12 volt house wiring

Re: 12 volt house wiring

Sorry Jay,

But I am probably going to ask you more questions than giving of answers...

First, here is a calculator that helps you to calculate wire size vs current. As always, if you are in the US, you also need to run these numbers up against the NEC (National Electric Code) and your local building codes. Again, also use the NEC codes to figure out which type of wires (THNN) and such for your application (based on temperature, humidity, current, conduit vs exposed vs exposed to sun).

http://www.wind-sun.com/smf/index.php?topic=1477.0

Second, there is no one answer for wiring a 12 volt home--there are no standard outlets and no good wiring practices that everyone follows.

So, just some things to think about. In general, solar electric power is expensive to generate and store so you will want to ensure that any electricity you use is done with conservation in mind.

For example, using CFL rather than incandescent lights, low power electronics (laptops vs desktop computers), and few twenty four hour loads (night lights, fans, pumps, computers, printers, radios, tv's, left on when nobody is around (I know--I am probably telling you things you know).

Next, because solar electric power is so expensive (and relatively inefficient)--it is generally not a good idea to use if for any sort of heating applications (for example, solar hot water by solar electric would need 4-5x the roof space and solar PV panels at $5.00 per watt vs solar hot water heaters at closer to $0.50 per watt). Using solar for electric ovens would be a last choice (you are in the middle of no-where and you can afford the solar panels/batteries). Using a microwave is probably OK for a quick heating of previously cooked food.

Next, distribution of power to your various appliances. The wiring calculator above will help you choose the wiring size based on the length of run and current. Generally in solar, folks aim for 3% loss (maybe as high as 5%) of solar power due to wire heating.

Now, the big issue... The reason folks use 120 VAC and 240 VAC to transmit power is because it needs so much less current (and less copper wire) to transmit power efficiently. For example, 12 volts vs 120 volts-- Power is equal to P=V^2 / R... If you use the same piece of wire for both, the amount of energy loss in the wire is (120^2 / 12^2=) 100 times more power loss in the same piece of wire when switching from 120 volts to 12 volts... You would need 100x more copper area to transmit 12 volt power vs 120 volt power.

Another way to think about it--normal home wiring can have a 5 volt drop from the circuit panel to the appliance... But nobody worries about 120-5=115 volts. But take 12 volts - 5 volts = 7 volts and nobody would expect the appliance to even function.

This is all a round about way of saying that when you design a 12 volt power system that you really need to understand where your loads are, where your batteries are, and where the panels will be.

For example, if you do want a 1,000 watt 12 vdc microwave oven--that is close to a 100 amp load. So--it would make sense to install your battery bank next to your kitchen area. And since there are no "standard" 12 VDC connectors--you are going to have to find/select something that works for you. You could choose to use "cigarette lighter" type outlets for loads less than 10 amps--but I find them to be clunky and not very reliable--plus if you have kids around--they are very easy for kids to stick metal objects into cause shorts/fires.

I am sure you have good reasons for wanting 12 vdc power distribution--but I would suggest that you also look into 120 VAC inverters (by the way, pure sine wave inverters are much better--and more expensive--than Modified Sine Wave inverters)--their losses are not that much higher and you would have the ability to use a good range of very energy efficient appliances. People with larger battery banks typically use 24 to 48 VDC because it does reduce the current (1/2 to 1/4 the current and 1/4 to 1/8 heating losses due to wire resistance) and use a 24 or 48 VDC inverter for standard power.

About the only thing to run on 12 VDC are small florescent lamps, automotive type radios and such...

I am at a bit of a loss--it sounds like you are building a home that plans on using a goodly amount of power--but you are trying to use 12 vdc as your power--which indicates more of a cabin or other possible issues (cost of inverters?).

It would be helpful to know your power requirements (watt*hours, kWatt*Hours, or Amp*Hours at what voltage). Just to give you an idea--A hardcore off-grid home probably uses a 100 kWhrs per month... If I try, I can get my old suburban home (near SF California) down below 200 kWhrs per month. Somebody in Arizona with AC--may be 1-2,500 kWhrs per month during summer. And that all still relies on using some sort of fuel/solar hot water/solar heating for heating/hot water/cooking.

Is any of the above helpful for you?

-Bill