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Comment by rwmj | Hacker News Reader

I would love to have my house wired with 24V (instead of 240V).

At the very least it would mean I won't one day be electrocuted by my power-shower.

A 240V mains socket (depending on jurisdiction) is expected to provide 3.6KW. That requires 15 amps of current, and (here in America) 14 AWG wire, 1.628mm thick.

To source the same amount of power at 24V, you need ten times as much amperage, 150A, and 1/0 AWG wire, or 8.252mm thick. Much, much thicker, dramatically more expensive, and harder to handle.

And even with 1/0 AWG wire, you'll see a lot of voltage droop between the main panel and the outlet, which will be tens of metres away. A 20 metre run, at 24V and 150A will drop 7.5% of its voltage between the panel and the outlet.

The reason why you don't really want that is because the line losses over even very short runs of copper wire at those voltages are huge. You'd need extremely thick wires (costly) to deliver usable power.

That's why houses that are powered by solar/wind backed up with batteries convert the battery voltage back to household AC, it's not just a convenience, it is also for reasons of efficiency.

Power delivered is Amperage x Voltage, if you want to consume 500 watts at 120V that's a bit over 4 amps which you can do with relatively thin wire, if you want to do it at 24V you'd be using 20 amps, requiring a 5 fold increase in diameter of the wiring! And that's just the beginning, then there are the troubles associated with DC breakers as well as wear and tear on a plug-in system designed for currents that high. Imagine a toaster or a water cooker designed for 24V, you'd need cables thick enough to do double duty as jump-starting hook ups.

I just wish there was a way to get 12V & 5V at ~1A in addition to the 110V outlets in my house, supplied by a large transformer in my basement. That way I could avoid wall-warts for all my electronic devices like my router, usb hub, phone charger, etc...

Even better would be a standard for 12V at 5A or 10A or something, to power all the other electronics in my house, like monitors, DVD players, etc, that have built-in transformers. But I'm not an EE, so maybe the losses from dozens of feet of low-voltage in the walls outweighs the efficiency gains from having a single large transformer...

This is a start: http://www.engadget.com/2009/12/05/10-usb-power-outlet-leave...

Unfortunately, it's a crapshoot to find outlets that don't leak power. Look for the ones with a mechanical switch to disconnect the USB ports.

You'd lose a lot of energy in the cables in the walls of your house that way.

The same reason they use very high voltages to transport the electricity and only step it down to 240 as close to the end-user as feasible.

I won't bore you with the physics, but you want to keep the amps as low as possible (which can be done by keeping the voltage high).

It is perhaps worth noting that 12/24V DC is just as deadly as 120/240V AC. There's naught deadly in voltage, it's the wattage that electrocutes you. A static shock is 6kV, as I recall, but the amperage is low enough that it doesn't hurt at all.

I'm a little sketchy on the details (been a while since I studied this sort of thing) but I can personally attest to losing muscle control in my right arm on a 6V circuit using only a standard (1Ah?) alarm battery.

> It is perhaps worth noting that 12/24V DC is just as deadly as 120/240V AC.

No it definitely isn't.

> There's naught deadly in voltage, it's the wattage that electrocutes you.

In fact, if you want to be precise, it is the current (Amps) that kills you. The reason why 24V isn't nearly as deadly is because at the resistance your skin normally has the current will be low enough that it does not get dangerous.

Up to 48V is considered a 'safe' voltage, but you can definitely feel it and DC is far more dangerous than AC (because with DC your muscles contract and stay contracted so it can be very difficult to disengage from the source).

If you lost muscle control over 6V then likely there were some other circumstances in play, for instance you could have been poking around in a spot where your skin had been breached or you may have had sweat or some other salty solution on your skin.

Real danger starts around 80V for DC and 150V for AC. Yes, you can get electrocuted at lower voltages but you'd have to really work at it.

Most high voltage circuits that you are likely to come in contact with have an internal resistance that is high enough that they can not source a high enough current to do damage, that's why you perceive high voltages as safe. But if a high voltage circuit is beefy enough that it can supply a couple of hundred milli-Amperes you're as good as dead because at that voltage your skin is no longer a strong enough barrier.

I recall an apocryphal story of a guy who got killed by a 12V car battery. It was a hot day, and he was trying to pick it up, but it was heavy, so he was using a couple of pairs of Vise-Grips. On the terminals.

Short the terminals of a 9V battery with your finger. Now unscrew an outlet cover and short the terminals within with your finger. Notice the difference. (Note: don't actually do the second one.)

People love love love to repeat this tired "it's not the volts that kills, it's the amps" line, while completely ignoring the fact that low voltage makes it really, really, really hard to obtain the necessary amps to be dangerous.

I agree entirely. Nonetheless, here's an interesting story about someone being killed by 9 volts: http://www.darwinawards.com/darwin/darwin1999-50.html

It's interesting both because 9V killed the guy and because he had to try pretty hard (stabbing his thumbs) to achieve it. So, while dangerous, not nearly as dangerous as e.g. 110VAC.

In my experience the overlooked risk is burns. People think low voltage dc is safe(er) (no electrocution) because of high skin resistance, and are amazed when there are a shower of sparks from a car battery (when something goes wrong like a tool falling across the terminals). In high school I did welding with capacitors that looked like pyrotechnics using a butter knife fom the cafeteria.

On a side note, I think I read women have lower skin resistance than men on average.. Something we were going to do a little design of expirment for in highschool but never got around to completing it because shocking people and playing with electricity was more fun.

Ohh.. According to this men have lower resistance. http://van.physics.illinois.edu/qa/listing.php?id=6793

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