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I'm not sure if there's a better place to ask this or not.

Why, at least here in America, is the standard electrical system set up for alternating current (AC) rather than direct current (DC)? I'm just completing a long-distance move and realizing how many DC adapters I have and how many of my devices require DC power:

  • All of my guitar pedals.
  • My alarm clock.
  • My external hard disks.
  • My USB car HDD dock.

... and the list goes on. If we're almost constantly having to use DC adapters to convert AC into DC, why isn't the standard DC, other than because people resist change?

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Note that your devices take a variety of input voltages. 5, 9, and 12 VDC are the most common. –  Jay Bazuzi Jul 4 '11 at 20:34
I've always wondered why we don't see parallel DC wiring in newer homes. A whole-house DC power supply. –  DA01 Jul 4 '11 at 20:55
Because Nikola Tesla and Westinghouse, beat out Thomas Edison in the War of Currents –  Tester101 Jul 5 '11 at 11:31
They are starting to build large computer data centers with a large AC-DC converters, then DC distribution to the individual machines. I wouldn't be surprised to see this technology start to make its way into homes at some point. –  KeithB Jul 5 '11 at 16:28
If you want the drunk version of Tesla vs. Edison: funnyordie.com/videos/ef668caf14/… –  Robert Durgin Nov 24 '11 at 0:42
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3 Answers

up vote 33 down vote accepted

Transmission/Functional reasons:

  1. AC is cheaper -- There is less power loss from generator to end user.
  2. High voltage is cheaper to transport -- Power loss is I^2 * R. R is a constant over any line. For the same amount of power (watts) P = IV. So we can decrease the power loss by increasing the voltage. - This leads us to transform-ability. I can easily run AC through a transformer to jump the voltage up or down (with a corresponding change in current). Doing so with DC requires several more steps.
  3. [addendum] If we were just lighting and heating with electricity, we wouldn't care AC/DC from a functional point of view. However, lots of things in our houses use motors, and AC motors are cheaper and last a lot longer than DC motors. However, when we were first wiring neighbourhoods, this wasn't a factor.

Safety reasons:

  1. DC Kills. If AC is a loaded pistol, DC is a loaded sniper rifle on a hair trigger. It takes so much less DC current to kill you then it does AC. A 15 amp AC circuit can give a nasty shock. A 15 amp DC circuit will kill you outright. EDIT: A bit more research shows that this is extremely debatable. Rather than delete it, I will put in this disclaimer.
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If there's a surge which hits a DC adapter, will it likely fry or blow up the adapter rather than the electronic, ie is that another safety concern? –  Naftuli Tzvi Kay Jul 4 '11 at 17:23
That's just due to the small size of the adapters. They're rated for 300-500 mA, and a voltage surge will drive up the current, and melt the transformer coil. You could buy better (more expensive) adapters that would be a bit more tolerant. The cheap solution is just get a power bar with surge protection. –  Chris Cudmore Jul 4 '11 at 17:27
The ability to step voltage up and down is key. –  Jay Bazuzi Jul 4 '11 at 20:33
Could you explain how DC is more dangerous? From my (admittedly limited) understanding of electricity, it's not whether something is AC or DC but simply the amperage that's the key variable in how deadly it is. –  DA01 Jul 4 '11 at 20:54
Your "safety reason" blurb isn't debatable, it's incorrect. According to IEC 60479 - Effects of current on human beings and livestock it takes about 4x as much DC current to kill a man than AC. This was also one of Tesla's pluses about DC during the "War of Currents." –  BlueRaja - Danny Pflughoeft Aug 6 '13 at 21:19
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To explore the voltage transformations further:

  • To change voltage in AC requires a transformer. Basically, two coils of wire and a chunk of metal.

  • To step voltage up in DC, first invert to AC, then run that through a transformer, then convert to DC. The resulting DC won't be smooth unless you add more electronics. Each step has some waste as heat.

  • Conductor size is proportional to current, commonly called "ampacity". More current requires thicker wires. Voltage drop is a factor of both current and distance. So, longer wires must be thicker. Thicker wires are harder to work with and more expensive, so higher voltage / lower amperage is vastly advantageous.

(Insulation is proportional to voltage, as is danger, so high voltage isn't a slam dunk.)

  • AC lends itself to multi-phase distribution. This reduces total conductor size (cheaper, easier to work with).

Multiphase is good for electric motors.

American homes typically get 240V split-phase + a neutral from the street transformer. Heavy devices (e.g. an oven) can work on both hots, for 240V. Light devices (my laptop) can work on one hot + the neutral. It works out nicely. See also: Multi-wire Branch Circuits.

Case study: My RV has a 12VDC power system. Wires have to be thick because amps are high. If I short my wedding ring, it will melt. We want to power big loads, like furnace blowers. RVs would benefit from 24VDC or higher, but we need mainstream cars to switch first; RVs will follow.

Case study:

Photo-voltaic (solar panel) installations on rooftops suffer because they produce relatively low-voltage DC. If there's a long wire run from the PV array to a battery bank or inverter, it loses a lot of energy as heat. Some are moving towards "microinverters" where each panel gets its own inverter on the roof. This reduces transmission losses.


I've heard there would be problems with ground loops if distributed DC to our stereo equipment, but I haven't managed to wrap my head around it.

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DC in cars: there have been several efforts over the years to raise the standard DC voltage in a car's electrical system. The most recent attempt was 48V. Some trucks and busses already run at 24V, which makes the cigarette lighter socket a problem if something is expecting 12V. –  staticsan Jul 5 '11 at 7:25
Some airplanes are at 24v DC now, yet have 12v cigarette lighter sockets... If they can afford the weight to do 24-12v DC conversion on an airplane, I'd be truly shocked (pun intended ;-) ) that they don't have it on the 24v trucks... –  Brian Knoblauch Jul 6 '11 at 12:27
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All of the answers to the question are correct. Basically, when Edison was first developing electrical generators on a power-grid scale, he hired Nikola Tesla as a protege, and Tesla, it is claimed, used the principles of alternating current and polyphase power to greatly increase the efficiency of electrical generators, which by Edison's original designs produced DC.

Basically, the big deal is that AC requires less work for more power (i.e. it is more efficient to generate). Think of an electric current in terms of a closed, pressurized water loop; water is put under pressure by some power source, which causes it to flow through hoses to some gadget that can use the flow of water to do mechanical work. The water, its energy spent, is then drawn back to the power source.

DC would be the equivalent of putting pressure on the water in one direction only, either by feeding it from a tank (similar to how a battery would work) or by using an impeller or other rotating pump (similar to a generator). Such a pump would move water inefficiently, as the pumping mechanism cannot be watertight. A one-way reciprocating pump would be watertight but would not move water constantly, which may be overcome (as in AC to DC converters) using a reservoir that will store additional pressure and then feed it in to the system while the pump is on its "backstroke". Any way you slice it, except in the case of a tank (battery), there is wasted effort in producing the current.

AC, by contrast, would be the equivalent of using a simple reciprocating pump to force water one way, then the other. As long as the devices expect the flow of water to reverse itself (or don't care), the design of the generator can be much simpler, and more efficient. The reasons for the efficiency gains are a little different when you do away with the analogy, but the analogy itself holds pretty well.

AC also has a few tricks up its sleeve that DC simply cannot replicate, which make it preferable to DC for large-scale applications. Perhaps most important is the ability to be "stepped up" as well as "stepped down" using a transformer. DC can only be "stepped down" using resistors, which basically transform electrical energy to heat and thus cause you to waste a lot of energy. Polyphase power, seen in the U.S. as 3-phase power, is more a solution to a problem of AC than a benefit (Three-phase AC allows the power grid to have near-constant overall voltage, overcoming the non-constant voltage of a single AC waveform, while using less wire than would be required to efficiently transfer the same overall power in a single waveform), but it provides the beneficial side effect of being able to "add" phases to each other for the same available current. In split phase, voltage is doubled while in 3-phase, voltage is multipled by √3. This is why residential is 120/240V (120 * 2) and commercial 120/208 (120 * √3).

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