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I am hoping this could be the right forum for this kind of question. I recently bought a house that is going to have major structural changes and an extension that will basically be new-build. I am pretty much a novice to all this.

I am thinking about the electrics, naively, and wondering what kind of cost savings and simplifications could be achieved by trying to run DC around the house.

My thinking is that

  • I want efficient (LED?) lighting (DC)
  • If necessary, perhaps I could pipe the light by cheap fiber optic, if that ends up being part of the solution
  • Most things I use around the house either run off DC or will be rechargeable via USB anyway.
  • I will still need a few AC sockets for things like the hoover, but not many

I am aware that low voltage DC will suffer voltage drop and power loss, and that fat cabling to compensate would make that more expensive, but what I am thinking is that because this is so early in the construction, are there any opportunities for taking advantage of decorative or structural elements as electrical conductors?

  • Can I use steel reinforcements, even in steel-concrete, as the DC distribution network?
  • Other structural and/or decorative elements such as aluminium bars, copper plates and etc., that when arranged could also serve as DC distribution

In short, in our current home it seems absurd at face value that we have all these little USB adapters and chargers lying around, transformers, wall-warts, bricks and etc., when it seems all I really need is a USB socket in a wall. I am wondering if there is any realistic way of achieving this that makes general sense.

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This question came from our site for electronics and electrical engineering professionals, students, and enthusiasts.

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    This has been asked many times before and the basic answer is that it may sound economical but it is not. Unless you'd use very thick cables a lot of power will be lost in the DC distribution lines. Thick cables are costly. Mains adapters are relatively cheap and in the end the whole setup will be more efficient using standard mains distribution than it can ever be using low voltage DC distribution. Another reason not do distribute DC is that if it breaks, everything DC is out. With small adapters etc, you just replace the faulty one. – Bimpelrekkie Sep 4 '17 at 11:18
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    You can't use DC power to avoid the installation of AC power. It isn't a technical issue; it's a question of what the building code and electrical code used in your area requires. – Dave Tweed Sep 4 '17 at 11:20
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    Just install these everywhere amazon.com/dp/B00J3PMU4C/ref=cm_sw_r_cp_apa_j4wRzbSCV90K1 – casey Sep 4 '17 at 15:16
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    @Harper When you do the sums, the OP quite clearly will be drawing high currents. An average room might use 6-8 8W LED downlighters depending on the room size. Say there are 6 rooms in the house, each with 6 LED downlighters, then you've got 288W. If you run the supply at 12V (the standard for LV lighting) then you've got 24A of current just on lighting, and we haven't even started on charging laptops and stuff. Let's just state as a fact that this will be a high current, and move on. The simple reason is it needs thick cables and higher-powered DC converters, and they both cost money. – Graham Sep 4 '17 at 17:55
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    @Jules: I didn't say you couldn't install DC. I'm just saying that the building code will require you to install AC as well. It seemed that the OP was hoping to avoid that. – Dave Tweed Sep 4 '17 at 20:21
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Power losses depend very little on the type of current used (AC/DC), and much more on the voltage, which you didn't specify. Running 120V around your house will typically result in ~2% of power loss in the wires. With 40V, you'll lose 9 times as much, or you'll need wires which have 9 times bigger section area. 5V is only viable for very low-power applications.

Using steel structure to run your household current is a bad idea, as galvanic corrosion will eat the steel away very quickly. Running the current though indoors decorative elements is also a bad idea: imagine your DC converter fails in a way that gets high voltage into the output. Or a lightning strike. Would you feel safe in a home where you have to stay away from walls during a thunder storm?

What you can do is buy a big mains to 5V power converter, and put it in a wall with several USB wall outlets. Then you'd get a spot where you can charge your phones without the need for all these wall-warts. Just make sure the power supply is still accessible in case it breaks and you need to replace it.

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    It's worse than that, with 40V and the same thickness wires as before you would lose 9 times as much power. – Peter Green Sep 4 '17 at 13:09
  • Depends what you're trying to drive @PeterGreen. Most of the loads OP is thinking about are going to pull 20 watts maybe. You can handle that effortlessly on 14AWG@12V. The anti arguments here presume OP is trying to run the whole house. Aside from ignoring 30 years of experience of actual, successful solar off-gridders, that also ignores OP's own stipulation that he's only trying to power things that are small. – Harper Sep 4 '17 at 14:47
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    I uprated all the answers and marked this one as correct as it addresses most clearly the ideas of using structural elements. Also, the idea of power converters and in-wall (or floor or whatever) USB slots seems pretty practical to me! Thanks for the idea. – Sentinel Sep 4 '17 at 15:03
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    @Harper - the first item on OP's list of things he's looking for is LED lighting. LED lighting for a typical house could easily run into hundreds of watts, which at 12V would mean he needed 5AWG (16mm^2) or similar. Useful calculator: solar-wind.co.uk/cable-sizing-DC-cables.html – Jules Sep 4 '17 at 15:13
  • I still can't help wondering though....if I went the route of hot air vent heating, could I use the aluminium pipes to push DC everywhere? – Sentinel Sep 4 '17 at 15:28
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It's an idea that initially sounds appealing but just doesn't work out in practice.

Unfortunately for a given acceptable power loss percentage the cable size required is proportional to the inverse square of the voltage. Running any significant power any significant distance at 5V is impractical.

Once you introduce an intermediate voltage (e.g. 48V DC systems) the losses in performing the conversion (which you generally want to be isolated to prevent currents from taking unexpected routes) from your intermediate distribution voltage to the final utilisation voltage tend to eat up any gains.

Using structural metalwork would open up it's own problems, such as poor electrical insulation, risks of shorts (a short on a high current low voltage line can be no laughing matter), galvanic corrosion and great confusion for anyone who works on the property in future.

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I can be very short. The most economic way is to use AC.
You will have to invest time and money for custom DC power distribution solution.

If you look at implementation challenges:
- DC power supply.
- DC fuses, you'd want them re-settable.
- DC sockets, rare.
- DC appliances.
- DC cabling.

If you don't want heavy cables, you'd need more voltage. Say 48 Volts, since that is still reasonably low, yet somewhat standard for telecom appliances.
Now, can you get LED's for 48 Volts? Or a phone charger? A PC supply?
You probably can, but you can probably buy years of AC appliances for the money.

Not to mention the time you'd have to invest.

Also, Dave Tweed mentioned in the comments that it might be against building code. Note that there are some DC standards, since there are CEE-form 42 V plugs.

  • 48V or 60V is probably practical for lighting, particularly if you're interested in strip lighting (which can generate some very cool effects). This is often available with voltages in this kind of range (e.g. heappyled.com/portfolio-item/…). Pretty much everything else would require DC-DC converters, which are just as expensive and inefficient as their AC cousins. Re standards for DC distribution, Power-over-Ethernet is another generally recognised standard that could be helpful. – Jules Sep 4 '17 at 15:26
  • @Jules 48-60V is also a little bit bitey, and is the level at which DC starts showing its dark side. Now you need to start thinking about arc suppression and the rules no longer treat it as "low voltage". I don't know know how they slipped PoE by the NFPA. The problem with all this voltage conversion is, it's not super practical. Look harder for ways to git-r-done at malleable voltages. – Harper Sep 4 '17 at 15:40
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It's a great idea - and you don't have to reinvent the wheel. It's already done widely, notably in off-grid solar homes, robustly wired RVs, tiny homes, people who want a comfortable home when mains power fails, etc. You can adapt existing concepts, but you have to know when to let go of them. When someone naysays, they are not letting go of the right concepts. It happens.

Justification

"Merely to save money" won't work. Your house still needs mains wiring in the legally required locations (mainly receptacles) because Code requires it, and you can't sell your house without it, which means you can't get or keep a mortgage on it. This means any DC system must be an "overlay" at least as far as receptacles. I see nothing in NEC that requires 120V lighting, though.

"Because I'm off-grid" is a superb reason. "Because it's cool" is fine - it's your house.

Because I want more is a fantastic reason. It's the 21st century for Pete's sake. Houses shouldn't lose power. All you need to fix that is an overlay system using off-grid solar tech, and prices for those are in free-fall, and you can get in as little as a few hundred dollars if you scale carefully. Panels, charge controller, batteries, you can have a very nice system. With a little more work the system can power refrigerator and a well-chosen furnace (or simply a non-electric furnace) and you stand a chance of being able to ride out storms. You won't be able to wash and dry clothes, but could watch Netflix on your TV. (notice most routers, Roku and many TVs take 12V input, and the central-office has a submarine-sized battery bank to keep telecom up at all times, and genny backup.)

Distance and Voltage

When dealing in low-voltage, distance is a very serious factor. This was Edison/GE lost the War of the Currents. So cottage vs. McRanchion matters. Corner vs middle of building, matters. You need to think about this. It's not a deal-breaker, just needs either smarts... or lots of feeder. I recommend "smarts".

Voltage is a huge factor, and your choices are 12, 24 or 36 volts. 12V has far-and-away the best selection of gear, with USB fobs for instance sold at literally any gas station. But it is the most difficult to haul distances, requiring thicker wire. 24V gear exists, but more hunting is required to find it. In return, you get 4x the transmission efficiency, which is a Big Deal. 36V doubles transmission efficiency again, but gear is scarce, so probably not worth it. Actually a lot of 12V gear uses switching supplies (e.g. LED drivers) which buck, and so inherently have the ability to work on 12V and 24V (and in some cases, 36). The question is whether they enable it and had the device listed for it. This bucking also makes them resilient to voltage drop within reason.

Anything over 36V is out of the question. It no longer qualifies as "low voltage" and DC can get rather bitey and arcey at those voltages. You don't want to mess with that.

You might as well put a desktop shortcut for a decent voltage drop calculator, because you'll be "living there". It's hard to find a good one that doesn't lie to you... most of the good ones are optimized for NEC mains installation, and have bugs when dealing with 12V. This is the best I have found.

Here's a concept to let go: 3% voltage drop. That's a mains electrical "meme" that guides a lot of really bad design, much to the delight of cable manufacturers. It is not in Code anywhere. What's in Code is the 310.16 and 240.4D current limits, and one thing I like about Southwire's voltage calc is it takes those into account while letting you choose any % drop. You are going to have voltage drop. Plan for it. But it's not the end of the world: likely your supply is 13.8V if a battery is supported, and even at 15% you're still at 11.7V. And bucking loads just won't care.

The grounding dilemma

Remember electricity wants to get back to source, not ground. In AC mains, we wire as an isolated system -- ground is excluded from the normal current loop, it only carries ground faults. There is also one neutral-ground bond in one place only which functions as a "fault catcher" and to keep it from floating/rattling at high voltage, which isolated systems are prone to do. That does not make neutral equate to ground. EE's hate this because they are used to GND/that little symbol being the symbol for the normal current return (what we call neutral).

What about a low-voltage DC system? Good question. The whole point of isolating mains is that it's hazardous - that doesn't apply to low voltage DC. Even if you built the DC system isolated, you'd still want to bond negative to the grounding system to keep it from floating/rattling. I can't honestly see a reason not to merge the DC system's grounding system and current return and building frame, provided it's rated for the current. There's no question on a RV, boat or train; they are vehicles and this is SOP. However in buildings, your steelwork is not as massive, nor bonded as well (vehicles are welded), and it may be a moot point anyway: you must use proper wiring methods and they don't make 1-wire Romex. You will surely be stuck dragging 2 if not 3 wires along regardless, so using the building for return may be moot.

The ultimate decider here is your AHJ, so have that conversation with him/her first. If s/he nixes it, run the negative wires. (AHJ = Authority Having Jurisdiction, i.e. your electrical inspector).

Topology

Think about every place you want DC power to go, and how much in watts (or amps if you have decided your voltage). You may be able to exclude large sections of your house entirely. Think about where to put distribution. Generally you want short runs to each load, you don't want to carry 12V long distances on thin wires. Every segment needs to go into the voltage drop calculator to help you make good wire-sizing decisions. I imagine most loads will be <4A going <20', so 14AWG wire will yield a <0.42V drop.

Every junction box needs to be accessible. But a distribution point only needs to follow the service panel rules if it has circuit breakers or fuses that you would reset.

Each circuit must be fused based on the thinnest wire in it: 15A for 14AWG and 20A for 12AWG. You are allowed to use larger wire if you want to.

Anytime a switch is involved, think about relays to reduce the carrying distance of power. Do the math both ways, but a relay can help, since the power doesn't need to travel the relay loop.

You have to break out the calculator, make the map and do the math.

Example:

enter image description here

This is just a quickie to convey the idea, much more could be done with it. This is with a long, narrow house with all the action on the ends, so 2 separate LV systems. Trying to run a fat trunkline down that long middle didn't make any sense, nor did running 15 "smaller" wires. There is some fat wire between the distribution points and battery. A choice was made to move the cable modem and router to the utility room to be near the battery, instead of wasting time trying to haul power to its difficult location in the living room. Easier to move phone and Internet lines. Since branches were kept short, #12 and #14 wire could be used, except for the bedroom-living room run for its lighting. There's only 40' of fat feeder and 20' of smaller feeder.

Load factors

One thing said a lot is "look how much power lights need." 6500 lumens was proposed for an "average room". I was like "you're really gonna want a dimmer on that." Fortunately DC light dimming is simple and reliable, unlike AC.

And why should all the lights in the house be on at the same time? Maybe the right answer is let them dim... intentionally engineer in some voltage drop, to warn the occupants not to leave lights on unnecessarily. That makes even more sense in an off-grid/battery scenario. where such carelessness would burn up battery. This is the kind of design decision you get to make.

Beyond that it really comes down to a calculus of "how much are you willing to spend on wire for proportional voltage drop". No problem getting fat wire - 4/0 mobile home feeder at $3.50/ft is more than you'll ever need.

Voltage conversion

Generally you want to do as little voltage multiplying/dividing as you absolutely have to. Inside a normal house, thicker wire will be cheaper than a choppy, lossy voltage pump.

That's especially true for 120/240 inverters - they are very lossy, especially when they are oversized, and they lose even with no load. A lot of off-grid/solar projects fail because the user fails to account for this enormous vampire load.

I make an exception for supply - solar, wind, generator, microhydro, "use your car battery to recharge your sagging batteries 3 days into the ice storm" etc. You probably can't do much about your power source being quite far from your battery. This is a great time to pump voltage up as high as you can, pretty much to wire insulation limits. This can be as simple as putting the solar panels in series instead of parallel, or winding the windmill generator differently, or using an inverter to pump your car's alternator up to 120/240V for haulage.

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Can I use steel reinforcements, even in steel-concrete, as the DC distribution network? Other structural and/or decorative elements such as aluminium bars, copper plates and etc., that when arranged could also serve as DC distribution

Decorative elements? Definitely not. That would expose voltage-carrying elements. They must be insulated.

Steel reinforcements would be a nightmare to get right. All the steel inside a slab of concrete is tied together, so it'd all be electrified. And with a large 5V supply driving that grid (say, 100A to get 500 W), anything touching the grid would be welded instantly.

Non-standard electrics also make a house harder to sell. You may know where the grid is buried inside the concrete, but will the next owner?

all I really need is a USB socket in a wall

Well, you can get those. USB-only sockets and combination 120V/USB sockets exist. These have an adapter built in.

USB socket USB socket 2

  • I disagree with your decorative elements carrying voltage comment. There are lamps powered by low-voltage DC powered by exposed bars on the market and quite popular in restaurants etc – Sentinel Sep 5 '17 at 7:28
  • @Sentinel Yeah, those do exist. My kid once managed to short circuit one of these while we were in a car dealership's sales office. Luckily there was a circuit breaker to cut it off, otherwise the short could have blown the DC converter. Another issue with exposed DC bars is that you need a DC converter which guarantees to never get AC voltage on the output, even when it's on fire or there's a cockroach inside. This is a safety level required of medical grade SELV equipment, which is not cheap. – Dmitry Grigoryev Sep 5 '17 at 8:51
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    @Dmitry Like father like son! Playing with sparks so early on :-) – Sentinel Sep 5 '17 at 11:03
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I'm not from your side of the pond but the ground rules are the same. As you pass current through a cable you lose Volts. Losing a couple of volts on a 110V circuit is a problem but no where near the problem as a percentage losing a couple of Volts is on a 24V system. Power is in Watts and at DC that's a simple Volts x Amps. To get the same power at 24V you need a lot more Amps and the more Amps you have in your circuit the more Volts or pressure you lose in your system. Apart from building codes and the like there are some very good reasons why we don't power circuits at 24Vdc.
Instrumentation on industry - that's different but only passing a few milliamps.

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In 10 years 5V USB charging is going to be obsolete. It actually already is. Most phones now are capable of fast chargers. Why would I charge my phone for 2.5 hours with a standard usb when fast charger takes 30 mins. I never understood the big deal of having the adapter plugged into outlet.

Stick with A/C

If you want something to brag about you can run some lights off a solar panel.

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