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James Curran wrote: I've long felt that we need a standardized low-voltage DC power source coming right out of our walls, much in the manner that we currently have 120V (or 220V) AC coming out of our walls -- to eliminate the need of all those "brick" power adapters cluttering our power strips.
The Three Gorges Dam in China have DC transmission lines that transport power to homes, businesses, and such. There's even a (short) HVDC line going from Pennsylvania to Long Island. Historically I think it's been a matter of AC's efficiency over long distance.
"One man's wage rise is another man's price increase." - Harold Wilson
"Fireproof doesn't mean the fire will never come. It means when the fire comes that you will be able to withstand it." - Michael Simmons
"You can easily judge the character of a man by how he treats those who can do nothing for him." - James D. Miles
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David Crow wrote: Historically I think it's been a matter of AC's efficiency over long distance.
I still see it coming into the house as high-voltage AC, with the longest run of DC being 10-20 feet within the house. The conversion would be done in the wall socket, so instead of two 110V outlets, it could have one 110V and two or four USB outlets.
Truth,
James
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James Curran wrote: ...instead of two 110V outlets, it could have one 110V and two or four USB outlets.
Not sure if this is what you are talking about, but I have several scattered around my house and shop.
"One man's wage rise is another man's price increase." - Harold Wilson
"Fireproof doesn't mean the fire will never come. It means when the fire comes that you will be able to withstand it." - Michael Simmons
"You can easily judge the character of a man by how he treats those who can do nothing for him." - James D. Miles
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Yes, exactly that. We have all the parts in place. We just need to convince people(*) to start using 'em.
(*) "people" being inventors designing new products that could use low voltage, and homebuilders, to put outlets like that in homes originally.
Truth,
James
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What's the problem?
My plan (currently being built) is to give my home a 12VC spine along the middle wall, with ribs to both sides, with fixed cables to all room lights. For plug-in consumers I standardize on BNC connectors, cables with BNC in one end, whatever in the other end. It will mostly be plugs with standard 5.5 mm outer diameter add 2.5 mm inner diameter, but I have a couple devices wanting 2.1 mm diameter, so they need another cable.
I am installing 3-pin BNC sockets in my power rails, having 'declared' that the left connector supplies 12VDC, the right one supplies 5VDC. For now, all my 5VDC consumers use USB charging, from a hub supplied by 12VDC (unless supplied by the 75W AC based charger). I am not certain that I will ever provide any advanced 5VDC supply though my spine/rib network, even if the cabling is there.
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AC-DC arguments are as old as the Edison-Tesla wars ...
Today (but at most for a couple decades) we have had DC-DC voltage converters with losses at a level that make them viable for general use. They are used in specific application, such as adapting the voltage from the PC power supply to that required by the RAM chips, but at the current, we do not have units converting from, say, a 750 VDC inlet to the house, to the 12 VDC expected by your tabletop radio, your USB hub or external PC disk.
For long-haul high-power lines, e.g. between Norway and Denmark or England, DC is the rule nowadays. But at the home level, it is still far easier to use traditional (or somewhat untraditional switching technologies) to transform household AC to whatever low-volt DC you need there and then.
Don't forget that required cable dimension is a direct function of the current, not the voltage. If you replace a 240V supply (regardless of whether it is AC or DC) with at 12V one, the current increases by a factor of 20, and you have to dimension you cables 20-fold! You will never get a 12 VDC power supply from you power company - that will be the day they come to install a railroad rail as the power connector to your house!
20-doubling cable capacity can only be done in short stretches. Like what I am now currently building in my home: I will have a 'spine' along the middle wall, both in the basement, first and second floors, with 'ribs' to the power consumers. The spines will be 6 sqmm conductors (I considered going down to 4 sqmm, but decided I didn't want the voltage loss), with 2.5 sqmm spines.
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James Curran wrote: So, the question for the electrically minded here: If all you had available to power your device was an USB cable, would that be viable?
Well, it totally depends on what you are powering, but for many things it is totally viable.
For example, I built a device that runs off my USB port that is a Arduino pro micro (with ATMega 32u4) which emulates a HID (human interface device) --in my case keyboard.
Take a look at this snapshot of it running on my computer[^] right now.
The item closest is the Arduino pro micro. The item at the back is soldered to the basic UART on the pro micro and is a Bluetooth (BT) device.
I can unlock my computer from my phone.
My Android phone (via app I wrote)
1. connects to bluetooth
2. sends password (encrypted over BT)
3. pro micro forwards the bytes sent as keystrokes on my computer (via HID)
4. unlocks my computer
It all runs off low voltage 5v powered off one USB port of my computer.
I even wrote this up as an article here on cp: Ending the Era of Weak Passwords: Never Type A Password Again (Never Memorize A Password Again)[^]
And even better here: Never Type A Password Again (The Continuing Saga)[^]
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Yes; these days, it does.
Years ago, some kid asked me to charge his phone on USB and naturally agreed. It was at 3% when he unplugged two hours later and started walking. (USB was up and coming those days).
USB, the "one universal" connector that fits all, comes in many shapes now. These days, my 10+ years old smartphone charges in an hour. I got a bunch of USB to USB converters, since USB on the PC isn't USB on the phone. If the kid would ask now, he'd have his phone completely charged in that time.
And got one of them "no plug" recharger-stations as a present. If recharches phones bloody damn quick, without connecting a cable! To me, that's magic. Or more realistic, it is physics I don't understand yet.
But from the looks, they can draw a lot of power, even if the PC is actually turned of. My keyboard does the same, it keeps displaying this RGB backlit even if the PC is "off". Push any button, and in seconds there's a login.
--
You could of course look up the power that USB3 provides, a quick Google would provide that. Including on which pin, and how much m-amperes.
Bastard Programmer from Hell 
"If you just follow the bacon Eddy, wherever it leads you, then you won't have to think about politics." -- Some Bell.
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One factor (although not the only one) killing SCSI was the bewilderment of cables. When I went to the computer shop to get a conversion cable for my eight SCSI 'standard'  plug, and the salesman made a sigh: You are lucky - we have fourteen different SCSI standards! - then I called it a quit. No matters what other advantages SCSI might have, I will never cope with fourteen different plugs, not even if they are compatible at a higher level!
The bewilderment of mostly needless USB cable variations point in the direction of the SCSI demise. We have created a mess that can't survive. The 'micro B' was a worthless sidestep of no benefit. The great marketing benefit of USB-C was 'you can put it in in any orientation that you want'. World-shaking news! Try to convince British guys that if they had stayed within EU, they would have been happy to get grounded AC plugs that could be turned 180 degrees when plugging in - that would be great, wouldn't it? (Those Brits never realized what they lost in Brexit!)
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trønderen wrote: Try to convince British guys that if they had stayed within EU
Unrelated, but try as they would, the EU is going to block it. If they succeed, more countries get the same idea, and thus we been blocking and torpedoeing all. We can't allow that to be a success.
trønderen wrote: (Those Brits never realized what they lost in Brexit!)
Those Brits are about to become Hong Kong near China. A free port, near a closed mainland. They'd grow, whatever our "deal". So, we hurt them as much as we can, hoping Italy doesn't follow.
trønderen wrote: we have fourteen different SCSI standards! - then I called it a quit. No matters what other advantages SCSI might have, I will never cope with fourteen different plugs, not even if they are compatible at a higher level!
Luckily, we have USB now.
Ehr..
Bastard Programmer from Hell 
"If you just follow the bacon Eddy, wherever it leads you, then you won't have to think about politics." -- Some Bell.
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Eddy Vluggen wrote: Or more realistic, it is physics I don't understand yet
Think transformer without an iron core, with one winding in the plate, and the other in the phone.
It's ridiculously inefficient, in the neighborhood of 50%.
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Hmmm... How'd I miss that... (I think I just looked under "General Discussion", forgetting there are other sections).
Truth,
James
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Well I'm more a hardware guy than softwares so I can probably answer this from a different perspective. While low power coming from the wall sockets is good for things like mobiles and some plug in lights and some other things you can get issues when say a TV or monitor need a supply that can be 'altered' to allow then to work (a flat panel screen uses 1000's of volts, scary but current is so low to be not dangerous unless you want flash burns). While displays can be made to work of low voltage they are not that robust or long lived. House hold appliances which need 230/240 volts at 10 - 13 amps forget about!. I think the issue is down to good for some(alot) of things But mains power is needed for other things to make them efficient. Also high power DC not safe in the slightest (despite Edisons claims) if there a short on a DC power line you usually get somke and damage to the circuit board and wires connecting it, AC a pop, blown fuse, maybe a skid mark! Also getting zapped but AC not good, I have the scars from an AC zap that if it was DC would have more than likely killed me. AC good, DC bad. So I think USB is really a non-starter. 
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Well, I don't see it as a complete replacement for household electric needs, just those which require low voltage/low amperage.
Truth,
James
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Having a 12VDC distribution network in your house has some distinct advantages.
First: I guess this depends on where you live, but around here, you can't do much more than replace a lightbulb without calling a professional electrician, and they are expensive. With 12VDC you can do all or most of the work yourself.
The risks of electrical shock, fires etc. are far smaller with 12VDC than with 230VAC.
Assuming that you draw the power from a (usually solar-powered) battery bank, you still have light during a power blackout.
For the myriads of small electrical equipment - active USB hubs, external PC disks requiring external power, small radios and similar electronics, LED strips, toys, chargers for anything from electric knives to flashlights ... - requiring low voltage power, you can get rid of these clunky charger bricks; you use a simple cable.
Yet, when I tell people that I am making a 12VDC in my home, practically everyone shakes their head and think I am crazy. A few of them know enough to ask questions in the style of "But haven't you been think of ...?" - the high current, high losses, required cable dimension, whatever. I give them my answers, which they usually accept as good enough, "But still, do you really think it is a good idea?" Yes, I do. "Well, I wouldn't..." So be prepared for some resistance from friends.
And try to reduce the resistance in the cables. Make a plan for where the cables should run - the shortest path possible.
One trick I will be using to shorten cables: They go straight to the drain (e.g. lamp), not making a detour to a wall switch and back again. Rather, in the connection point for that lamp, you put a two-coil, bi-stable relay. The two coils are connected on the one pole to the 12VDC, the other to a thin control lead that you can pull anywhere, to as many on-off-switches you want. Switches are "call button" type: A brief press on the "On" button activates the one coil, pulling the relay to the "On" state. A press on the "Off" button activates the other coil, pulling the relay "Off". (I actually use 3-position toggle switches returning to middle position, no connection. Pressing at the top, the "on" control line is shorted to ground, pressing at the bottom shorts the "off" line to ground.) The control lines need only carry enough power to draw the relay coil, which is usually not much, and only while you press the button.
Along that control line, you can put as many ways to short to ground as you like. E.g. when you open you main door, a contact strip may turn light on in you entrance. You can have switches for turning the light on/off both at the top and bottom of the stairs. You could have a switching transistor do the shorting, the transistor base controlled by a computer. (From a PC, you can use a USB-to-LPT adapter, switching one control line through each data pin, or you could have more direct signals from some SBC like an Arduino.)
I was much in doubt about what kind of sockets to use for plug-in devices, but settled on 3-pin XLR connectors (I mistakenly wrote "XLR connector" in an earlier post; it was supposed to be XLR!), prepared for 12VDC, ground and 5VDC - but I am not sure that I will stick to 5VDC. It will probably be 24VDC - my solar panel battery bank is 24V, and some equipment both in fans and light are available in bigger capacities as 24VDC models.
So go ahead with your low-volt DC network!
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Being an hardware guy too, just want to add to what Glenn said...
Electrocution voltage is smaller for DC than for AC, meaning that you can be electrocuted with a much smaller voltage (considering the same current).
Lower voltage implies:
1) higher current for the same power, which in turn implies:
1.a) thicker cabling
1.b) greater heat losses because the current is continuous and the wires, besides heating more from the higher current, do not have time to cool down. In AC, the wires cool as the current goes toward zero in every cycle.
1.c) more expensive protection circuitry since it is harder to detect anomalies. As an example, in DC you do not have phase nor frequency that can be used for detection. Nor can you rely on the return path, which is fixed at zero, unlike in AC where short-circuits can be detected on the return path when using circuit breakers (not fuses).
1.d) more complex and expensive conversion systems
2) since with DC there is no signal on the return path (negative wire) and the current is higher, there are higher EMI (electro-magnetic interference) whenever a device gets connected/disconnected/changes power drawn. On AC the signal on the positive wire and the signal on the negative wire almost cancel each other.
With that said, there are wall sockets that have a transformer inside of them to convert the 230/110VAC to 5VDC and these are used and are safe since the transmission is still low power AC. But you have to choose them thinking on the future since many devices powered by USB consume more and more current.
If you are considering transmitting DC power from a central location throughout your house I advise against it, as you will be spending more money, both short term (cables through walls) and long term (power losses), and increase the risk of accidents (like fire from overheating cables).
Some years ago, I had a colleague that had installed 12VDC distribution lines throughout is house to power a refrigerator, some tools and an "emergency system" (he didn't specified what that was) and had a fire precisely due to wires overheating. He ended having to pay a fine for 1) having an installation that did not comply with the law (in my country it is forbidden to have DC distribution in domestic buildings) and 2) for not having protection systems on that circuit. He also had to pay for repairs on his house and on his neighbors houses who were also affected by the fire. His insurance did not cover any costs as the installation was illegal.
DC distribution only makes sense for very high voltages (over 100KV) since losses in AC, at those voltages, become higher.
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Yes, it is possible to do it the wrong way. It is possible to do it the right way, too!
ElectronProgrammer wrote: Electrocution voltage is smaller for DC than for AC, meaning that you can be electrocuted with a much smaller voltage (considering the same current).
Like 12VDC ...
I am surprised that you are at all allowed to sell 12VDC adapters, car batteries and PC power supplies delivering 12VDC! 
If you set up a home 12VDC network, you will of course protect every single branch with fuses dimensioned for the expected load on that branch. Do you know what kind of current a lead-acid car battery can deliver? Compare that to the fuse protecting a LED strip. I honestly do not fear for my life, even if I am now installing a 12VDC network in my home.
Besides: In practical use, how would I get at that 12VDC to be electrocuted? Sockets, plugs and cables for 12VDC are as well insulated as for 230VAC. I won't be touching it. Only when installing new branches, new light fixtures, will I have direct access to the 12V. And why should I be less careful when doing this installation, compared to working with 230VAC?
Lower voltage implies:
1) higher current for the same power, which in turn implies:
1.a) thicker cabling True, if we are talking about the same effect. But are we? A 12VDC network is not for feeding vacuum cleaners and dishwashers, but for LED lights, charging mobile phones and power feeding your external PC disk. Fifty watts of LED light on a single branch will brighten up your living room!
Look at the cabling for your 230VAC supply: A 1.5 sqmm is considered enough to carry 10A, or 2300 watts. Use the same cable with 12VDC: 120 watt is a lot for 12V based applications. At least in Norway, new houses are wired with 2.5 sqmm cables, handling 15-16A, or almost 200 watts if you use similar cabling for your 12VDC network.
1.b) greater heat losses because the current is continuous and the wires, besides heating more from the higher current, do not have time to cool down. In AC, the wires cool as the current goes toward zero in every cycle. Fascinating idea: Giving the cables an opportunity to "cool down" every 10 milliseconds somehow makes it easier to dissipate the heat, when given the same resistance and total power loss. Honestly, I am somewhat sceptic to your theory.
c) more expensive protection circuitry since it is harder to detect anomalies. As an example, in DC you do not have phase nor frequency that can be used for detection. Nor can you rely on the return path, which is fixed at zero, unlike in AC where short-circuits can be detected on the return path when using circuit breakers (not fuses). Do you have similar consideration for the cable between the 12VDC adapter plugged into the wall outlet and your external disk? We are talking about an extended version of that! The systems you are talking about are "grid related", of no relevance to, say, an active USB device powered from a solar battery bank.
1.d) more complex and expensive conversion systems Whatever "conversion system" e.g. that USB hub has for providing 5VDC USB power from the 12VDC input is internal to the hub. And if the external power is provided by a 230-to-12VDC adapter plugged into the grid outlet, or provided by the 12VDC distribution system of my house, the task doesn't become more "complex and expensive".
2) since with DC there is no signal on the return path (negative wire) and the current is higher, there are higher EMI (electro-magnetic interference) whenever a device gets connected/disconnected/changes power drawn. We are talking about devices drawing a handful of watts, at most - not of trans-ocean gigawatt cables, where your concerns are certainly valid. But not in a private home where you switch on a LED light of 5-10 watts!
there are wall sockets that have a transformer inside of them to convert the 230/110VAC to 5VDC Sure, but during a blackout, you are blacked out. One major reason for a 12VDC home network is to prevent that. Also, you need to call an electrician to make any change to your setup.
If you are considering transmitting DC power from a central location throughout your house I advise against it, as you will be spending more money, both short term (cables through walls) and long term (power losses), and increase the risk of accidents (like fire from overheating cables). The right way to do it includes - as you also say - doing it from a central point in your house, with a star of cables spreading from there. There probably won't be that many users per "star beam"; you need no monster cables! Obviously, you protect every cable with fuses well below the capacity of the cable, so that it will never overheat.
In my own setup, I have 4 sqmm four-armed star at each floor, with side branches to each specific lamp, fan or outlet using 2.5 sqmm cables. Calculate the power losses: In my house, the maximum cable distance from the center is 8 meters, typically half of which runs through 4 sqmm cable, half through 2.5 sqmm. To put it short: It won't cause you to throw out your old electrical heaters!
Some years ago, I had a colleague that had installed 12VDC distribution lines throughout is house to power a refrigerator, some tools and an "emergency system" (he didn't specified what that was) and had a fire precisely due to wires overheating. To put it bluntly: So it wasn't done the right way. Even a low-voltage installation requires some knowledge of how to do it. Overheating due to incorrect dimensioning, and lack of overload protection with fuses, is just as bad with low-voltage installations as with 230VAC installations. Finding cases where unqualified 230VAC installations have caused fires is not difficult!
Another side is that you should know where 12VDC is a viable alternative. Anything involving motors is on the dubious side.
He ended having to pay a fine for 1) having an installation that did not comply with the law (in my country it is forbidden to have DC distribution in domestic buildings)Here (Norway), you can have a distribution net up to 200 watts, which covers a whole lot of the needs. It must be available for inspection, and wherever a cable goes though a wall, it must be in an a pipe of non-burning material (for all practical purposes: metal).
and 2) for not having protection systems on that circuit. That is a cardinal sin! So it wasn't done right. Bottom line: Do it right!
DC distribution only makes sense for very high voltages (over 100KV) since losses in AC, at those voltages, become higher. Sorry, I cannot agree. You cannot simply transfer the issues of gigawatt power lines into the home, and state that they apply equally in that context. If you insist on doing so, you will have to be prepared to explain how the same arguments count against 12VDC adapters. Or explain what makes the major difference between the AC adapter in the wall outlet, with a two meter cable, and the central 12VDC provision with an eight meter cable.
modified 12-Feb-21 16:39pm.
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Ignore that last phrase. It was for another post. Must have mixed them. Sorry 
trønderen wrote:
Look at the cabling for your 230VAC supply: A 1.5 sqmm is considered enough to carry 10A, or 2300 watts. Use the same cable with 12VDC
That does not look correct. You can't just assume that because a wire can stand 10A AC it will also stand 10A DC. The power loss to heat is different.
To carry 10A DC at 12V you need a thicker wire and I'm not sure the 4sqmm wire you mention are thick enough.
I did a quick search and found the two calculators below that seem to confirm my suspicions.
For the numbers you give 12V DC, 10A load, maximum 8 meters you need a 16sqmm wire otherwise you will have a voltage drop that may be significant. And in DC, that voltage drop will translate to heating of the wire which, in turn, might degrade the wire insulation. The second site actually mentions heat.
I have inputted some numbers and a 4sqmm seems to support approximately 2A DC.
<a href="https://www.solar-wind.co.uk/info/dc-cable-wire-sizing-tool-low-voltage-drop-calculator">DC Cable Sizing Tool - Use The Correct Sized Cables - Free Calculator</a>[<a href="https://www.solar-wind.co.uk/info/dc-cable-wire-sizing-tool-low-voltage-drop-calculator" target="_blank" title="New Window">^</a>]
<a href="https://faroutride.com/van-build/tools/wire-calc/">WIRE GAUGE CALCULATOR: What Wire Size (AWG) Do You Need? | FarOutRide</a>[<a href="https://faroutride.com/van-build/tools/wire-calc/" target="_blank" title="New Window">^</a>]
As far as I know, here in Portugal, 12/24VDC power distribution can be used in commercial buildings only. Residential buildings can only use 12/24VDC for signaling (very low current used in things like smart home systems) and must have a separate tube from power.
To have something like a solar panel or a battery pack, one must use a converter to 230VAC.
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Sorry for bringing some realism into the panic ...
Voltage loss (hence power dissipation) through a cable is given as dU = (2 * I * l * rho) / A, where I is the current, l is the cable length, rho is a material constant which for copper is 0,0175 Ohm * A/m. A is the cross section of the conductor.
Filling in for I = 2 Amp, l = 4 meter, and A = 4 sqmm gives a voltage loss of (2 * 2A * 4meters * 0.0175 ohm*sqmm/m/) / 4sqmm = 0.07 V, through the maximum spine length in my house.
If, at the very end of a spine, I make ma maximum length rim, using 2.5 sqmm cables, the voltage loss is (2 * 2A * 4meters * 0.0175 ohm*sqmm/m/) / 2.5sqmm, or an additional 0.112 V, for at total voltage loss of 0.182 V. So with a 2A load, I will experience approximately 1.5% voltage loss along the cable. Given the 2A current - about 24 W - more than a third of a watt may be dissipated from my eight meters of cables. Alarm! Look out - Danger! Fire hazard imminent!
Most consumer points do not run from the very end of the spine, and do not have four meter ribs running from the spine. This 4 + 4 meters is an extreme worst case. For most situations, both cable lengths and currents will be (way) lower.
I can understand that there may be people who do not understand the need for power cables to dissipate a third of a watt over eight meters, thereby causing fires. Yet, others may prepare for it, and lay the cables to provide the necessary cooling for effects at this level.
BTW: Norwegian mountain cabins, far away from any 230VAC supply, have for decades had solar panels with battery banks providing 12VDC for light, small fridges, ventilation and other purposes, without converting the 12VDC battery power to 230VAC, but using the 12VDC directly. It would make no sense converting it to 2230VAC only to plug in a 230VAC-to-12VDC to get it back to the power required to drive the light and the cabin style cool box.
(Note to the OP, James Curran: Here you see the kind of arguments you must be prepared for if you decide to go for a low-voltage installation in your house!)
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trønderen wrote:
I = 2 Amp, l = 4 meter
You didn't said 2A before. You said "almost 200W" which gives 10A.
The specification of the setup should be as detailed as possible. Which was my point in the misplaced last paragraph (I managed to track down the other post and pasted that paragraph at the bottom of this post).
trønderen wrote:
It would make no sense converting it to 2230VAC only to plug in a 230VAC-to-12VDC to get it back to the power required
I agree. And I think modern houses should be built with a DC distribution network besides the AC one, given the increased proliferation of low power DC devices. This way, that network would already be certified for use since houses are inspected before being granted permission for habitability. Until this happens we should stick to what our local law allows.
I also think houses should be built with a network for internet where one can connect either an Ethernet cable or have small fixed converters to Wifi with smaller range and end this signal fight where a single router must fight with its 200 neighbors. But that is a conversation for another topic.
I know my English is not the best and my explanation capabilities are bad, as I am not used to speak to people, but I am trying to help (and apparently failing). You seem more qualified than I so I will just stop trying.
Best of luck with your setup James.
In my previous post that says
"DC distribution only makes sense for very high voltages (over 100KV) since losses in AC, at those voltages, become higher."
should have said
"If you really think you need a DC distribution system make sure your specifications of what (with corresponding voltages and maximum currents) connects where (how far from the source) is well defined. Then either hire a professional that does DC installations or ask someone with knowledge to validate your setup before starting to build it, preferably someone with knowledge about your local laws."
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ElectronProgrammer wrote: You didn't said 2A before. You said "almost 200W" which gives 10A.
True - you were the one setting the limit for 4 sqmm cable at 2A. So I made the calculation of heat loss for what you consider the maximum handling capacity of a 4 sqmm cable. You can easily repeat the calculation for 10A - in fact, the voltage loss is proportional to the current, so for the 4 m of 4 sqmm cable, the loss is 0.35V, and for the 4 m of 2.5 sqmm cable 0.56V.
However, we are here talking about extreme worst case examples. In my house, no 12V equipment lies at the end of a 4+4 m cable stretch; the typical is half of that. Furthermore: No single branch will carry 10A (or even close). As I wrote in an earlier posting: You will of course protect every single branch with fuses dimensioned for the expected load on that branch. -- That is even if the cable dimension would allow for a higher current than your expected load. If you cause a shortcut, the fuse should blow, rather than the cable overheat. This is part of knowing how to do it.
ask someone with knowledge to validate your setup before starting to build it, preferably someone with knowledge about your local laws Our Norwegian laws set a limit on the total effect of you 12VDC network at 200 W, and you must stay below 50V. Cabling must be available for inspection everywhere; if they go through walls or ceilings, they must run in metal pipes (in principle: Non-burnable material, but for all practical purposes, that is metal). I am replacing cornice with similar-looking cable gates with fronts that can be flipped off to get at the cables. (Side remark: The cabled Ethernet runs in the same cable channels.)
12VDC networks have a long tradition in faraway mountain cabins in Norway, and we have a multitude of shops selling all sorts of parts for setting it up: Cabling, fuse boxes, water pumps, plugs and sockets, lead-acid batteries and solar panels. You can buy all sorts of lamps, fans etc. made for 12VDC, intended for cabin use. We just don't use it much in regular homes. It is easy to find people with a good understanding of what you can and should do, and what you shouldn't, and the common knowledge level is high. We know because we need to know. We have two big Norwegian language websites, named ByggeBolig.no (like BuildingAHome) and ByggeHytte.no (like BuildingACabin). In both forums, various issues related to low voltage power is regularly brought up. Although the majority of posters are DIY guys, several professional craftsmen, including electricians, are active and provide professional advice.
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Ah Ok, don't take my mains. Another issue is the grounding of things often cheaper USB devices use a common ground to the supply to make things easier and cheaper to make. Be careful when using these things as you can get a tingle, which ca,n if things go south, result in mains being applied. While it is a good idea to some extented (Low Volts, Low Amps) the legislation needs to be much tighter. It is the wild west with some products (the company I worked for ordered all office staff USB fans, I did a test and found the earthing/grounding issue told them, they were not happy!)
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I'm listening to a webinar where every third word is 'y'know'.
It's driving me insane.
cheers
Chris Maunder
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Like y’know folks, ... like folks y’know, ...
When a language with million word vocabulary goes down to 3 (or maybe 4, sad) 
Mircea
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