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Bigwooo
April 28th, 2009, 8:27 PDT
I have an off grid home in Mexico where I知 installing a whole house water purifier. The system incorporates a 12v-25w UV light. In order to conserve power, I want to install a switch to turn the UV on and off in each of two bathrooms and another at the kitchen sink. I would like to install a 4 way switch between the two 3 way switches to accomplish this.

My questions are:

Do standard switches (3 and 4way) work with 12 volt?

Even though I致e worked out the voltage drop and wire size needed, do the switches themselves cause enough voltage drop to be concerned about?

Assuming I can use regular AC switches, do I treat the positive wire like the hot wire in AC, and the negative like neutral in AC? Of course I値l run a separate system ground wire.

Assuming this will work, can the negative wire be significantly shorter than the positive wire, or should they both be the same length? The water purifier is next to the sub panel, and since the negative wire isn't used with the switches, could I hook the negative from the breaker directly to the light at 6 feet long, while the positive goes through the switches and is 40 feet? (note: my system ground will run to the ground lug on each of the switches)

Thanks,

Wooo

niel
April 28th, 2009, 11:40 PDT
the problem is arcing with using ac switches on dc. if the current is kept small it is possible to use the switches and they are often found to be cheap compared to their dc counterparts because of their vast availabilities. as a rough guide divide the switch ac capacity by 10 for dc, but expect to replace it in the future too.

mike95490
April 28th, 2009, 12:02 PDT
12V, 25W = 2.1A
So your standard switch (while not usually rated for DC) should function. You may have to replace every couple of years, as they wear via arc & pitting.

Of greater concern, is your UV water purifier. The UV tube takes several minutes to warm up to rated output power. They are usually designed for "always on" operation. You may not get enough power to reliably sanitize the water in a off-on-off system. Also, when off, you could get bad water diffusing thru the purifier tube, contaminating the clean side.

There are other purifiers that use the UV tube to generate ozone, which is bubbled thru a water column. The action cleans the water, and slowly pumps it into a usage tank, but once shut down, the protection vanishes, and water can grow bad levels of bacteria from the 1% that was not destroyed. (either case ozone or UV).

Also, if you fine filter the water first, you can run (gravity drip) though a carbon block filter which has small enough pore size, to filter the nasties.

Bigwooo
April 28th, 2009, 14:24 PDT
Thank you Mike and Niel

12V, 25W = 2.1A
Of greater concern, is your UV water purifier. The UV tube takes several minutes to warm up to rated output power. They are usually designed for "always on" operation. You may not get enough power to reliably sanitize the water in a off-on-off system. Also, when off, you could get bad water diffusing thru the purifier tube, contaminating the clean side.
Also, if you fine filter the water first, you can run (gravity drip) though a carbon block filter which has small enough pore size, to filter the nasties.

I didn't know there was a warm up period. That's a biggie.

I like the idea of Ozone, but the ozone generators I've seen draw about 240 watts. That's a lot of additional panels and batteries. I think I just might size my solar to support the additional 25 watts x 24 hours for the U.V. Don't need any nasty's in the water.

It seems like I keep adding conveniences that require just "one or two more panels". One more panel equals more cables, more batteries.....Just part of living off grid I guess ;)

If anyone is interested here's the water purification system: http://www.waterfixercompany.com/model1000.html

RandomJoe
April 28th, 2009, 14:40 PDT
Voltage drop at 12V is annoying! :p

What I would do, if you decide to go ahead with it, is use the switches to turn on/off a 12VDC relay at the purifier. That way your wiring to the switches only carries a few milliamps - and for that matter you could use MUCH smaller wire for the longer runs to the various rooms. And you probably won't have any issue with pitting the switch contacts.

Bigwooo
April 28th, 2009, 14:42 PDT
Voltage drop at 12V is annoying! :p

What I would do, if you decide to go ahead with it, is use the switches to turn on/off a 12VDC relay at the purifier. That way your wiring to the switches only carries a few milliamps - and for that matter you could use MUCH smaller wire for the longer runs to the various rooms. And you probably won't have any issue with pitting the switch contacts.

Thanks, I think with all the above advice I've given up on the 12volt idea. They offer a 120 volt system and if I do decide to install switches It will be much easier.

mike95490
April 28th, 2009, 15:51 PDT
here's a 55W ozone system.
http://www.tripleo.com/
Same thing - UV tube, plus a fancy aquarium air pump

But, same problems, when off, there is no residual protection.

BB.
April 28th, 2009, 17:23 PDT
For drinking water (as opposed to sink/washing water)--I wonder if something like a biological capable (down to viral) filter (http://www.sawyerproducts.com/products.htm#002) would work (does not filter out minerals/salt)?

Throw a standard sediment filter in front--it would seem to be a low power/low cost/reliable filtering media for home use.

I purchased one for camping/emergency use--I also have the standard chlorine/etc. chemical/filters too. Don't know much more than what I have read about the product.

-Bill

rplarry
April 29th, 2009, 7:28 PDT
Bigwooo
I also live off grid in Mexico. What I did for drinkiging water is to install a 45 gallon drinking water tank and a shurflo pump that supplies my refrigerators ice maker and then I Teed off that line and ran a tube to the kitchen sink where there is a dedicated drinking water faucet. We can buy 5 gallons of drinking water in town for 13 pesos, so not too expensive. For the rest of the house I have a 2000 gallon "pila" which I add a little chlorine to and use that water for washing clothes, dishes etc. Works well for me.
Good luck with yours,
Larry

Cariboocoot
April 29th, 2009, 11:00 PDT
UV type purifiers also want at least 5 micron pre-filtration. You may be better off going with a "particle" whole-house filter which uses no electric and a ceramic cartridge type for drinking water.

The main question is: what's in the water that needs to be removed?

Check this Rainfresh site for some ideas: http://www.rainfresh.ca/productinfo/productguide.html

Bigwooo
April 29th, 2009, 15:46 PDT
Thank you for all the suggestions. There's a lot more out there than I realized.

The system I'm looking at has a 5 micron, then .5 micron filter before the U.V. light. I want to be sure any bacteria/virus's are destroyed. The source of the water is good, but it has to be trucked in and the trucks that deliver leave little to be desired.

We're just far enough from where we can buy 5 gallon bottles of water to be inconvenient. We have a 9000 gallon underground tank and it's not shaped properly to use ozone to clean the entire tank.

I want to filter and disinfect the shower and sink water just to be sure no one gets the nasty's. I will definitely look harder at the 55w ozone and the "Rainfresh UV system. I don't think there's any way to get the water flow I need (at least 3.5gpm) while still killing the bacteria without drawing power for either UV or Ozone.

mike95490
April 29th, 2009, 16:06 PDT
If you have your own tank, you can add chlorine to it, and then carbon filter you drinking water. (and still shower & wash in semi-clean water)

Lefty Wright
April 30th, 2009, 15:46 PDT
If you change to 120VAC this won't come up, but there is one thing I must mention.

You asked about running the negative wire separately from the positive wire to save length.

Don't do it. When running DC always run the negative and positive wires in the same raceway so the lines of flux cancel out. If you run just one wire of a DC circuit by itself you are creating an electro magnet. This can cause heat and losses.

And when running DC if you enter a metalic box with each wire going through separate holes you must saw a slot between the holes to break the current flow that would be induced by the magnetic lines of force emitted by the DC wires.

AC can do this to a lesser degree - in breaker panels, for instance, where the installer left the hot wires long and wound them up and stuffed them into the panel after landing them on the breakers. This causes heat and since circuit breakers are thermo devices the breakers can trip at low current levels.

I have made several service calls to places having nuisance trips caused by this. When I put my hand on the metal panel enclosure I could feel that it was very warm.

After cutting the wires to the proper length and re connecting them to the breakers the heat and tripping problem went away.

BB.
April 30th, 2009, 17:00 PDT
I am not so sure that you need to run DC in pairs to prevent heating/losses (such as through two separate holes in a electrical box). There is no constant reversal of fields, so there is no, constant, induced current flow.

AC wires--parallel runs is a must (to prevent induced current)--and is actually in the code too.

Now, there are many good reasons to run + and - wires together for DC (and AC too)... The big one is that if you take two separate paths (even in open air where there is no metal to heat)--you are creating a loop antenna. This antenna can broadcast electrical noise from your equipment (receiver interference), and make the equipment more susceptible to receiving electrical interference too (radio transmitters, lightning induced current, coupling between AC and DC circuits, etc.).

Also, placing the wires next to each other (and even providing a twist--such as is done with CAT 5 Ethernet cabling)--also lowers the overall impedance of the circuit (better response to step load changes--less overshoot/undershoot/inductance, less chance of "ringing" / oscillations in attached loads/circuits, etc.).

-Bill

Bigwooo
May 9th, 2009, 19:50 PDT
Wow lefty and BB,

Great information I didn't know. I'm not an electrician, but I'm going to have to do a lot of the installation myself as there's really no one qualified near where I'm building in Mexico. I've been teaching myself about installing electrical and solar, so far no problems on what I've already installed, but that may be beginners luck ;). The more info the better :D