If DC adapters are so prevalent, why is AC used in construction by default?

Question

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?

Answer 1

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.

Answer 2

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.

Aside:

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.

Answer 3

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).

Answer 4

You are correct. And if that interests you, I encourage you to innovate in that area.

All the things you listed are extremely small loads, smaller than 10 watts. Many of them want 12 volts DC directly. You would not be wrong to put a second 12V electrical system in your house that runs those small loads.

Those would include the following, and also consider this: it's surprisingly easy and cheap to provide battery back-up for that 12V system, topped up by solar power. Now those parts of your house are blackout-proof.

• Lighting -- LED lighting makes this easy.

• Chest freezer - modern Energy Star freezers are so efficient that most off-griders don't bother with special 12V freezers anymore, and run a common (but well chosen) freezer off an inverter.

• Ditto Refrigerators, but they tend to be a much bigger load since their insulation is thinner and their doors are opened a lot more often. That may require upsizing the system considerably.

• Sump pump

• Radon air handler

• Internet routers - most of them are already 12VDC. The telephone company's infrastructure has massive battery back-up; cable TV can't say the same.

• Phone/tablet charging: use car chargers, sold at every gas station.

• TV - many TVs allow 12V input.

• Charge battery-powered workshop tools.

• Thermostats and the relays they control.

• Heat: add an auxiliary wall or floor furnace that doesn't require electricity (not even for the remote thermostat).

• Hot water: Gas hot water heaters require very little (or no) electricity. On-demand gas heaters use a small amount, but only when you're using them.

You see where this is going: in a blackout you can be cozy, warm and watching Netflix.

Here are some loads you can't easily run off a 12V system because the energy requirements are too large.

• Air conditioning and dehumidifying

• Forced-air furnaces which are, paradoxically, pretty much every system in the snow-belt. This is why it's so hard for people there to do blackout protection this way, because this fattie is at the top of the "critical load" list. Those no-electricity furnaces aren't even sold in the snowbelt!

• Using electricity to make heat (house heating, water heating, drying or cooking)

• Clothes washing and gas drying (the motor loads are considerable)

• Dishwashing (notably the water-heating and heat-drying parts)

• Power tools

Larger 12V systems can handle it: The wiring can't and here's why: Power (watts) is volts x amps. Volts go down, amps go up. Amps decides wire size, which quickly hits impractical numbers. Your 240V/30A air conditioner becomes 12V/600A. I've hooked up 600A electrical service, the wires are HUGE and very expensive ($60/foot). Doesn't work. Even a 1500W hair dryer (now 12V/125A) requires essentially welding cable.

A larger 12V system will invert to 120/240V right at the battery, and distribute around the home with normal wiring.