# How to run 12v lights through 110v wired house?

1. Jun 13, 2011

### ulhbuilder

Hello!
I have waaaay to much time on my hands due to an illness and bought a bunch of items on ebay to play with and keep my time occupied.

I bought a bunch of 12v smd led car bulbs and want to power them through my existing 110v wiring, I can handle fitting the bulbs into the existing fixtures and my house has a backup system of 12v batteries, inverters, transfer panels, etc. I figure I can have an electrician split out the rooms I want to use them in and run those rooms directly from the DC battery bank.

The questions I have are:
Will the 12/2 and 12/3 romex work for 12v? I know DC loses alot in longer runs and they could be 50-100 ft runs when you figure distance and height...

If the circuits also include the wall plugs, that doesn't work to well as 12V! Any suggestions?

Are there any tiny devices/inverters that can be placed into a light bulb socket to convert from 110v to 12v? Obviously I can buy 12v bulbs made for 110v outlets, but I want to "adapt" the 12v bulbs to work from the 110v.

Best would be to change the circuits to 12v, but I am open to suggestions on the above!

Thanks for any help offered, If this is not feasible I have an off grid cabin that runs on 12v so I can use them there, problem is in my current situation, I can't get there!

I also bought a bunch of TEC's (Peltier) to play with, any suggestions? LOL

Thanks again

2. Jun 14, 2011

### MATLABdude

Welcome to PhysicsForums!

Unfortunately, most LED bulbs still aren't terribly bright, although you can buy CFLs and full out fluorescent tubes that run at 12V--you find a lot of these in campers and RVs, as well as off-grid houses / cabins / caves (just kidding on the last one, but I'd bet that there're probably some fall-out shelters that do just that...)

This probably applies even more so to lights meant for car interiors.

I'm not familiar with any screw-in style 120VAC to 12VDC adapters that then allow you to screw in 12V lights. And will automatically bypass when 12VDC is present instead of 120VAC. That's probably the big problem: having both 120VAC and 12VDC and expecting the lights to automatically switchover (or be able to use both).

That said, how about having a parallel "emergency lighting" system (or even day lighting for when electricity is most expensive and lighting demands are lowest)? You could still have the 'fun' of running romex (and cutting out knockouts for electrical boxes, running wires through walls, patching drywall, etc.)

Losses (and you have losses with 50/60 Hz AC, which at that frequency, isn't all that different from straight DC) are a function of cable diameter. Wikipedia has a chart of the resistances of differing gauges (milliohm per ft or per m):
http://en.wikipedia.org/wiki/American_wire_gauge#Table_of_AWG_wire_sizes

So assuming a hundred foot run of 14 gauge (and good, low-resistance connections, and no high-resistance kinks), the resistance from one leg of the wiring would be 1/4 ohm. For the sake of calculations, I'll assume you have some 3W LEDs. Using the formula $P=V^{2} / R$, you can model the LED as a resistance of (roughly) 48 ohms: your wire resistance is pretty negligible in comparison.

In comparison, most house-hold low voltage (12VAC for your central vacuum, HVAC, door bell, etc.) uses 18 gauge wire, and is run throughout your house.

EDIT: Most of these low-voltage systems are also pretty low current: usually less than an amp.

Last edited: Jun 14, 2011
3. Jun 19, 2011

### ulhbuilder

I thought before I asked this was an uphill battle but thought I would ask the experts and experienced!

I never considered running new wire, boxes, etc., as you stated it's too much work for too little return.

And I verified that my rooms are wired to circuits, not just lights vs. outlets... crap, woulda been so easy, but unless you know what you want at time of installation... lol

So I am left with no options besides changing fixtures with 12v convertors or store bought led 110 light bulbs, which I won't do again due to cost... sigh... lol

So,
Since I have bought a bunch of led lights off fleabay such as "Pure White G4 24 SMD LED Marine Lamp Light Car Bulb 12V", "2 X Car H7 68 SMD LED White Headlight Bulbs" and the sockets, can I hook these up in outlets, triggered on/off with standard 110 light switches?

Do I need any resistors, diodes, power supplies, etc.?

I am a newbie and am not making any attempts to sound more knowledgable than I am, (so please speak in simple terms with words in a few syllables!)

They will run off a 12v marine battery bank, I also have an inverter to 110, but don't want to run them through that as it wastes alot to go 12v to 110, and then to 12v.

I do know a very little about 12v systems as I have one at my home and at my cabin, which is still snowed in as of today, shade side of 14,000 ft peaks in a narrow box canyon, hopefully in 3-4 weeks!

Thanks again,

4. Jun 19, 2011

### ulhbuilder

And thanks about the TEC's, I was thinking of some kind of power generator to help recharge batteries, as I just told you about my cabin, "cooling" is not really needed! lol I know solar panels are more efficient but thought there is some way to generate some power out of the ones I have!

Thanks again

5. Jun 20, 2011

### Dmytry

I would run secondary wiring for 12v... figure out how much current you are going to use, and what is the distance, then you can calculate the minimal wire cross section so that the voltage drop is <0.5v.
For the voltage drop V under current I (in volts), the resistance is
r=V/I
If the wire length is L, and cross section area A, and the wire is made of copper with resistivity of p=1.7E-8 ohm*meter, the full resistance of wire is
r=p*L/A
and solving for A:
A=p*L/r
[That is all in meters and square meters]
Other circuitry: well you may want charger for your battery, and a power supply to power the LEDs without batteries. The simplest way to do that would be to use relay switch between battery and the LEDs, diode between power supply and the LEDs, and connect relay coil to the power supply so that when power supply is on, relay is energized and disconnects the switch.
Or you can use a manual switch that connects either battery or power supply. But not both because the power supply is not a dedicated charger.

6. Jun 20, 2011

### Skaperen

It's not the DC that loses alot ... it's the lower voltage. It would be the same for AC at the same voltage. When the voltage is lower, the current has to be higher to get the same power across. The voltage drop across a resistance (e.g. the wire, slight as it may be) is in proportion to the square of the current.

At 120 volts a 60 watt resistive load would have 240 ohms. Assume your wiring has 1 ohm, and you have a total resistance of 241 ohms. The current will be 0.49792531 amps. The load will get 119.502074 volts. That's not really bad at all.

At 12 volts a 60 watt resistive load would have 2.4 ohms. Assume your wiring has 1 ohm, and you have a total resistance of 3.4 ohms. The current will be 3.52941176 amps. The load will get 8.47058823 volts. That would be terrible.

Even if you upped the size of the wire by 10 times (for 10 times the current), it gets better ... 2.5 ohms, 4.8 amps, 11.52 volts to the load ... but still not so good. You would have to up the wire size 100 times to have the effective performance of 120 volts with just 12 volts.

That's the I2R rule for you.

An alternative to upping the wire size by 100 times is cutting the circuit length by 100 times. That means you just can't go so far. That leaves cutting your loads. That seems to be your intent with the interest in LEDs.

But LEDs do have one thing going for them. They are current driven devices, instead of voltage driven devices, if you are dealing with the individual LED components. A solid state current limiter could be made to work over a wide voltage range and drive the LED with a configured current. This would be easier for DC than AC. Then the voltage wouldn't affect the light if it stayed in the design range. You still have a substantial fraction of the power being wasted in the wiring, though. Also, adding like (same current, same age) LEDs in series lets you operate on an even higher voltage. How many LEDs does a given lighting situation need?

Lower voltage, higher current, does work better for certain incandescent lights, and so that is often used in many situations. But that usually involves AC with the transformer to drop it down to 12 volts being placed as close to the load as possible to minimize the loss in the low voltage part.

I don't want to discourage you from low voltage, but you need to have the appropriate expectations with it. And that means using a lot less power. Of course, you can do that with higher voltage, too. In many cases, using a higher voltage can save more electricity than the lower voltage. You just need to do the maths for each case to figure out what is best.