Planning on wiring our new home for DC

mattbatson · January 2012

I'm wondering if anyone knows of any picture tutorials or threads where someone has documented the in's and out's of setting up an off grid cabin/home with DC appliances, DC wiring (receptacles, etc), and all the solar components, etc?
We are in the planning stages of outfitting our soon to be erected quonset hut home to be run soley on solar power. I plan to use DC lamps, DC fridge, DC TV, etc that have been taken second hand from RV's. I'm hoping to avoid the losses from inverting my expensive solar power

I may have a small inverter for things like laptop and such...but the big items, like the well pump and fridge and so on will be all DC.

I'm still determining my power needs, and from there will plan my system (with the help of advice from you guys on here

)...but now I'm looking for info on DC wiring in a home.

thx

BB. · January 2012

Re: Planning on wiring our new home for DC

Look for boating websites and books... Pretty much all 12 or 24 volt DC, batteries, and chargers/alternators. About the only thing missing are solar panels--and that is not too hard to address here.

Electricity for Boaters - BoatSafe.com

And from the Working FAQ Thread:

Regarding Solar Books:

What's a Good Beginners Book?

From the above thread:

FL SUN wrote: »

This is a link to a PV textbook I find very informative. It was a requirement for the FL contractor's exam I took last month. It even has a very good interactive CD with a bunch of informative extras. A quick search on-line shows this book goes for about $75.00 USD everywhere.

Don't forget nothing compares to OJT when it comes to installation. It's always best to apply in the field what you've learned from a good textbook first.
http://www.powells.com/cgi-bin/biblio?show=HARDCVR W/COMP MEDIA:NEW:9780826912879:75.00

TnAndy wrote: »

Rick,

Here a good "textbook" on solar......it actually IS a textbook for some solar courses, and is set up that way with practice questions at the end of each chapter, but it's also a good self guide as to the basics of components and how they mesh together to make a system. Worth the money, IMHO.

Solar Book Here

Some information on NEC and how it applies to solar power:

PV and the 2005 NEC -- Reference Document

And, of course, the NEC Code Book (current edition or version that is used by your locality).

-Bill

PS: Our host also has a Book they recommend:

Book - The New Solar Electric Home

PPS: From another poster:

KeithWHare wrote: »

I recommend "Photovoltaics: Design and Installation Manual" from Solar Energy International. This does a pretty decent job of explaining everything except for battery banks.

Keith

PPPS: From another website, I saw thisBoatowner's Illustrated Electrical Handbook recommended.

PPPPS:

rollandelliott wrote: »

according to wikipedia once it is adopted into law by a particular govt agency it becomes public knowledge

http://bulk.resource.org/codes.gov/ga_electric.pdf

is one link

there are many more specific to certain states/cities here

http://bulk.resource.org/codes.gov/

mattbatson · January 2012

Re: Planning on wiring our new home for DC

http://asheville.craigslist.org/rvs/2797699465.html

thinking of just using this

and then just wiring it like a regular RV?

westbranch · January 2012

Re: Planning on wiring our new home for DC

you will want to get very familiar with this calculator
http://forum.solar-electric.com/showthread.php?29-voltage-drop-calculator

HTH

BB. · January 2012

Re: Planning on wiring our new home for DC

Also add... 12 volts is real seductive as a battery bank voltage. Lots of 12 volt devices and appliances--Just plug it into your solar powered battery bank and away you go.

The bad of 12 volts is actually quite a bit. A 100 amp of current (large wiring) is only about 1,200 watts... If your devices pull more than 100 amps of 12 VDC (well pump starting), you should be looking at higher voltage battery bank.

And sending 12 volts any distance is difficult/expensive. For example, your battery voltage may drop to 11.5 volts during normal operation while your typical DC device/inverter probably will cutoff by 10.5 volts--Giving you about 1 volt of wiring drop.

A 120 VAC circuit, you can drop 10 volts and still have everything run. Plus, the voltage drop is about 1/10th that of 12 volts because of the 1/10th current of running at 120 VAC (power=voltage*current; 10x the voltage drop, 1/10th the current).

I would put your "heavy current" 12 volt loads very close to your battery bank. And any loads that are across the room or farther away--I would really suggest looking at 120 VAC inverters. For a small cabin, if you can keep your AC loads low--the MorningStar 300 Watt TSW 12 volt inverter is really nice (it does include a "search mode" and a remote 12 volt inhibit switch).

-Bill

waynefromnscanada · January 2012

Re: Planning on wiring our new home for DC

From my experience, I'd have to say that in the end, all things considered, unless your loads are quite small and your cables short, the losses you fear from using inverters will be small compared to the losses involved in 12 volt wiring. That, and considering the extra costs of all but "toy" 12 volt appliances, you'd be further ahead with regular household high efficiency appliances, and put the saved money on extra solar panels/batteries etc.

mattbatson · January 2012

Re: Planning on wiring our new home for DC

BB. wrote: »

Look for boating websites and books... Pretty much all 12 or 24 volt DC, batteries, and chargers/alternators. About the only thing missing are solar panels--and that is not too hard to address here.

Electricity for Boaters - BoatSafe.com

And from the Working FAQ Thread:

thanks for the suggestions!
I'm going to order some of those books

mattbatson · January 2012

Re: Planning on wiring our new home for DC

waynefromnscanada wrote: »

From my experience, I'd have to say that in the end, all things considered, unless your loads are quite small and your cables short, the losses you fear from using inverters will be small compared to the losses involved in 12 volt wiring. That, and considering the extra costs of all but "toy" 12 volt appliances, you'd be further ahead with regular household high efficiency appliances, and put the saved money on extra solar panels/batteries etc.

hey,
the living space will be 20x20...half of the area of the quonset hut...and i figured that there wouldnt be any long runs of DC wiring?
Also, I can find used RV fridges for under 300 bucks, and new 12V DC TV's for 300 or so...etc...

But I will make sure I go and price everything for comparison purposes before committing to this.

mattbatson · January 2012

Re: Planning on wiring our new home for DC

BB. wrote: »

Also add... 12 volts is real seductive as a battery bank voltage. Lots of 12 volt devices and appliances--Just plug it into your solar powered battery bank and away you go.

The bad of 12 volts is actually quite a bit. A 100 amp of current (large wiring) is only about 1,200 watts... If your devices pull more than 100 amps of 12 VDC (well pump starting), you should be looking at higher voltage battery bank.

And sending 12 volts any distance is difficult/expensive. For example, your battery voltage may drop to 11.5 volts during normal operation while your typical DC device/inverter probably will cutoff by 10.5 volts--Giving you about 1 volt of wiring drop.

A 120 VAC circuit, you can drop 10 volts and still have everything run. Plus, the voltage drop is about 1/10th that of 12 volts because of the 1/10th current of running at 120 VAC (power=voltage*current; 10x the voltage drop, 1/10th the current).

I would put your "heavy current" 12 volt loads very close to your battery bank. And any loads that are across the room or farther away--I would really suggest looking at 120 VAC inverters. For a small cabin, if you can keep your AC loads low--the MorningStar 300 Watt TSW 12 volt inverter is really nice (it does include a "search mode" and a remote 12 volt inhibit switch).

-Bill

Ok, sounds like my system will be too large for 12V.
Alright, it sounds like my 12v DC plan for the house isnt going to work...especially since we will be running our well pump off of it.
thx

mattbatson · January 2012

Re: Planning on wiring our new home for DC

westbranch wrote: »

you will want to get very familiar with this calculator
http://forum.solar-electric.com/showthread.php?29-voltage-drop-calculator

HTH

hey, I had to rebuild my laptop after getting hit with a virus and dont have microsoft office installed right now.

I imagine the calculator you are referencing shows some very large gauge wire needed for 12v systems?

Sounds like it was a bad idea, and I'll be going back to just running a more typical system with an inverter...24V or some such also...
thx

BB. · January 2012

Re: Planning on wiring our new home for DC

It really does come down to power management and what a "12 vdc" appliance really expects. A Kill-a-Watt meter (or similar) is great for measuring AC appliances. And for smaller DC appliances, one of these small AH/WH meters would be very handy.

For example, say you have a 120 watt load drawing 10 amps at 12 VDC. A 14 awg cable (standard home stuff) 25 feet (from battery and up wall 8'; 10' over; and 7' down) will give you a 1.5 volt drop (generic voltage drop calculator). That is too much drop for most 12 volt devices (around 10 volts with loaded/discharged battery bank).

The same load at 120 VAC and 120 watts, 25', and 1 amp would give you 0.15 volt drop.

On the other side, to properly recharge a flooded cell battery bank, your bank voltage will be as high as 15-15.5 volts or so... Many "12 volt" appliances are designed for around 14.2 volt maximum (car alternator set-point of 13.8-14.2 volts typical). There have been some "12 volt adapters" that have failed when subjected to >14.5 volts. And many inverters will cutoff at >15.0 volts (that is a pain :roll:).

In any case, for many smaller devices, it ends up being fairly easy to just stick with 120 VAC power bricks than to go through the issues of 10.5 to 15.0+ volts that a DC battery bank and wiring is nominally subjected too.

If you have other limitations (such as not much room for a solar array and/or theft problems)--you might be a bit more efficient with 12 volt only appliances. But, the newer AC appliances are (in general) much more efficient than they used to be even a few years ago. You pretty much need to measure and compare the 12/120 VAC power usage between devices and see how each performs. It is difficult these days to generalize what is the most efficient.

-Bill

mattbatson · January 2012

Re: Planning on wiring our new home for DC

BB. wrote: »

It really does come down to power management and what a "12 vdc" appliance really expects. A Kill-a-Watt meter (or similar) is great for measuring AC appliances. And for smaller DC appliances, one of these small AH/WH meters would be very handy.

For example, say you have a 120 watt load drawing 10 amps at 12 VDC. A 14 awg cable (standard home stuff) 25 feet (from battery and up wall 8'; 10' over; and 7' down) will give you a 1.5 volt drop (generic voltage drop calculator). That is too much drop for most 12 volt devices (around 10 volts with loaded/discharged battery bank).

The same load at 120 VAC and 120 watts, 25', and 1 amp would give you 0.15 volt drop.

On the other side, to properly recharge a flooded cell battery bank, your bank voltage will be as high as 15-15.5 volts or so... Many "12 volt" appliances are designed for around 14.2 volt maximum (car alternator set-point of 13.8-14.2 volts typical). There have been some "12 volt adapters" that have failed when subjected to >14.5 volts. And many inverters will cutoff at >15.0 volts (that is a pain :roll:).

In any case, for many smaller devices, it ends up being fairly easy to just stick with 120 VAC power bricks than to go through the issues of 10.5 to 15.0+ volts that a DC battery bank and wiring is nominally subjected too.

If you have other limitations (such as not much room for a solar array and/or theft problems)--you might be a bit more efficient with 12 volt only appliances. But, the newer AC appliances are (in general) much more efficient than they used to be even a few years ago. You pretty much need to measure and compare the 12/120 VAC power usage between devices and see how each performs. It is difficult these days to generalize what is the most efficient.

-Bill

very informative as always sir :cool:

thank you

I'm ordering the hosts solar power book, a used NEC book off amazon, and then a used photovoltaic systems by dunlop off of amazon also....should get me started for now.
thx

samuel · January 2012

Re: Planning on wiring our new home for DC

There is also always the option of using both 12V DC and 120V AC.

At my family's cabin we started out with an inverter. Then I started to experiment with 12V DC and have since added three 12V LED exterior lights, two on switches, one on a motion sensor. Recently I added 1200 lumen of 12V LED light to the kitchen (on a timer/switch) and am in the process of adding four 12V automotive-outlets in the stairway for a "charging center" since we turn the inverter off when we're out of the cabin during the day and set our inverter to search mode at night - this makes it economical to charge our AA and AAA batteries, cell phones, iPods…etc for an extended period of time with the inverter off. Plus I hate the thought of going: battery to DC-to-AC inverter to AC-to-DC power adapter to device.

I'd never switch the entire cabin over to 12V lighting, but it can nice to have a few 12V lights in a few convenient areas. Food for thought.

mattbatson · January 2012

Re: Planning on wiring our new home for DC

samuel wrote: »

There is also always the option of using both 12V DC and 120V AC.

At my family's cabin we started out with an inverter. Then I started to experiment with 12V DC and have since added three 12V LED exterior lights, two on switches, one on a motion sensor. Recently I added 1200 lumen of 12V LED light to the kitchen (on a timer/switch) and am in the process of adding four 12V automotive-outlets in the stairway for a "charging center" since we turn the inverter off when we're out of the cabin during the day and set our inverter to search mode at night - this makes it economical to charge our AA and AAA batteries, cell phones, iPods…etc for an extended period of time with the inverter off. Plus I hate the thought of going: battery to DC-to-AC inverter to AC-to-DC power adapter to device.

I'd never switch the entire cabin over to 12V lighting, but it can nice to have a few 12V lights in a few convenient areas. Food for thought.

that is an idea, thx
I may do that instead...I see myself changing things as I go, ha ha.
cant wait to get started after I do a little light reading.

I've read your blog with your cabin in the UP...great read...very imformative

oil pan 4 · January 2012

Re: Planning on wiring our new home for DC

Instead of having a centeralized battery bank with heavy conductors going every where why not decenteralize the batteries. Have 2 or 3 banks that are right next to loads?
Of course everything would be interconnected.

techntrek · January 2012

Re: Planning on wiring our new home for DC

You will still be dealing with long runs with low voltage - such as from the panels to the various battery banks.

mike95490 · January 2012

Re: Planning on wiring our new home for DC

oil pan 4 wrote: »

Instead of having a centeralized battery bank with heavy conductors going every where why not decenteralize the batteries. Have 2 or 3 banks that are right next to loads?
Of course everything would be interconnected.

I think that's recipe for a nightmare. battery boxes to build & vent. fuses all over the place. where to start when you smell smoke....

techntrek · January 2012

Re: Planning on wiring our new home for DC

And interconnecting several remote battery banks will cause them to be discharged/charged unequally, ending in premature failure.

JoanTheSpark · January 2012

Re: Planning on wiring our new home for DC

One more option..

You could use a 48Vdc system (batteries, etc) and then use DC-DC converters (voltage drop a quarter of that of the 12Vdc system).

I got to admit though, those 48V to 12V DC-DC converters aren't common and electronic running directly of 48Vdc isn't either, so it's expensive. And the only upside would be that you 'save' one conversion step.

PS:
IMHO most of the appliances are using DC motors and electronics anyway these days, so they convert the 120/230Vac all the time.
With more and more renewable energy being harvested and stored decentralised as DC it will only be a matter of time till we see and can buy appliances that are made for something like 200-400Vdc input. You're ahead of your time by approx 20 years :cool:

BB. · January 2012

Re: Planning on wiring our new home for DC

There is a newish standard around that is ~340 VDC that is being used for computer racks. The idea is to save one conversion step for industrial sized UPS systems.

However, I am not sure it will ever truly take off in computer rooms. And for home use--I don't think it will ever.

DC current is very good at sustaining arcs (makes for great arc welders). Switch gear and fuses have to be much larger/heftier to break DC current without arcing over and failing.

Also, DC cannot be "electrically isolated" easily. With AC you just use a transformer and you can take 120/240 VDC and have have very safe 12 VDC to power your laptop computer, etc.

And, transformers provide current limiting. A pole top transformer for home distribution circuits is designed to output a maximum of 10,000 amps into a dead short. All fuses/breakers that are used in your home panel are designed to "interrupt" 10,000 Amps maximum.

With large battery banks, it is very easy to get many times more than 10,000 amps into a dead short--and there is no "easy/cheap/reliable" way to limit that surge current.

Plus, it is very difficult to measure leakage/ground fault current in the 0.005 amp range (as used for GFI outlets to reduce the chances of getting a shock in the bathroom/kitchen/outside). Again, this is very easily done with AC and a small transformer.

So, for off grid systems with typical home/cabin power requirements--I think it will just be better to bite the bullet and get a good quality inverter to run most of your loads. And design the system to take care of the ~85% inverter efficiency by using a bit more solar panels + batteries.

-Bill

icarus · January 2012

Re: Planning on wiring our new home for DC

Without reading all the posts, my experience suggests, don't bother with the DC wiring except for Trully needed dc loads.

For the ~$300 that a Suresine 300 inverter costs, you can power nearly anything need very efficiently. Additionally 120 vac cfl bulbs are more efficient than lower voltage dc incandescents. We wired our house with dual circuits, and now, the only dc circuit we use is the one for the radio, the phone and the water pump. All those could be just as well powered with 120 AC except the pump.

Tony

Ps my guess is that it won't be any cheaper in the long run, and you will be much happier going with 120 vac for most loads.

oil pan 4 · February 2012

Re: Planning on wiring our new home for DC

The AC vs. DC thing was solved over 100 years ago.
Its just a pain to make DC work.

Planning on wiring our new home for DC

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