How does three-phase electricity works?

[Zeiss Ikon]

That thread pretty much agrees with what I've posted above -- you can get 3-phase in America if you're willing to pay for it, but it's rare in residential.  It's fairly common in commercial and industrial settings, because they use enough power that 3-phase makes sense (three times the power with only 50% more wire/connection).  The trend in residential is to use less power, rather than more (I get a conservation-oriented letter from my power company every month, reminding me that I use a lot more than the average power consumption for my neighborhood -- which surely has a lot do do with a couple aquariums and half a dozen computers that run 24/7, and a partner who gets cold easily but can't take excessive heat, either).

[wumpus]

I'd be curious if anyone is wiring houses to use DC power (probably 115VDC), at least in some circuits.  Solar generates DC, and other forms of available power (wind, water, etc) are unlikely to produce proper 115V@60Hz (or your local power requirements).  You can efficiently split a portion of power into a DC load (think a shunt regulator, only instead of wasting the power feed it into your inverter and/or batteries).  DC should work fine for most LEDs (be careful), plenty of computer power supplies (check first before buying), and presumably ovens/ranges (may require checking/hacking for the controls).  One catch is that the amount of power needed for lighting has drastically fallen (switch to compact fluorescents) and fallen again (to LEDs) and is likely an afterthought.  Any decision would likely depend on using DC for the air conditioning (anyone planning this far ahead is probably using a ground-based system, so only those need be concerned) and possibly refrigeration.

A quick read through the UL and NEC codes implied this was legal (carefully mark what is what), but I don't have access to them and aren't exactly familiar with them (I had a short job testing things to UL specs).

I'm expecting USB to continue to become a major electric outlet, but even if they allow >20V (depending on the code you can get up to 30-40 volts before the "real" regulatory problems kick in) you will always have the original AC vs. DC problem that the lower the voltage the greater the loss is over the wires (although  this would be another case where changing from an AC-DC supply to a DC-DC converter should work better: much less strain on the capacitors).

[StrandedonEarth]

As I understand it, the problem with residential high-voltage DC is a safety issue. Electric shock contracts muscles. With single-phase AC, there's that moment of zero current, which gives the shockee a chance to let go. A DC system doesn't give that chance, you're stuck to the wire until the power is cut off. I suppose having a ground-fault-interrupt breaker can mitigate that, but getting caught between both legs of the circuit without passing enough current to trip a breaker would still be a problem.

But i don't see any reason why modern houses couldn't be wired with 5-12VDC for LED lighting and USB power/charging outlets.

[Zeiss Ikon]

The few milliseconds of near-zero current during the AC cycle isn't long enough for human senses to even detect, never mind for human reflexes to react.  An AC shock will lock muscles every bit as solidly as a DC shock, and is somewhat more likely to kill due to the reaction of the heart's pacing system to 60 Hz current.

Home DC is impractical at present because of 120 years of AC history.  Everything in your house runs on AC, and very little of it can switch to DC.  Most circular saws will run about as well on 120 V DC as they do on 120 V AC, but a refrigerator, electric clock, microwave oven, fluorescent lamp (including Compact Fluorescent units that replaced incandescent bulbs), and most LED bulbs won't work on DC.  The fridge and clock depend on a type of induction motor, fluorescent lighting depends on transformers to create the high voltage needed to create the arc inside the tube, and most LED lamps also use a transformer (there are probably some that use a DC-capable switching supply, but those cost more than a transformer/rectiifier setup, hence are less favored).  Even your computer power supply and the charger for your cell phone most likely require AC (though they could be designed to run on DC, they generally aren't).

The only equipment that works equally well on AC and DC is incandescent lights (old style bulbs with a tungsten filament inside) and "universal" series-wound motors (AC motors with brushes are this type).  A power tool that has brushes and no electronics (soft-start or speed control) will probably run just fine on suitable voltage DC, though some will suffer switch damage over time.  Many hair driers are in this category, though often the "low heat" setting will just be "cold" or "high heat" because they use a diode to cut off half the AC wave to the heating element to reduce its output (which you get depends on the polarity of your DC power).  An older electric range will work (except that the clock will burn out almost instantly) -- resistance heating doesn't care about DC vs. AC.  A modern inductive cooktop will likely be destroyed, if it doesn't just fail to even start up (I'm not familiar with the electronics in those).  Radiant cooktops may or may not work -- the heating elements are probably fine, but they use electronic controls instead of simple switches like a 1970s vintage range; those controls might depend on AC input.

[wumpus]

20 hours ago, Zeiss Ikon said:

The only equipment that works equally well on AC and DC is incandescent lights (old style bulbs with a tungsten filament inside) and "universal" series-wound motors (AC motors with brushes are this type).  A power tool that has brushes and no electronics (soft-start or speed control) will probably run just fine on suitable voltage DC, though some will suffer switch damage over time.  Many hair driers are in this category, though often the "low heat" setting will just be "cold" or "high heat" because they use a diode to cut off half the AC wave to the heating element to reduce its output (which you get depends on the polarity of your DC power).  An older electric range will work (except that the clock will burn out almost instantly) -- resistance heating doesn't care about DC vs. AC.  A modern inductive cooktop will likely be destroyed, if it doesn't just fail to even start up (I'm not familiar with the electronics in those).  Radiant cooktops may or may not work -- the heating elements are probably fine, but they use electronic controls instead of simple switches like a 1970s vintage range; those controls might depend on AC input.

Except that everything that requires AC and isn't an induction motor requires it because there is a transformer between the the mains and the full-wave-bridge rectifier.  Of course, sufficiently old (non-switching) systems absolutely relied on that transformer to get the voltage right, but today it is often a bulky and unneeded relic (and there is the possibility of a half-wave rectifier from the unbearably cheap manufacturers).

I'm not expecting outlets to be wired this way, more likely things that are wired directly (such as a central heat pump or lighting.  But possibly including things that could have their plugs altered because of just how rarely they are moved such as refrigerators and ranges.

Power tools really shouldn't be using brushes.  They limit the tool's lifetime and are almost certainly more expensive than transistors.  You'd be surprised just how cheap a motor will be brushless.  Of course the issue with a transformer in the initial supply matters: DC won't pass it and simply bypassing it will give you the wrong voltage (and you probably need the inductance to make the rest of the power supply work).

[Zeiss Ikon]

52 minutes ago, wumpus said:

Power tools really shouldn't be using brushes.  They limit the tool's lifetime and are almost certainly more expensive than transistors.

I repair power tools for a living.  Brushes may cost more initially than transistors, but when you need to replace a $120 electronics unit (including the trigger switch and stator) to make your brushless drill work again, tell me how much cheaper transistors are than brushes (replacing worn out brushes might cost as little as $10 if you can unscrew the caps and swap old for new yourself).  In a world where tool abuse is about as common as showing up for work, brushless tools are not (yet) cheaper than tools with brush motors, in terms of cost per year of operation.

That said, brushless tools are becoming nearly universal in battery powered tools, but running brushless from (universally available) AC requires either rectifying and regulating voltage, or stepping down and rectifying to produce power that the control electronics can use, even if they're sending line voltage to the stator coils.  A couple manufacturers have produced brushless AC powered tools, but so far, they are less powerful and more expensive than conventional brush-motor tools in the same class -- and, again so far, they aren't as robust.

If you had high power (high voltage, or high current capacity) DC in your house, brushless might be the way to go, though most everything you already own would likely be permanent magnet brush motors, because brushless haven't been mainstream long.