I am often untimely in publishing these notes, but this time I have outdone myself. I do have an excuse, though. It involves a dog, a motorcycle and 3 broken ribs. The dog and the motorcycle were not injured, but I went splat. Man, I crack myself up, sometimes! (Get it?)
The purpose of this project is to get water from a spring at one end of my farm to a storage tank on the hill near the other end. Avoiding the expense and maintenance of batteries, the water will be pumped by solar during the day and used as needed. I currently use a century-old water-powered pump to do the job, but it does not provide as much water as I could use at times for lawn watering or cleaning up the crop dusters’ equipment. Plus, sometimes I just get in my mind I need to try something different. Water is not only the end use product, here, but it is also the power storage medium for keeping the pressure tank at the house topped up.
While I am still on the injured list, I am trying to catch up on delayed chores. I have a few detail items to obtain, but will probably pull wire for the project this week. You may never install a solar-powered water pumping system, but if you do anything with solar, you will need wire and some knowledge of a few concepts involved with it.
- The insulation of the wire has to be suitable for the purpose. Voltage rating, insulation type, UV resistance, etc. If you have a 600 volt series string of panels, you don’t want to get bit using 300 volt wire. If it will be exposed to sunlight, you don’t want the insulation to fall off. That sort of thing.
- Ampacity. 100 amps of current through 20 amp-rated wire = fire. Ampacity is important.
- Voltage drop. We call wires conductors, but they really are resistors. The wire may be able to carry 30 amps, but if you use 300 feet of that wire at only 10 amps, you might not get all of the volts at the source end to the destination end.
Wire is going to be a big deal in this project. I went with my trusty assistant, Luscious Long, to find out just where the best place would be to put the panel and how much wire would be involved. (Luscious and I usually go on archaeology treks miles into the woods, so this is like a walk in the park.) The pump will be the only place it can be. In a dense jungle swamp, of course. Closest opening is in a narrow corridor, bordered by tall trees east and west. First south view with clear east and west is 350 feet. Luscious and I slogged through the jungle with a 100 foot tape to get the numbers right. You don’t want to buy 280 feet of wire and need 350. Avoiding splices is good, too.
Oddly, buying a lot more on a full roll may be cheaper than buying just what you need, by the foot. I saw this yesterday in budgeting a project for the museum. I can buy the 200 feet I need for that project at $3 per foot ($600) or I can buy a 250 foot spool ($467). On the other hand, for my pump motor, I will need a few feet of motor cable and a couple of bucks a foot will be cheaper than buying a 50′ spool. You have to check the options.
Now let’s consider the wire type. Buy copper. It is expensive, but it conducts well and does not corrode as badly as aluminum. Beware of CCA, or copper coated aluminum. If you order cheap wire on ebay or from Chinese suppliers, that’s what you might get and it can quickly turn to white dust. For this project, it will have UV exposure and it will be in a wet environment. First thing to come to mind is Direct Burial cable. It has a heavy insulation and good UV resistance. The problem I have with that is it comes with an extra conductor. You buy a roll of 10/2 (10 gauge and 2 conductors) and you get 2 main conductors plus a ground wire. I neither need nor want a ground conductor and the extra expense. I could connect the ground wire and one of the main conductors for a little less voltage loss. In a code situation, where inspections would be involved, it would probably be a good idea to check with the inspections office for what would make them happy. This is a farm project, where inspection is not involved, but we want it to work, last and not set the woods on fire. I’d be ok if it electrocuted a cottonmouth moccasin.
Direct Burial cable tends to be kind of stiff as it is solid. We can’t bury this wire, so a little flex would be nice. Where can I find outdoor wire that is just what I want? There is another option.
You know how all the houses in the big house neighborhoods are lit up by lights in the shrubbery, so you are reminded that their house is bigger than yours? Those lights in the flower bed are connected by a 2 conductor cable called Landscape Wire. The stuff is half buried and half just strung along the ground, so it is good for UV and underground moisture. Systems use low voltage, below 30 volts, but the wire is rated to 300 volts. It is exactly 2 conductors and no ground. That looks interesting! It comes in long spools and in lots of sizes, too.
The motor controller I am using suggests an input of around 30-60 VDC, my 475 watt panel puts out a max of 53 volts and lugs down to 45 at Power Max with around 11 amps. The pump ad says the pump uses 140 watts, or around 6 amps at 24 volts (Power = Amps X Volts or Amps = Watts/Volts). That will be maybe 3 or 4 amps coming in from the panel. Most any wire will carry that current, but because wire has resistance, we need to consult a wire voltage loss chart to determine what gauge wire we will use. We start with 53 volts and have lots more watts capacity than we need, but it would be nice for the pump to run on a cloudy day, too.
Don’t pay any attention to the % column because the tables are made for 120 or 240 VAC service. Look for actual volts drop. The chart I am using says if I run 10 amps through 150 feet of 10 gauge wire I will lose about 3 volts. I am using twice as much wire and half as much current, so it still should come to about 3 volts. Since the panel can make up to 11 amps, 11×3 is only 33 watts lost from a 475 watt panel to run a 140 watt pump. Should work great, even on not so sunny days, or early in the morning and late in the afternoon.
Well, call me a little jaded about panel specs, motor specs, startup current and the completely ridiculous term “Sunshine State.” I might be able to get by with 12 gauge wire by using a couple of 36 volt panels in series to push a little harder, but why not just leave a little extra with which to work? One of my basic assumptions is, “They always want more.”
Where to get the wire? Honestly, a lot of the time it is hard to beat Home Depot. For the same price they will deliver it to my porch and I don’t even have to go to town to fetch it. They don’t carry solar panel wire, last I looked, but everything else is covered well or they can get it in a few days. However, I check Ebay for surplus deals. I found a 500 foot spool of 10/2 for $300, a bunch cheaper than storebought. If you buy an Ebay deal, just make sure it does not come from China, where they might conveniently fail to mention that the wire is CCA, if that is the case, and it usually is.
Do you know a guy named Murphy? As in Murphy’s Law? While studying system specs is important in planning, specs are sometimes dubious or subject to change. When the pump arrived, I immediately opened the manual for close study (not really, I checked out the goodies, first) and discovered a different specification. The manual covered the company’s full line of pumps, but the only one in the manual for a 24v pump said it used 340 watts. That’s not a lot of power, but it is a lot of difference and with some consequences.
Did I get the right pump and the wrong manual? Did the advertised spec indicate no-load and the one in the manual indicate full load? Am I going to have to fall back and regroup? I will spare you the math, but I calculated what it would take in power to raise 1300 liters of water 50 meters and it looks like the 140 watts is right for full load. I suppose I’ll just have to see what will happens when I turn it on, but I did some more quick and dirty math to determine if I should be worried.
340 watts is now 14 amps to the motor and now maybe 8 or 9 amps from the panel. That won’t hurt the wire and when the sun is good we have plenty of go-power, but morning, afternoon and cloudy times will suffer. (I suspect the motor controller will run the motor at a slower rate under less than ideal conditions, giving plenty of runtime when the sun is not at full power.) Buying the 10 gauge wire will save the project if the book specs are right. Marginal condition performance can be salvaged by substituting a bigger and or higher voltage solar power source. I am lucky enough to have a collection of spares to afford such games. While I will concentrate on getting the pump properly wired and plumbed, I am not going to put a lot of effort into mounting the planned 475 watt solar panel until I am sure it is the best one for the job.
That’s where we are now. I’ll put on my snake boots next week and pull wire through the snake-infested swamp. A trip to Home Depot should get me the rest of the miscellaneous fittings and things I need. I’m anxious to try this thing!
–Neal