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I have a couple deep cycle batteries that trickle charge from a solar panel, for emergency use. Right now the only way I get useful power from them is to use a 12VDC -> 120VAC inverter, e.g. the type that can hook up to a car battery.

I'd like to semi-permanently install the batteries in the basement and set up a couple 12-volt circuits for daily off-grid use. E.g. maybe installing some 12 volt LED lamps, or removing the wall warts on some of my electronics.

Is there any standard for wiring residential 12 volt circuits? Is it even legal? I can't find anything on wire gauge recommendations, outlet and plug shapes, etc. I've seen some generators, e.g. this one, have a 12VDC receptacle with two angled blades, but I can't find anything like that at the usual retailers and I'm not sure if that's a standard receptacle shape or not.

Does anyone have any experience or guidance on where to get started for 12-volt DC residential circuits?

(This is more a side project to help reduce my electric usage and take advantage of the solar panel I already have than a serious source of emergency power. I'm not interested in a generator.)

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There are POE devices that push 25 Watts at 50+ Volts with Cat. 5 cable, so I doubt it's illegal. Running DC does interest me. Since most of our electronics run on DC, getting rid of the wall warts and inverters could save some serious power. –  Edwin Jan 24 '13 at 19:53
    
This is truly a question ahead of it time! And a very good question. –  ppumkin Jan 25 '13 at 10:41
    
DC through ethernet is very low power. At 12VDC handling more realistic power requirements, Ampicity is king. Figure the amount of copper necessary (AWG/Circular Mils) to carry the current and oversize it quite a bit to give you low resistance so you're not heating copper. –  Fiasco Labs Dec 18 '13 at 4:23

6 Answers 6

What you're describing is basically what you'll find in an RV. However, it's not likely to be as useful as you might wish.

The "12V" in the name of a lead-acid battery is nominal. The actual voltage might vary between 10V and 15V. Most electronics can't tolerate that kind of variation. Audio components can suffer from ground loops. Even though many devices in our homes run on DC today, AC is still a good choice for power distribution.

In an RV, most of the 12V goes to lighting and fan motors, which simply get brighter/darker or faster/slower, within that range of voltage. The variation may not be ideal, but it still works.

Since your voltage is 1/10th that of conventional house wiring, your current will be 10x as much. That means the conductors will need to be much thicker - more copper, and more expensive.

Once you have both 120VAC and 12VCD in your house, it's annoying to switch back and forth between 120VAC and 12VDC. Whatever you set up, consider a 12V converter for when you have shore/grid power, so your 12VDC devices can be used all the time.

A widely-used standard for DC outlets is the "cigarette lighter" plug. If you install them in your walls, I recommend labeling each one with the max current.

Be sure to put fuses on each circuit, at your distribution block. And a master fuse as close to the battery as possible.

When a lead-acid battery charges, it releases hydrogen gas. If you're going to put that system indoors, you need ventilation to clear the gas, and prevent an explosion. Alternately, you can use sealed lead-acid batteries ("SLA"), but they're more expensive.

Be very careful to keep your two wiring systems separate. Don't ever run them in the same conduit or junction box. Labels on the low-voltage will help avoid mixups in the future.

The cheapest, simplest, most reliable way to provide emergency backup power to a home is a generator with a transfer switch. Even RVs with batteries and solar panels will have a generator.

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I was under the impression that many electronics were not that sensitive to input voltages, because they did their own regulation internally. See also: electronics.stackexchange.com/questions/13910 . I don't have personal knowledge one way or the other. You make an excellent point about the current (and therefore resistance). I'm mostly planning on low-power devices (probably under 100 watts total) so I'm not too worried about it, but it's definitely a valid concern. –  Henry Jackson Jan 24 '13 at 21:25
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The nominal voltage argument seems a little far fetched, as you would likely put a regulator on the power source. –  Edwin Jan 24 '13 at 21:54
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Yes, a voltage regulator will address the overvoltage concern - although it won't help with undervoltage. –  Jay Bazuzi Jan 24 '13 at 23:36
    
Okay, use a buck-boost converter or whatever it takes to get voltage to within your equipment's tolerances. The voltage variance of 12v marine batteries does not determine the usefulness of wiring dc in the home. –  Edwin Jan 25 '13 at 1:25
    
Why would I want to avoid running 100v/220v and 12v in the same conduit?(That is why there is a colour codding standard invented) If it is data sensitive like, audio, network they suffer from electromagnetic interference. –  ppumkin Jan 30 '13 at 9:19

Allot of DC devices at home run at various voltages from 22Volts down to 5Volts. So just running a 12Volt wire is impractical because you would have to regulate the voltage up or down - Just like with 110/220Volts now. That is why you cannot find a receptacle for 12 Volts.

But

You can use a 5Volt receptacle :)

enter image description here

OK- But what has that got to do with 12Volts?

Everything! You can run an Ethernet cable to transport your 12Volts throughout out the house which typically uses 24AWG. To be safe lets use 26AWG specification for current calculations.

AWG Lookups

26AWG by standard should handle 0.3Amperes. At 12 Volts that is no more than 3.6Watts per pair. If you decide to use an Ethernet cable you will have 4 pairs you could use giving you a total of 14.4Watts per etrher cable. That is enough for basic applications that need 12volt! But if you find thicker ether cable like 22awg you can raise this to 10watt per pair.

enter image description here Then at each receptacle you can buy a 12v - 5v DC-DC converter (£1.50) and your USB plugs will be standardised and power via your 12Volt supply.

You may ask why not just run 5v directly from the batteries? The same reason you have 110v/220v in your socket. Over distance you loose power and the lower the voltage the thicker the cable you need to transport more power. So step down regulators guarantee near constant 5v at the source while you can supply several places with the same cable guaranteeing more power on smaller core cables.

But LED lights do not run at 5v! - Only at 12v!

No they do not run at 5v. So the only way to properly run devices that require 12volts in your home is NOT using a receptacle but wired in for good. Exactly the same way your 110/220v lights are wired in(all this is just downscaling) . You should then buy the properly colour coded wires and a larger core copper cable like

Generally for consumer wiring the standard should be,

  • 220v brown/blue (yellow+green earth)
  • 110v white/black ?
  • 12Volt red/black
  • 5Volt yellow/black

Many people will comment that above statement is wrong. Well, consumer wiring is the wires we get with TV's, kettles etc and that is what many countries follow. But standards vary from country to country and really old buildings pre 1950 in most countries do not follow any standard unless they were specifically rewired.

You would then just run insulted cable at a suitable gauge directly to your static LED lighting, even in your existing conduits! Just make sure you everything colour codded so the next fella can SEE the difference. Even labelling 12v 110/220 is good practice but not necessary.

But I really want 12v at my reciptacle This is an updated part thanks to another answer

You can buy these 12volt standardised plugs and connect them up directly to 12volt. It will require a bit of DIY but you can buy blanking plates, drill the correct size hole and fit these in. But i'll tell you what. They sure are ugly and I personally would avoid them.

enter image description here

Safety

It is good idea to make a Distribution Board just like the ones used for high voltage and use circuit breakers. With cat Cable (26AWG) you want 0.3A to be safe - the rest is up to what cable you decide to run.

But even a DIY one like this will work very well and ensure safety against short circuit, over current with the ease of monitoring consumption. You can even run 110/220 into this DB and incorporate a fail over, in case your batteries run down, so you could then have a simple sensor, relay and a 12volt 2~5a power converter.

enter image description here enter image description here

Just one thing to remember - If you are using acid batteries do not leave them on concrete! Always place them on top of timber or elevated. A strange thing happens that acid batteries discharge when placed on concrete/ground and should not be close to earth. Not sure about dry cells.

This also looks nice and easy and uses car type fuses.

enter image description here

But using proper circuit breakers rated for Direct Current should be used as they offer reliable protection for various things, but cost allot more.

Not following standards

You might think to yourself, that you can just run 12volts directly into a USB receptacle. Sure, it will work. But what if your sons/daughter/wife friend comes round and plugs in their iPhone without asking. Ooops, its getting hot in there.

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MK battery corp says this: Many people have the impression that when batteries sit on concrete, energy “leaks out” or they are ruined. The short answer is that letting modern batteries sit on concrete does not harm or discharge them in any way. In fact, concrete is generally an excellent surface on which to place a battery. The electrolyte in a battery sitting on an extremely cold floor with very hot air around it could stratify, causing damage from sulfation; whereas concrete provides good thermal mass to buffer any temporarily extreme temperatures in the battery compartment. –  Philip Ngai Jan 30 '13 at 17:19
    
the keyword being modern batteries. What is the definition of modern battery? –  ppumkin Jan 30 '13 at 20:28
    
Anything you can buy today! –  Philip Ngai Jan 30 '13 at 21:00
    
All batteries that do use plastic instead of tar for the package. –  Gunnish Jan 31 '13 at 11:17
    
Wierd- When I used to work a while for a auto body repair shop - I experienced this discharge with the so called modern batteries you can buy today? A new fully charged battery that stood overnight on concrete was D-E-D the next morning? So I still stick with my opinion. Dry cells wont do this I know –  ppumkin Jan 31 '13 at 11:24

In the US in most jurisdictions, house wiring of any sort still falls under the National Electric Code. It has a brief section on low voltage power distribution, AC or DC less than 50V nominal. Wiring must be at least 12 Ga copper or equivalent, with overcurrent protection. The system must be grounded. All work in a neat workmanlike manner. And a few other things you may wish check up on but probably not applicable to what you describe. It appears pretty open ended as to what methods you should use.

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12V is fairly common in off grid (not connected to utility power) systems. If you do a search for "off grid 12v", you will find lots of references around setting up such a system.

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The main problem is that the lost power is \$ P = R I^2 \$. If you want to be able to use high powered equipment you will need really large cables if you want to be able to run the cables for several meters. Those cables are expensive and might not be easy to even get a hold of depending on the required resistance/length.

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His list of planned loads is "12 volt LED lamps, or removing the wall warts on some of my electronics". These are not high powered loads unless he is growing marijuana or has some other need for extremely high levels of light. –  Philip Ngai Jan 30 '13 at 21:02
    
If you would use LEDs in your whole souse it would still be > 100 W and long cables. If you want to change a laptop (say 19V 4 A) that would add up 15A (at 12V given that the charger converts to 19V at an endpoint), which requires quite think cables if you want them trough your house without any significant voltage drop. –  Gunnish Jan 30 '13 at 22:43
    
I don't consider a laptop power supply a wall wart. Even for a laptop power supply, 19V@4A does not become 12V@15A. And if you are trying to save energy, you're not going to be burning > 100 W of power on your lights. –  Philip Ngai Jan 31 '13 at 1:44
    
LEDS for the whole house will need some power even at optimum efficiency. I got the 15A from the sum of 100W + 19*4. What do you consider a wall wart? for me its anything that converts the voltage that is outside the machine using the power. –  Gunnish Jan 31 '13 at 11:15
    
A wall wart is something that looks like it touches the wall, a box with the electrical contacts built into it. I have never seen one rated for more than 3A@12V and I have never seen a laptop powered by a wall wart. There is always a cord from the laptop power supply to the receptacle. –  Philip Ngai Jan 31 '13 at 19:37

I have been planning to install receptacles in my home for 12 volt dc. I decided to use a 15 or 20 amp outlet that is designed for 277 volt, the configuration is nema 7-20R or 7-15R. It looks like a regular 120 volt receptacle but the hot and neutral slots are set at a 45 degree angle so I won't accident plug into 120-240 volt. use the silver terminal for white-, and gold terminal for black+. I plan to use a 10 gauge cable to each receptacle and a 20 amp automotive fuse at the source to protect the whole cable. each outlet will have its own line back to the battery.

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