. But not for a pump drawing several amps.

SWER is used for power distribution, where power is supplied on a single wire at (for example) 15000V. Only a few amps are required to deliver considerable power, and _very_ good grounding electrodes are installed. With say 10A of current (150 kW) and 5 ohms of resistance, the voltage lost in the ground resistance is only 50V, a loss of 0.3%.

-Jon
 
  #4  
Old 08-12-05, 09:55 PM
toolboy
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dont forget about voltage drop !!!!

question, from your description it seems that you intend to connect a 12V DC motor to a 12V battery with 400ft (200 each way) of #4 copper wire. is this correct?

How big is this pump? (watts or Horsepower)

you can expect a healthy voltage drop, and with only 12V to start on a fresh charge, and if my quick and dirty calculations are correct (#4 wire approx 0.24 ohms per 1000ft .... 0.1 ohm total wire resistance for 400 ft .... say the pump draws 10 amps ... thats 10V drop on the wires and 2v at the pump) you may have a problem

how big is this pump? perhaps i am wrong in my understanding of where the pump is located, in the house or at the lake?
 
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Old 08-12-05, 11:36 PM
Solarfix
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The pump is at the lake. I understand now why the earth cannot be used as a common ground with rebars as a grounding electrode.

But I don't understand the results of your calculation. I agree with the resistance of .24 ohms/1000 ft. (or .1 ohm/400 ft.) of #4 wire but, using the old standby formula, V=I*R, I get 10 * 0.1 = 1 volt drop, leaving 11 volts to run the pump. (The real numbers are a little different as the batteries are usually kept charged at a little over 13 volts and the pump draws 8 amps.)

Is there an error in my calculation?
 
  #6  
Old 08-13-05, 08:27 PM
toolboy
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no. no error in your calculation. the error was mine, sorry (my only defense is that it was pretty late when i posted and uuuh ...)

that still seems like a very very long run for such a low voltage system, but if it works, thats great. ;last night my mind was racing with al kinds of crazy ideas to increase your power transmission voltage. but, simplicity = reliability. the last thing u need is a lot of complicated malfunctioning equipment when all u want to do is get some water.
 
  #7  
Old 08-13-05, 08:44 PM
Solarfix
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Thanks

Thanks to all for bringing me up to speed on the grounding situation.

Larry
 
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