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Looking for a direct current (DC) wiring resource  RSS feed

 
Nicola Marchi
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I was wondering if anyone had a book, website, or other form of information that discusses DC wiring for an offgrid system. Everything I've found seems centered around an inverter.

Wiring a PV system to batteries and using straight DC seems so much more efficient and worth the hassle of finding DC appliances.

I even imagine a little creative wiring could solve charging my laptop since I've seen laptop car chargers.

If there's any glaring flaw in my idea please point it out to me, but i'm hoping for some resources to get an idea of the scope of a future build.
 
Bill Kirby
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I'm pretty new here but the RV industry has been doing things with DC for many years. I've lived in an RV in years past with a couple of solar panels to keep my batteries charged and never needed an inverter or AC for any reason. There are many DC appliances available and there are also other alternatives. Have you ever seen a propane fired curling iron? The wife used one for a long time. It worked great. Just some thoughts. Start your search by checking the things that are available there. You might also take a stroll through a large truck stop. You wouldn't believe what's available to be used on 12 volt systems in trucks.
 
Brett Hammond
Posts: 76
Location: Maryland, USA
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Ok, I know this is an old question, but just in case someone else is interested, the problem with DC is that you can't run lots of power(watts) any significant distance with DC unless you use huge (I.e. expensive) cables. It is not problem for a 20 ft boat or travel trailer, but if you want to run a toaster or iron off DC in a house where the appliance is 75 feet from the batteries, you are going to pay several dollars per foot of cable to get it there without significant loss. Not only is AC cable much less expensive, but AC appliances are much less expensive too. Spend a few hundred bucks for a cheap inverter. They will run most anything. I have one in my truck I bought from Walmart for $50 and it runs everything I have tried so far. Or take that $2k you were going to spend on DC cables and buy a nice sinewave inverter.
 
Ray Holt
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Location: Bristol UK
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A good resource for DC wiring is a small book ' 12 volt solar power's by Michel Daniel published by Permanent Publications in the UK. Don't be put off by the small size and dodgy line drawings, I used this as the basis for wiring our house 12volt circuit (we're off grid and have inverters and a back up generator for the large load 240v appliances - washing machine, toaster, blender etc). He has some good ideas in the book for running some common electrical items direct from 12v - I've used his suggestions for our TV, stereo, laptop etc.
 
frank li
Posts: 193
Location: Michigan
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This was the base starting point for my endeavor. There are more up to date publications, but i still like this.

https://solarconduit.com/shop/consumer/the-solar-electric-independent-home-book.html
 
frank li
Posts: 193
Location: Michigan
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Brett Hammond wrote:Ok, I know this is an old question, but just in case someone else is interested, the problem with DC is that you can't run lots of power(watts) any significant distance with DC unless you use huge (I.e. expensive) cables. It is not problem for a 20 ft boat or travel trailer, but if you want to run a toaster or iron off DC in a house where the appliance is 75 feet from the batteries, you are going to pay several dollars per foot of cable to get it there without significant loss. Not only is AC cable much less expensive, but AC appliances are much less expensive too. Spend a few hundred bucks for a cheap inverter. They will run most anything. I have one in my truck I bought from Walmart for $50 and it runs everything I have tried so far. Or take that $2k you were going to spend on DC cables and buy a nice sinewave inverter.


Remember voltage matters and many people are under the heresay notion that dc is no good over long distance. I battle electricians on this weekly. They will not hear it even if you show them the calculations and meter display at both ends, then its "too expensive", except its not!

True, 31.25 amps at 48vdc nominal or a rough 52vdc conveys the power of 15a 120vac and would require #4 copper for 75' ($.56/ foot x2 $84!plus conduit if inside) and 2.23% loss on that circuit at max load. Very many appliances that require high power do not require it for long duration so we can edge right around 2% or even close to 3% loss and still sleep at night.

Once you have figured wire cost and cost and difficulty of that scenario, ac does look way better a "no brainer", but, do not forget voltage, voltage matters and so does practicality.

What if your example allowed room for higher voltage. 120vdc circuits would have the same loss characteristics as 120ac circuits, we do it with pv charge control all the time. Doing it twice could be an issue that relegates it back to impractical under many conditions-land.

So what can be done is place a battery and high voltage dc charge controller near the load. Eminently practical for some projects a no-go in others...so we chat.

9,000 watts from 20 amps at 450vdc over 75 feet is 1.68% voltage drop on #14 copper conductors.
 
Rufus Laggren
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Location: Chicago/San Francisco
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DC requires relatively rare control fixtures. You can use AC switches for a while in a DC circuit but AC circuit breakers or fuses would not be so good an idea. Why? Because AC goes "off" every 1/120 second which makes it _much_ easier for a switch or circuit breaker to actually break the flow in the circuit. DC wants quite badly to continue flowing and will arc, or try to, every time, even at 12 volts. This shortens the life of controllers greatly and also tends to weld them closed (they fail "on") unless they are purpose built for DC. IOW expensive. While a failed switch may just be an annoyance, a failed circuit breaker could be more serious.

The same "don't want to stop" problem exists when people touch a wrong place and get a shock - with DC it's harder for them to break the circuit they're making. Up to 32volts this may not matter too much. Above that it becomes something to consider. Large PV systems can run 200+ volts DC making them one of the more dangerous electrical installations for their size an voltage.

Each connection (place where one wire joins something else) loses voltage. This is the bane of low voltage and applies to low voltage AC as well as DC; but since most DC systems are low voltage (under 50volts) it's particularly relevant to DC circuits. IIRC figure 1/10 volt loss each connection for clean well made connections. It goes down hill from there. Since each resistance (connection) generates heat as more current passes and low voltage circuits need much more current to carry energy compared to 120 volt circuits, bad low voltage connections can generate lots of heat - not good - as well as contributing to "brown out" for large loads. Good connections, while not actually hard to make, require serious, routine and continuous care. That means, in effect, good DC connections _are_ hard to make...

Electrical circuitry is made of myriad complex parts and constructions that are not and were not no-brainers. But we've got a mature industry now that's solved most of the problems. We produce the parts needed for our AC infrastructure in quantity and, more or less, adequate quality at a low price making it a far more functional choice at this time and place than DC equivalents. Low voltage is used where the circuitry is close to people, in one way or another, where people can and do easily come in contact w/the working parts of circuits; and where there can be a water problem. Eg. vehicles. And, of course, in today's wonder gizmos that use semiconductors running at very low voltages.

Rufus
 
frank li
Posts: 193
Location: Michigan
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Insulated low voltage dc( 120vdc) is unlikely to be any more dangerous in most power distribution and appliance uses... if you do not bridge it.

Dont get me wrong, high voltage DC arc fault in the switches or other equipment is no fun especially at high power levels.

Dc rated switches and breakers are sometimes expensive but good ones are REALLY good. 150vdc, 2 million cycle-life at full load, magnetic-hydraulic breakers from midnight can be had for a few dollars and control and automation suppliers gotta have solutions that are smaller and cheaper. There is already a shift to high voltage dc  or dc in general as we utilize battery technology to greater extents.

Lawnmowers and cars are good examples of potentially dangerous dc powered devices that are a-ok approved for use by anybody  without certification or training beyond reading the owners manual, which everybody is doing, for sure!

Touch safe is the standard for devices operating at dangerous voltages or any voltage, usually.




 
Rufus Laggren
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Location: Chicago/San Francisco
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I read that after several scary incidents part of first-responder's training now is how to not die while cutting people out of electric vehicles.

Rufus
 
frank li
Posts: 193
Location: Michigan
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Its easy, dont cut the fat, blaze-orange cables!
Seriously though, they have to train to extract people from ANY car in a way that is always dangerous. Fire fighters have equipment. They have insulating clothes and equipment to deal with cutting and smashing an ripping apart all kinds of structures webbed with all kinds of high voltage. The first thing they will do is turn it off or cut the pack cables with insulated tools and face shields. No problem. DC is not evil, just mis-understood. All electrical charges and fields are potentially dangerous.
 
frank li
Posts: 193
Location: Michigan
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Ac is more troubling to me where it is bonded to "protective" earth and the entire building electrical equipment grounding system.

Have you ever spliced hot utility lines? You gotta be careful.
It is easier to protect myself from a known positive and negative, isolated dc system with disconnects everywhere (photovoltaics installation) i know exactly where the power is and i keep it isolated, just like ac systems, without all the extra precautions and protective equipment.

I can stand on my feet and climb on metal structure that is bonded to ground and handle live pv leads at 1000v, safely and without major concern, kind of like plugging in a vaccum cleaner, but safer. Touch safe! Same thing inside enclosures. It is done by isolating power.
 
Matthew Lewis
Posts: 59
Location: Canada
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We live in a tiny house wired for both AC and DC power. We have 100% DC lighting, several DC outlets and our water pump and furnace are DC as well.

We even were able to find a TV and Blue Ray player that weren't marketed as 12 V but ran off of 12v power supplies at the wall outlet rather then internally.

The main reason we decided to do a dual system was to save on the inefficiecies of converting DC to AC and then back to DC for led lighting and other appliances. Also to not have an inverter wasting power idleing 24 hours a day. We still have an inverter but will only turn it on when needed.

For those of you interested in more information on the wiring in our tiny home I have a few videos. One of the best is this one:



Or check out our blog: http://www.canadianrenegade.com

And if you have any questions feel free to ask!
 
Peter VanDerWal
Posts: 73
Location: Southern Arizona
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In the USA, the National Electrical Code includes sections describing the requirements for DC wiring.  The main issue is that switches, fuses and breakers have to be rated for DC for the reasons described above. And (the tough part) everything has to be CERTIFIED for DC by a nationally recognized testing lab (Like UL, CSA, etc.).  This means you can't (legally) use cigarette lighter outlets, or automotive switches for DC in your house.  Rules that apply for RVs don't apply for permanent housing.

The good news is that Square-D QO and QOB circuit breakers are all rated for at least 48VDC, these are much cheaper than buying special DC rated circuit breakers.  If you dig around a bit you can find DC rated light switces, that look just like regular light switches (but they tend to be more expensive).
You can use standard AC outlets for DC, however they must be a different style from any AC outlets in the structure.

I.e if you use normal 120V outlets like these:


Then you could use something like one of these style for the DC circuits:

As long as you aren't using the same style for any 240V AC outlets.

The point is to use different style outlets for different voltage and AC vs DC, so that you can't accidentally plug a DC appliance into an AC outlet, or a 12VDC appliance into a 48VDC outlet, etc.
 
bruce Fine
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inexpensive source of wire is your local scrap yard, some of the independently owned places will sell you stuff by the pound
 
We've gotta get close enough to that helmet to pull the choke on it's engine and flood his mind! Or, we could just read this tiny ad:
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