It has to do with the high voltage and the impact of voltage loss.<p>† I&#x27;ve accumulated a lot of stuff I&#x27;m glossing over at the end.<p>Let&#x27;s consider a 1 square millimeter cross section wire. Your power lines will be bigger, but it scales up with the area.<p>Electricity facts refresher:<p><pre><code>  ohms measure resistance to electricity movement
  amps measure how many electrons per second you are jamming down a wire
  volts measure how hard you are jamming the electrons down the wire
  watts measure power, this is ultimately what you care about

  volts = amps * ohms
  watts = amps * volts

  this is true for AC and DC
</code></pre>
Your copper wire has a resistance of about 17 ohms per kilometer. This wire is allowed to take 3 amps of current when used for power transmission. That makes the voltage drop over a kilometer be 3 * 17, 51 volts.<p>This voltage drop doesn&#x27;t depend on the voltage of your power line, just of the current running through it. If you are Mr. Edison with a 110 volt generator, then you have lost almost half of your power just transmitting it one kilometer.<p>If you are the Pacific DC Intertie sending power from Washington state to Southern California, you are operating at 1000000 volts. You are losing 0.005% of your power per kilometer.<p>The whole AC&#x2F;DC thing was because in the early 20th century it was easy to change the voltage of AC power but difficult and expensive to change the voltage of DC power. You could pump AC up to higher voltages for transmission, and bring it back down for domestic use.<p>␄<p>† Notes follow:<p>The current limits I used for the wire are for common electrical engineering work on devices, I don&#x27;t know the limits for power transmission lines, but…<p>I sort of doubt they use copper for transmission lines, too expensive, so the resistances will be higher. Wikipedia to the rescue: The Pacific DC Intertie uses aluminum wire, reinforced with steel, with a 644 mm^2 cross section. (Not clear if that is the aluminum of the whole thing.) They are also pushing 3100 amps which puts us in the ballpark, 1000 times the current in 644 times the cross sectional area. They are operating above the power transmission guideline I picked but below the limit for chassis wiring.<p>Also for transmission lines, the much thicker wires will not be able to dissipate heat as well as the tiny wire I used and there will be a derating of their current capacity from that.<p>High frequency AC gets weird, it travels on the outside of the wire and the cross sectional area doesn&#x27;t scale, but 60Hz isn&#x27;t going to do that much.<p>If you think &quot;I know, I&#x27;ll wire my home&#x2F;datacenter&#x2F;yacht with 12v DC power!&quot; Then you <i>really</i> care about voltage loss. If you want to move 2000 watts 10 feet with low loss you will be using two copper conductors about the diameter of your thumb. This is why you see 48v used on things like Power Over Ethernet. That is the &quot;low voltage&quot; limit for some regulatory agencies, and they want to get as much power of the tiny conductors as possible.