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Home Forums General Discussion AC or DC?

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This topic contains 11 replies, has 5 voices, and was last updated by  elnav 5 years, 9 months ago.

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  • #36947

    eric
    Participant

    Maybe this question has been asked already. We’re getting off the grid and are deciding on which direction to go with our power system. We’ll have a small cabin maybe 600 square feet or so to start, and would like the simple luxuries/amenities, LCD big screen TV, computers, fridge, microwave, washer & dryer, etc. I’m familiar with the disadvantages of DC systems, cable size, distance, etc. Will be installing a wind/solar combo.

    My question is, should we go with an AC or DC system? What are the pros and cons of each system?

    #41989

    MRGUMBY
    Participant

    I do not know why some folks seem to think DC is a disadvantage.

    In reality, you run one or two sets of monger welding cables to your power-sucker devices and the rest of it is really of very little concern…or much different in cost. Certainly a LOT cheaper than a huge, perfect sine wave inverter would be.

    IMO, 12 volt, because of the availability of low cost devices, is not a disadvantage at all.

    Taking the inverter out of the mix…along with the heat and built-in dependability issues it adds, just does not seem like a disadvantage. Quite the opposite actually.

    In a 600 SQ FT cabin, to me, a 23 inch TV would be just fine…and there are a ton out there that either have a 12 volt input or are very easy to modify inside to run on 10-14 volts.

    I bought a Emerson from WalMart for my truck and ran it on a 300 watt Cobra inverter. It used next to nothing and the inverter ran the DVD player, the TV and the dish network box without any issues.

    Using one of the computer prologic aplified speaker systems for the surround sound would be a great way to go, since they run on 12 volt from a wal wart. (At least the Cambridge systems all did)

    your question is huge and there is not enough info to answer it, but I hope I have tossed in a few tidbits of value

    #41994

    chowan
    Participant

    i like DC but a big decision maker for you may be whether the cabin is already wired? all the apliances you mentioned are big energy users so you are likely going to want a big ac system.

    if i was building the cabin i would probably do a dual system which would give me the option to turn of the ac.

    #41997

    eric
    Participant

    We’ll be building the cabin from the ground up, which is why I was asking. I’ll be doing most of the wiring myself.

    #42005

    elnav
    Member

    Simple luxuries ?? your list of appliances makes this statement an oxymoron.

    If you are at all concerned with efficiencies I suggest redoing your power calculations.

    My uncle in law has an even longer list of comfort appliances including an ensuite jacuzzi tub but then he owns a logging company so the $8000 annual fuel bill was no great deal. He simply scheduled one extra tanker truck delivery to his off grid homestead. For the average person however, it makes a huge difference whether you go AC or DC. However I can’t condense 20 years experience into a few short paragraphs to explain it to someone who is not necessarily a trained electrical engineer. Too many concepts to explain when you cannot assume the other person has the same technical background and experience. I squared R explains losses but voltage transformation in DC is far more involved than with AC.

    As Chowan says all your simple amenities are big AC power users so whats left to power with DC? The time and cost of running a second parallel wiring system simply is not worth it. Many of my designs involve triple voltage systems, sometimes four voltages. Unfortunately the nature of the products being used dictate this. However I do not recommend this approach.

    #42007

    northern person
    Participant

    I have my cabin almost completed and wish I had done all my lighting in DC. My cieling fan, water pump, and deep freeze are all DC. I had a couple areas completed that I cant go back and change wiring to DC as I have AC outlets connected to these circuits. I hear people talk about cable sizing issues but this doesnt add alot of costs unless your project is huge. Lots of 12 volt appliances avialable which last forever

    #42008

    eric
    Participant

    @elnav – Thanks for the advice, and yes, I was being slightly factitious with regard to our “simple luxuries”. I like my luxuries. I do plan on redoing my power calculation, since I’ve not really sat down and done much calculating as of yet. Hence my question… AC or DC? ;)

    It seems though from the answers above, that AC would be the way to go with such large power needs. Which simply means more money upfront, which I don’t mind, as long as I know I have the appropriate amount of power for our off-grid luxury.

    My question was more about which was more feasible, buying DC appliances where possible, or would the cost of DC be prohibitive over an AC system and regular appliances.

    Is it possible to have both? AC and DC? (and no, you are correct, I’m not an electrical engineer)

    If these seem like basic questions to some more experienced people, I will apologize for asking stupid questions.

    #42009

    elnav
    Member

    Yes it is possible to have both AC ande DC. I have been designing multi-voltage systems since 1996. Northern person made the comment that there are plenty of 12V devices and they last forever. This is one statement I will challenge on technical grounds. I once worked for a charter company and was forever replacing 12V flourescent lights. When I had a dozen or so lights in the junk bin I would try and repair them by either swapping parts or getting repair components. Failure was invariably caused by over heating. 12V products are mostly made for a market place that is very cost conscious. Very few people are willing to pay $129.95 for a light fixture But they will pay $29.95

    Volume production also means the unit cost is lower than limited volume production. This explains why 120V AC appliances are comparatively cheaper than their 12V DC equivalent.

    Lastly it takes power to do work. If you compare 10 watt devices the 12V product will use 10X as much current as the 120V product doing the same function. Current is what produces heat. This increased current results in more heat internally in the 12V device which typically leads to failure sooner rathe than later. This is equally true for lights, fans and coffee makers. I spent a lifetime working in the manufacturing industry including 4 years as a Quality Assurance Manager. Analyzing product failures wa a daily job function. In addition, as a repair technician, I saw first hand what worked and what did not. I agree new advances in LED technology can make DC lighting feasible. It is too new for us to have any statistical database on what if any effects, LED light will have on eyes over many years exposure. I do know LED lights gives me eye strain in as little as one hour.

    I also know normal electrical connectors commonly used in all appliances tend to overheat and fail sooner in 12V DC appliances. Sometimes they even start fires.

    Someone in another thread proudly declared that they were using 26Ga cat5 data cable for power wiring. Evidently this person has over looked the fact you can start a fire with 12V. Think about the cigarette lighter in your car. That outlet is fused at 8 amps. Not exactly high power but when something is high resistance it will reach hot enough temperatures to start a fire.

    I have done my share of bitching about stupid standards but I also recognize when a standard is there for a good safety reason. I have the education, training, and experience to judge when a short cut is safe and when it is not.

    What scares me is when people lacking this background blithely assumes something is safe just because it looks like it might work. Low voltage DC is a case in point. Someone runs telephone wire for one LED light. No problem but later on someone decided to add another or maybe two 12V devices to this light power circuit. How often do you think they bother to make a power calculation to ensure it is safe. For that matter how often do you see people running 12V wiring without fusing?

    You do not have to take my word for it. There are plenty of books out there that will tell you the same thing. I am guessing people are asking questions here because they haven’t the time or inclination to read all those books.

    #42016

    MRGUMBY
    Participant

    elnav, your post mentions 12 volt causing fires. While I get what you said and why you said it, I think it is very important to clarify things.

    DC done poorly can cause a fire…

    So can propane in a cabin hooked up with an aging rubber line used to connect to the bottle and running under the crawlspace. Rubber lines are for camping hookups that you can see and replace when they get cracked. Under a cabin, where you cannot see them…they can rupture and fill the entire place with gas in about five minutes. If sleeping inside, you would likely die from inhalation or when the proper mix was achieved and it hit the woodstove or pilot light…>>>BOOM!<<<

    120V AC plugged into an inverter with five 14gauge extension cords run and taped under throw rugs (I wish a had a 10-spot for pictures or video of every setup like this I have seen in an off-grid cabin) will cause fires too. More setups than not (that I have seen) using small inverters are not set up properly. Plug adapters, extension cords, no grounds, no ground fault device…etc.

    Anything done unsafe can kill you.

    DC, done correctly, is no more dangerous than AC, propane, or any other modern tool we use.

    If you take something like this:

    http://reviews.cnet.com/radios/cambridge-soundworks-i525/4505-7875_7-34134420.html?tag=lia;rcol

    The wall wart transformer that comes with this unit converts wall current to 13 volts.

    (In fact, most of the audio gear I have seen converts AC to low voltage DC inside.)

    If your batteries were within 30 feet of the cabin, using lets say 4 gauge fine strand wire for the 30 foot run to the cabin for a max load of 25amps at 12 volts. Then into a dist block for the various devices= (LED lighting, laptop, cell phone, LED TV and radio or amplified speaker system…added up correctly and wired with proper fusing for the load capacity of the wire used.)

    =you would not use this setup over an inverter?

    When I do the math, I find the wire and fuse blocks cost a lot less than a quality inverter and a bigger system to cover the losses in conversion.

    While what you say is the truth about low voltage requiring more amperage for the same work…it is kind of misleading, since the panels or the windmill stores your power in 12 volt batteries to start with.

    I just would not want people to leave this thread afraid of 12 volt DC.

    …because the truth is, if you do it right…you can have the basics for less money using DC properly.

    #42017

    MRGUMBY
    Participant

    Eric,

    Can you make an actual list of your stuff?

    For my own setup, it is not logical to try to run the audio/video system from DC.

    But it is pretty high-end stuff that is quite power hungry.

    I am going with a dual system, but I have pretty much decided to run the generator while doing laundry and watching a movie/playing my cherished vinyl.

    Lights, Computer, water pump, small TV, cell phone, router and microwave boosters are all going to be available 24/7 and will be DC.

    Thus far, I can run all of this for three days on three fully charged Wal mart special 115AH 160reserve wet cell marine batteries before dipping below 12.5 volts. The only thing I have not actually run was the water pump…but I did a load simulation with a flood light whenever we turned on the faucet to make up for it. (This seemed to use a lot more than I thought BTW…a shower will be a generator startup and top-off event in our earthship.)

    The DC fridge vs propane is a question thus far unanswered. Up front costs are more for the panels and batts to go the solar route and I have not sharpened a pencil to see if the lack of a propane bill would offset this investment yet.

    I think before you make the decision to go big AC, you need to list what your gear is. How big is the TV you will have?

    Audio gear?

    Have you looked at the fridge setups out there? If you get one “normal” sized, they are ungodly expensive…like 5 grand…but a small apartment or RV sized unit gets more affordable.

    Lights too. elnav is not joking about people adding “just a little something extra” to a circuit that carried a moderate load for ten years without incident and then that little added load made smoke and fire…knowing what the amperage requirements are is everything.

    +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

    Anyone who is going to wire a cabin “home brew” should read and fully understand this page.

    To avoid a fire…besides not running a wire where it will chafe or damage the insulation and short out…this is the most important thing to comprehend when dealing with DC.

    Car amplifiers or water pumps or computers…dc load and fusing is all the same and this page breaks it down very well.

    http://www.bcae1.com/fuses.htm

    ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

    #42018

    elnav
    Member

    Anything done poorly can be unsafe. At least one poster suggest 120V AC is more dangerous (a personal perception) than 12V DC Many people are familiar with 12V DC in their vehicle and we all know familiarity breeds complacency. People will work bare handed on energized 12V circuits. I do as well, but I also have been shocked if my skin is sweaty on a hot summer day. Many years of having to work safely has convinced me to follow procedure all the time. Despite this I have been accidentally shocked from time to time.

    Most of the people reading these forum posts are not professional electricians or installers. They do not attend monthly safety meetings and in most cases they have not read a dozen or more text books on the subject.

    My comments can in no way completely pass on all that I have learned (sometimes the hard way) in a lifetime.

    Yes you are correct that if something is done right it will be safe and last for decades. HOWEVER one second of inattention or a forgetful moment could also mean disaster. I know of one situation where a guy was wiring up a light in his boat with the power still on. His hand slipped and the two wires shorted some six feet from the battery. No fuse and the 14Ga wire over heated causing the insulation to melt and char. This released toxic smoke that overcame him before he could take three steps to reach an open door and fresh air. He died!

    I myself have done something similar during a forgetful moment.

    Fortunately I am here to tell you about it. Learn from my mistakes.

    Don’t bother to tell me how you already know it all from the books and therefore you are better and safer than me. When I worked for a power utility I saw a former lineman every morning who was missing his fingers on one hand and half of his foot. He knew better and he trusted the crew chief who said it was safe to work. No one knows exactly what went wrong but this guy got maimed despite the safety rules. It was a sobering reminder to all of us to be very careful and double check every move we made. At the utility the most scary place my partner and I worked in was the DC battery room.

    Evidently some people have not realized that most of the wind turbines and a lot of solar panel systems now operate at voltages above 12V. This tells me they are not necesarily keeping up to date and chances are they do not keep up on safety issues either. I am simply trying to point out what may not be the safest approach.

    My only reason for suggesting wiring as if for a 120V AC system is because it is the most common and there is a lot of self help books on how to do it right. If you still feel 12V is your best choice then run 12V in the wiring. If you have followed recommended practices ( code) then it will be safe.

    Running extensions under a carpet is definitely poor practice. A couple of years back I attended a job site where an inverter system was supposed to be installed. The floor was a mess of extension cords. Someone stepped on a cord and there was a bang, flash and the lights went out.

    Extension cords get warm when carrying current. PVC insulation softens when warm. When you step on it the hot wires push through the insulation and can short out. NFPA forbids running cords under a carpet for this reason. They have seen too many house fires started this way.

    Most off-grid residents do not have fire insurance and live too far from a fire department. Go ahead take a chance. Someone always wins the lottery. Many more qualify for the Darwin award nominations.

    #42019

    elnav
    Member

    A couple of posters provided links to audio gear. One link was all about fuses.

    That was good as far as it went. However, be mindful that audio is not the same as power wiring. A 300 watt inverter will pull close to 30 amps when fully loaded. 30 amps requires #10 ga wiring. It takes very good workmanship to make reliable and long lasting 30 amp connections. the best ones are bolted not push on. Industry standards say push on connections should not carry more than 10 amps in continuous service. Automotive 30 amp circuits usually have something better than the typical consumer grade push on connections. Have anyone noticed that newer cars have bolt on connections, fusible links and circuit breakers instead of the older style most of you are more familiar with?

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