12V only system

[rniles]

I'm wanting to plan for an off grid system, I don't like the waste from converting a DC to AC ...so I was wondering if it's feasible to just stay 12V.

PV system to charge a bank of batteries, then wiring 12V only to power a fan, lighting, and a few 12V Cigar Sockets throughout a Little House. There seems to be plenty of 12V appliances that plug into the 12V cigar soclets; will be using a laptop and recharging little electrical goodies (cell phone, ipods/ipads, or things like that). Other things I would have is 12V/Propane fridge.

Am I missing anything or haven't thought of anything? ..especially cons to doing something like this?

Or if there's another thread that covers 12V only systems, please point me in the right direction. I've read http://countryplans.com/smf/index.php?topic=6059.0 and a lot of posts by MountainDon (and others).

Thanks

[peternap]

It depends on what you need. I'm on a 12 volt system but I have AC for some applications.

The big problem with 12 volt is that when you do need 120, the lines have to be huge to the inverter because of the massive current draw.

The big advantage is that there are a ton of RV and auto items available for 12 volt, alternate generation methods are a snap and inverters are cheaper.

You really need to ask yourself if you're prepared to change your lifestyle enough to be satisfied with 12 volts. If the answer is yes, go for it.

[rniles]

Would it be possible to run two wires from your batteries ...one shorter for large watt usage (like inverters) and one longer run of low watt usage (like lights, chargers, etc)?

http://www.windsun.com/Hardware/Wire_Table.htm

I like these charts, they help with the usage, wire size, length calculations.

And what AC appliances do you feel would be on your must need list?

[Phssthpok]

http://www.marxrv.com/12volt/12volt.htm

[MountainDon]

Question: will this be a new build that is not only off grid but off the radar? No inspections, permits or anything? If it subject to permits, etc that could be an issue when it comes to things like automobile power outlets. They and other 12 volt stuff is not NEC approved. If the build is NOT subject to all rigamorole, then 12 VDC may be very usable. Only you can tell if it can work for you, or if you can work within what sould be viewed as the limitations of low voltage DC only.

I would make a list of everything you want you use. If you don't already own the item look around and see what is available for the job and if it will serve you wants.

Be aware that electric switches rated for AC use are normally AC only and will burn out when used on DC power.


There were a couple of things that discouraged me from going DC only. For a while I was thinking DC and some AC. In the end, for my wife and I, I went AC with a pure sine wave inverter. After having it running for a couple winters now I am glad that I did.

We were going to have a microwave. That was an absolute. If a m-wave is in the future believe me when I say we found a microwave that depended on a generator a PITA. Lots of RV experience. Back then I decided an inverter that was just big enough to run a microwave would be a great idea. The first inverter I bought (square wave) would not run the m-wave. It would hum, the turntable turned but the magnetron produced no microwaves. It (and some other m-waves) can be really fussy when it comes to waveform. Not all, but some. In the end the inverter (pure sine wave) cost more than the m-wave, but everything worked. If one does not want a microwave this does not apply.


On the refrigerator; many (most?) fridges that can be operated on electricity or propane use a heater element to heat the absorption system. Resistance heating of any kind is not efficient on electricity when the power is stored in lead acid batteries. There are some refrigerators that use an efficient compressor technology; they are expensive and electric only as far as I know. We have a Servel propane fridge; works good. The AC heater on it works, I tried it, but a power hog off grid. Good if there is a power grid and the propane runs out. IMO.

I guess the bottom line is really how many electrical devices and what type are going to be used. My own feeling is if the power needs can be fulfilled with two six volt golf cart batteries and small PV module/array then go for it. If that won't supply enough battery power, without generator use, then go for an AC system. Maybe I like my assorted electrical gadgets too much. I know I really do not like to be forced to run the generator just to permit some appliances to be used.

[rniles]

Some good thinking points, thanks MountainDon!

I'm trying to keep electrical use low ...simplicity, I think, outlines my thought process and I may not get want I want but I'm fishing none the less.

If automobile power outlets (cigar sockets) are not NEC approved, then are we forced into AC electrical systems by code? Is there a different DC receptacle that can be used? These DC power outlets (cigar lighter socket) are typically used in boats (even live aboard), and most automobile have multiple sockets now (my wife's car has them in the front seat area, rear seat area and even in the trunk (hatchback) area). RVs have them as well.

I think the RV's DC system is the example I'm trying to follow (minus the AC portion) and hoping that with a DC only system, less energy is required to be produced and less energy would be used.

(thanks for the fridge information ..I didn't think of that ...I wonder if the fridges Truckers have in their rigs would be work looking at)

[MountainDon]

Many use 240 VAC 15 or 20 amp receptacles and cord plugs. They are classed a NEMA 6-15 or 6-20 receptacles and plugs

NEMA 6-15 

NEMA 6-20 

Sometimes hard to find in box stores.

http://www.levitonproducts.com/catalog/dept_id_969.htm?sid=520CB8ED84E1DD56113D97281E622F5F&pid=1208
http://www.levitonproducts.com/catalog/dept_id_971.htm?sid=520CB8ED84E1DD56113D97281E622F5F&pid=1208

Cars, RV's and boats don't have to meet NEC, only buildings.


The catch is the building can not have any 240 volt receptacles of that type in it anywhere.

If you had no 120 VAC in the building at all, not even the wimpiest of inverters you could likely get away with using 120 Volt receptacles and plugs. Not sure, but I think that would fly if there was no AC at all.


On the switches a toggle house type switch that has a hard click may be suitable for DC. Not easy to find. The switch contacts have to be heavier than in an AC switch. That's because AC current makes breaking the current flow easier as the current turns off and then reverses 60 times a second. DC flows one way and makes a large spark. The spark will erode and sometimes weld the contacts on an AC switch. A good reason to keep them away from batteries and flammable gases.

For my few DC switches I used some switches made by Carling switch. Not NEC approved for home use, but the inspector never revived an invitation.

I've also never been 100% happy with most of the 12 volt auto plugs and receptacles I've had in trucks and RV's.

[peternap]

I forgot some people are above radar
so, everything Don said.

For the record, I do not use the auto style connectors. I make up power cords and use the 220 outlets.

There are also breakers that need to be rated for DC. What Don said about switches, goes for breakers also. Going from memory, I'm using Square D breakers that they acknowledged are good for DC but are not rated for it.

[MountainDon]

Square D, QO or QOC breakers are okay for DC applications.

Their "AIR" value is actually a little low. It could be possible that in case of a bad short circuit the breaker might not let go fast enough to prevent contact fusing. So if one wanted to be extra cautious or wanted to impress an inspector one would install a large DC rated fuse between the batteries and the breaker box.

It's on my list of improvements to make someday.
"AIR = ampere interrupting rating"; 50,000 ( or was it 25K ?  )is recommended for DC with batteries where they could supply a huge rush of current with a big dead short. So I have been told by the techs at Outback.

(also moving this general topic)

[considerations]

As my project has progressed, I've accumulated a "few" items no longer needed.    I have a 30 amp DC breaker laying about.  If you find a need for it let me know.  It needs a home and is basically brand new.

[MushCreek]

As for energy required, it's roughly the same, regardless of power type. Watts are watts. If the voltage is lower, the amperage has to be proportionately higher, so it's always the same number of watts. As an example, 10 amps at 120 volts equals 1200 watts. To get 1200 watts at 12 volts, you need 100 amps! That requires thicker wiring, so the 12V could end up costing you more. I just don't want you to think that 12V 'uses less power'.

[rniles]

As my project has progressed, I've accumulated a "few" items no longer needed.    I have a 30 amp DC breaker laying about.  If you find a need for it let me know.  It needs a home and is basically brand new.

Thanks considerations ...I'll definitely keep this in mind if/when the time comes.

[rniles]

I just don't want you to think that 12V 'uses less power'.

Wasn't really thinking that ..was more thinking of the energy loss through conversion from DC to AC and needing larger solar/battery requirements

Like ...can I rethink the way I do things to get by with less electricity.

This is something I'm hoping to think about ..more or less, I was thinking of wiring a small cabin the way you would wire an RV ...just for some low voltage lights and small item electrical uses (like recharging small electronic devices).

My hope is that this would keep my solar/battery requirements down.

In my camper, I have replaced all bulb with led bulbs ...by single 12v battery lasts forever now ...the only thing that sucks juice is the fan from the propane heater. It's an older fan (1987), so the blower uses more electricity.