off grid grounding and OL protection

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nadprosper

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Hi Guys,

Some artist friends of mine are installing an art piece in the middle of the Texas dessert that consists of a lit metal welcome sign 20 feet off the ground mounted on an iron post. The power for the lights comes from a solar panel that goes to a charge controller then to a battery and then to an inverter to get 120V and then to 7 1 watt 120 volt LED lamps. My questions are this.....should they ground the electrical system and how? ( on the 12V side or the 120V side or both ) What kind of fuse protection should they use and where ( on the 12V side or the 120V side or both ), is GFI protection a good idea ( keep in mind that the intention is minimal maintenance so trips out to the middle of the dessert to reset a GFI would be a drag ).

Thanks in advance
 
It seems inefficient to start with low voltage DC, then convert it to high voltage AC to operate what are really low voltage DC devices.
 
Never the less, the question is about grounding and protection. The reason for going with 120 V was because it allowed us to us a 1 watt lamp that gave us a 25 watt ( equivalent ) light source...... minimizing the size of the battery and solar panel which was an important consideration.
 
nadprosper said:
Never the less, the question is about grounding and protection. The reason for going with 120 V was because it allowed us to us a 1 watt lamp that gave us a 25 watt ( equivalent ) light source...... minimizing the size of the battery and solar panel which was an important consideration.
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It doesn't work that way. The use of the inverter will require more solar panels and batteries than if you design a system to directly supply the LEDs.

As far as the grounding you need to look at 690.41 for the DC side 250.20(B)(1) for the AC side. Look to 690.9 for the required overcurrent protective devices.
 
Thanks for the NEC references.

Just to check my thinking....... I found 1 watt 120 Volt LED lamps.....we are using 7 of them.....totalling 7 watts for the entire load. Add in a 15 watt panel and a roughly 34 Amp hour battery and an inverter. This means that on a day with 12 hours of sun the lights can stay on 24/7 and never deplete the battery ( 15 watts/12v=1.25 amps an hour charging the battery vs. 7watts/12v=.58 amps of load x 24 hours=14 amp hours of load a day leaves me with an excess of 1 amp hour of charge ( 1.25 amps for 2 hours = 15 amp hours of charge ).

The smallest wattage 12v LED I could find was 3.5 watts making my total load 24.5 watts. 24.5 watts at 12v is 2.04 amps meaning my total amp hours of load are 49 ( 2.04 x 24 hours ) meaning my 15 amp hours of charge cannot keep up.

Now, there may be a product out there that I did not find, but based on the parameters dictated by the size of the sign and simplicity of installation....we were limited to one solar panel and we chose standard base lamp holders for our light source.

Please let me know if my math is crazy.....

Thanks
 
I'll bite. What's up with a welcome sign in the middle of the desert?

GFI or GFCI?

Are you suggesting there will be people around this sign and that they might touch it (in the middle of the desert), therefore GFCI might be needed? Or is it for the Lizards and snakes?
 
Be aware that _all_ LEDs are low voltage devices.

A 120V LED lamp is using some combination of multiple LEDs in series and drive circuitry to operate the low voltage semiconductors with a high voltage supply. Sometimes this drive circuitry is so well integrated that the device looks like a single component (ACRICHE LEDs, which I belive put _many_ LED junctions on a single semiconductor die, making a high voltage device that can be run efficiently from the mains with a simple series resistor, for example).

Some of these 120V lamps depend upon the quality of the supplied AC waveform; they may not take kindly to the output of an inverter. Others will barely notice the difference; the only way to know is to test it.

I have found that manufacturers often _greatly_ exaggerate their lamp equivalence claims. You mention '1 watt lamp that gave us a 25 watt ( equivalent ) light source', a claim that I would not trust unless you have special circumstances. If the manufacturer does not give actual lumen numbers, then the equivalence value is marketing BS. If they do give actual lumen numbers, then there is a still a good chance that they are wrong.

-Jon
 
Solar Design Comments

Solar Design Comments

Why operate lights 24 hr/day?

Your solar design will not work, the 15-watt output of a PV panel is noon on a clear day, oriented to the sun. Your battery will not last very long with an undersized PV module.

A reliable PV system design to operate a 7-watt, 12-volt DC load all night using a solar controller with darkness detection will need a 30-watt PV module and two of your batteries (better is one larger battery). If you must use AC light fixtures, then you need an inverter. There are no 7-watt inverters. For a similar load I use a Xantrex Mobile-Plug 75 inverter (now out of production, but available surplus) that has a 1.5 watt loss and 80% efficiency. This would require a 45-watt module.

These small inverters are not UL listed and not suitable for normal grounding. They are essentially 60/120V inverters without a neutral, each side of the receptacle is 60 VAC with respect to the negative of the 12-volt supply. If you ground the apparent negative, you kill the inverter. When I use these for restrooms, I use a 2-pole CB and mark the circuit as non-standard.

I do not know the format of the 1-watt LED you have selected. I recently discovered that my local RV trailer repair shop was carrying white LED backup 12-volt lights for trailers that are very bright for their approximately 1-watt draw.
 
Thanks for all the info!!

Sign in the desert? Art Baby....Art!!

I all ready tried to post a long reply, but I somehow lost it.......in short, BillK, why do I need a 45 watt panel to run a 75 watt inverter, I thought the inverter ran off the battery and the panel just charged the battery..... Also, the inverter we have ( a 300watt go power ( modified sine wave :( ) has a three prong output. If I ground the negative terminal of the battery, does that mean that a) the inverter will fry and b) the inverter isn't grounded? We were thinking of running hte output of the inverter to a subpanel which would be grounded. Bad idea?

Thanks
 
Solar Design Comments-2

Solar Design Comments-2

The size of the required PV module is the result of energy calculations for the worst time period defined by 'days of battery backup' at the intended site using solar and temperature data. Too complex for a simple posting. I have software and the required data to do these calculations. The only way to get reliable operation is to design it right.

If you use a 300-watt inverter, its inefficiency will use more energy than your LED lights. Also, if you use a solar controller with darkness detection to turn the inverter off during the day, the surge into the capacitors in a 300-watt inverter may destroy a low power solar controller's load control function. You can use a power relay for the switching, but that is more cost and adds to the load. I know the 75-watt inverters I use are compatible with a 20-amp solar controller.

Grounding depends on the inverter, study the installation instructions. If the inverter is UL listed (most small inverters are not), then it will be designed such that the neutral is bonded to ground and the battery negative is also grounded (or can be grounded). Real easy to make a boat anchor out of an inverter by doing it wrong.

Plan on grounding the frames of the PV modules. See NEC Art 690, it covers PV systems including 12-volt.
 
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