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I've recently begun playing a lot with tiny programmable computers/controllers (like the Raspberry Pi and the Arduino), and I'm planning to distribute several throughout my house as various sensors. So far as I have been tinkering with them I've been using a wall-wart transformer with a USB plug to provide them with 5v (like the one I use to charge my cellphone every night). The problem with doing this throughout my house is that I do not have an AC plug wherever I would need a device.

I've been thinking about running a 12v "bus" throughout my attic, which would allow me to branch off power for devices wherever I need it. This way I would just need a 5v regulator to pair with each of the devices. (The only reason I would run 12v and not straight 5v is because I already have a large 15-amp 12v switching power supply which I could use)

Is there anything special I should consider with this solution? I feel much safer running 12v around in my attic than I would running my own mains power lines. Seems like a no-brainer on the surface, but I may be overlooking something.

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You may want to check with your local building inspector. They don't see this sort of thing often and you might inadvertently violate building code. – slightlynybbled Mar 29 at 19:04
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National Electrical Code, and Local/State Electrical Code have requirements for proper Low Voltage wiring. You don't need to be a licensed electrician most of the time to install it, but you do need to follow the code. – Passerby Mar 29 at 19:24
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I'm wondering if this question might be a better fit on diy.se as the experienced members there seem to be very familiar with the regulatory codes which @Passerby refers to. – brhans Mar 29 at 19:26
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Instead of providing 12V power, you may want to provide Power over Ethernet so that way would you get the network capability and power with a single cable. – Eric Johnson Mar 29 at 20:25
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@user2813274: Thanks. I think PoE would be too expensive for my application though (sensors throughout my own house), and I do not think confusion is an issue - I'm the only one touching it, and if I ever move, I'm taking my sensors with me! :-) – loneboat Mar 29 at 21:15

A few things come to mind:

  1. You need to protect the bus from overcurrent. Even if your power supply has its own built-in protection, I would use an additional fuse (or circuit breaker) at the output of the power supply. This way you can be sure the maximum current won't get away from you. This leads to:

  2. You'll need to use beefy wire. In the USA, 15A circuits are wired with #14AWG, minimum. If you want to use thinner wire, you'll have to fuse each leg appropriately.

  3. If you do use #14AWG wire (or your local equivalent), don't use the typical cable used for household AC! Although it would work out technically, it would cause major confusion and ambiguity. You don't want anyone expecting 12VDC and getting Mains voltage (now or in the future).

  4. At 12V, the current draw can quickly add up. Keep this in mind as you add devices. You may want to swap in a 24VDC power supply in the future. It is a common industrial standard, gives you twice the power over the same wires, and still falls into the "low-voltage" category.

Adding to Point #4: If you choose local 5V converters that accept a range of input voltages (including 12V and 24V, of course), then you won't have to change anything if you bump up the supply voltage.

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Great point about not using similar wiring to my AC. I have a bundle of Romex which I did NOT plan to use, but that was only because of how expensive it is. Avoiding confusion is another great (and better!) reason though. – loneboat Mar 29 at 19:20
    
Would "bouncing up" just entail a transformer on my power-supply's end? Or a new power-supply entirely? – loneboat Mar 29 at 19:21
    
I was referring to a whole new power supply. Stepping up DC is not as trivial as AC, since a simple transformer doesn't work with direct current. And a 15A DC-DC converter is going to be pricey :) – bitsmack Mar 29 at 19:27
    
I only mentioned it because you might want to spec your individual voltage converters to be able to run off of 24V in addition to 12V. – bitsmack Mar 29 at 19:28
    
Okay, thanks! I think I'll stay with 12V for now, since I only have a few very low current devices to power. Good to know that 24V would be a good step up in the future! – loneboat Mar 29 at 19:28

On the face of it it should be OK but the reason we use 120 / 230 V is because the current is so much lower. At 12 V your currents will be 10 or 20 times higher and your cable size will be correspondingly higher to avoid high voltage drops.

Your 15 A, 12 V PSU is capable of delivering 15 x 12 = 180 W into a partial short circuit. This is an obvious fire hazard so good wiring practice is a minimum requirement and, maybe, using a star topology with current limiting on each leg would provide some additional safety.

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Can you clarify what you mean by "at 12 V your currents will be 10 or 20 times higher"? I have some large-ish cable I intend to use (single-core, I think it's 10 or 8 gauge). – loneboat Mar 29 at 19:17
    
And by current limiting, do you mean putting a fuse in-line with each leg? – loneboat Mar 29 at 19:18
    
Yes, there's a reason Tesla chose AC instead of DC. – Tim Spriggs Mar 29 at 19:19
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@TimSpriggs This is high voltage (240/110 V) vs low voltage (12V), nothing to do with AC vs DC. – FakeMoustache Mar 29 at 19:25
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Thanks for all the comments, chaps. 12 V is 12 V. At least we all agree what a volt is. I think we should drop the arguement. What will the neighbours think? – transistor Mar 30 at 9:46

Things to consider.

  1. How will you convert from 12V to 5V? Linear regulators will get very hot and waste a lot of power if they are asked to deliver nontrivial current. So you will want to use some kind of switching converter for more power hungry devices (A pi is a LOT more power hungry than a simple microcontroller).

  2. How thick will the wires need to be to avoid unacceptable voltage drop (which wastes power, can also cause startup problems with switched mode converters and can cause ground potential differences when can be a problem if you have any non-isolated communications links between the devices).

  3. What will you do about overcurrent protection? lower voltages mean lower electric shock risk but low voltage high current supplies can be a fire risk.

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Was considering using something like this to regulate: ti.com/product/LM7805C – loneboat Mar 29 at 19:31
    
Thats a linear regulator. Start trying to use it to power a high current device like a Pi and you will end up with a lot of heat to get rid of. – Peter Green Mar 29 at 19:37
    
Someone above referred me to switching regulators to alleviate that. I will research those before I implement anything. Thanks for your comments! – loneboat Mar 29 at 19:42
    
switching regulators with reasonable efficiency cost about $1 for a 2A regulator PCB assembly at [any on-line marketplace]. – Jasen Mar 29 at 21:39
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Yep, to achive the same percentage volt drop at a given power level in a 12V system and a 120V system supplying the same ammount of power the cables in the 12V system need to be a hundred times bigger. – Peter Green Mar 30 at 15:59

Any cable must be fused to avoid fire hazard.

Consider using 48v. That's the highest voltage considered a 'safe' low voltage by most authorities. It eases the cross-section requirements on cable compared to 12v. As it's found in telephone exchange cabinets and pro-audio desks, switching regulators to get down from that to any other voltage are readily available.

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+1 This is also the standard for PoE (Power over Ethernet), but limits are fairly low. One thing the OP definitely has to do is to separate the mains and low voltage wiring. Not in the same conduit and not in the same junction boxes. – Spehro Pefhany Mar 29 at 19:56
    
any cable capable of carrying more than the short circuit current of the power supply need not be fused. The cheap buck converters on e-bay top out at 35V in, so a lower voltage may be preferable. Personally I'd use 24V if I was using all new parts. – Jasen Mar 29 at 21:42

Do not run your 12v in the same conduit as any other lines (120, 240 or phone lines). It is dangerous and illegal.

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