Off Grid Wiring

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I wrote a little program to calculate power loss in wiring. I couldn't find a free program to calculate power loss in watts for a given wire size in all AWG sizes, so I wrote one...

This program is handy to see how much power is lost in the coils of an alternator stator, in the lead in wire from a wind generator or solar array, battery bank cables or any run of wire. Just remember you have to double the length of your wire length run to account for current in both directions in a standard DC wiring system.

Click Here to download the program.


Batteries:

I'm going to start the wiring section with my thoughts on battery storage. Last time around I went with 12) 6 volt golf cart batteries. They worked fine and were the inexpensive "Sams Club" batteries. I'll go with those again, just for the price point, being so much cheaper than name brand varieties. I won't need as many batteries this time, for two reasons. First, my daily power requirements will be much less. Second, I'm not so sure over sizing a battery bank is necessarily the way to go.

The conventional wisdom is to size the battery pack so you don't use more than the top 20% of the full charge. Battery life is greatly extended by over sizing, and limiting the depth of discharge to 20% to the occasional 50%. But looking at it realistically, if you reduce the battery pack to half the size you need to keep discharge at 20%, you're still only discharging 40% of capacity, and I doubt that would reduce the battery pack life by half, but even if it did, you'd still be at a break even situation on amp hours in and out per dollar invested. Plus, you'd have half the initial investment, which means you'd have a net dollar savings from interest on the half battery pack you didn't buy.

The cost of maintaining a bank of batteries always bothered me. By maintaining I mean depreciating out the cost of the investment. In my current situation, I'm looking at a bank of 6 batteries, which should cost me about $480. If I get 40 months out of those batteries, that's a cost of $12 per month. Not terrible, but still a cost that's need dealing with. My local electric company made the decision to go off grid easier. They started a monthly "meter fee" of $8 a couple years ago, then they raised it to $10, and now $12 a month. So the meter fee now completely offsets the cost of depreciating a battery pack. Off course, at the same time this meter fee was going up, the cost per kWh was going up as well, some 40% in the last 5 or 6 years.

The batteries are rated at 220 amp hours, with 6 batteries that's 1320 amp hours capacity, or 7820 watt hours. Assuming I use 1,500 watt hours a day, that's discharge level of 19.2%. Assuming I have spells of little sun or wind, that still gives me 3 or 4 days of no charging before I get down to the lower limit of 80% depth of discharge. In the real world, there's always some charging going on even on cloudy days, plus, it's pretty rare around here to go days without any wind. Providing I keep my power consumption down, the 6 batteries should give plenty of reserve and good long term life as well.

12 Volt Battery State of Charge Chart

State of Charge Volts Volts*
100% 12.7 12.7
90% 12.5 12.58
80% 12.42 12.46
70% 12.32 12.36
60% 12.2 12.28
50% 12.06 12.2
40% 11.9 12.12
30% 11.75 12.04
20% 11.58 11.98
10% 11.31 11.94

* Source Trojan Battery company


Wiring:

The bad news is we are in recession, the good news is some of things that got very expensive in the past couple of years, got cheaper again. Copper is a good example, it's dropped to half what it was a year ago. Thank goodness too, since going with low voltage systems means heavy gauge wire! I've been adding 12 volt circuits to my living spaces for the past year. I'm simply using standard interior single strand wire. I'm also using regular 120 volt wall outlets, I intended to use an oddball 220 style plugs and outlets, but when I saw the price I decided to just go with the standard style. The down side to this is you could accidentally plug a 12 volt device into a 120 outlet. So I label all the wall outlets as 12 volt dc.

With the 12 volt outlets wired with 12 and 14 gauge wire, they won't carry a lot of current without significant voltage drop. So these outlets are only intended to run low power devices of 5 amps or less. Most outlets will only ever run a CF light at 13 watts or a small fan of around 15 to 18 watts. For larger 12 volt appliance I'll have to run heavy gauge wire, probably 6 gauge depending on the length of the run.

Some of my overhead light fixtures that are wired to wall switches were also converted to 12 volt. I ran a heavy main line to my breaker box, pulled the light only circuits and connected them to the main line. I understand standard 120 volt ac wall switches don't hold up well to low voltage dc. They are supposed to arc as the contacts are made or broken, wearing out the contacts. But I've seen other people make the change and after 20 years still haven't had a switch fail. So I'll take my chances for the time being, checking those switches from time to time to see if there is noticeable pitting.

Right now I've got my entire 12 volt load on a 30 amp dc breaker. I would think this would suffice, at least it should until I start adding high draw 12 volt appliances. I recently started adding a few incandescent 12 volt light bulbs. CF bulbs are great, but in cases where the bulb is turned on for short periods, it makes sense to use incandescent bulbs, the CF's burn out quickly when used for quick on and off use, plus the incandescent bulbs are cheaper and don't use much energy when only turned on for a minute or two.

The solar panels have a buried pair of 6 gauge wires running from a junction box at the array into my work shop. From there the 6 gauge wire goes into a 30 amp breaker box, outputting to a pair of 3 strand 14 gauge wires going to the charge controller that feeds the batteries. At some point as I increase my solar array, I'll have to add more copper to those circuits to keep line loss down.

My new, small wind generator runs wild ac into the building through a pair of 10 gauge wires. The 10 gauge terminates at the rectifier diode. The nice aspect of running the wild ac into the house is that I can short the ac leads, effectively shutting the wind generator completely down. In the past I used a pair of 2 gauge copper wires from the wind generator into the house. If I go with a larger generator, and/or when I move the generator farther from the house, I'll have to increase the wire size. I know people recommend a fuse on the wind generator. But I'm leery of that. I'd hate to see a fuse blow and the wind generator over speed and destroy itself. The way it is now, the 15 wire in the stator should burn through before the 10 gauge wire going to the batteries. I'll suffer a lost stator if need be...