Hacker News Reader

Comment by xanderlewis | Hacker News Reader

Why are toasters better at 240V? Can’t you just pull more current if you’re only at 120V (or whatever it is in the US) and get the same power?

I guess there’s some internal resistance or something, but…

View on HN

Correct. You can get the same power with half the voltage by doubling the current.

The trouble is the wires. A given wire gauge is limited in its ability to conduct current, not power. So if you double to the current, you'll need to have roughly twice as much copper in your walls, in your fuse panel, in your appliance, etc.

Additionally, losses due to heat are proportional to the current. If you double the current and halve the voltage, you'll lose twice as much power by heading the wires. For just a house, this isn't a lot, but it's not zero.

This is why US households still have 240V available. If you have a large appliance that requires a lot of power, like an oven, water heater, dryer, L2 EV charger, etc, you really want to use more voltage and less current. Otherwise the wires start getting ridiculous.

This is not to say that higher voltage is just necessarily better. Most of the EU and the UK in particular has plugs/outlets which are substantially more robust and difficult to accidentally connect the line voltage to a human. Lots of people talk about how much safer, for instance, UK plugs/outlets are than US plugs. If you look at the numbers though, the UK has more total deaths per year to electrocution than the US, despite the fact the US is substantially more populous. This isn't because of the plugs or the outlets, US plugs really are bad and UK plugs really are good. But overall, the US has less deaths because we have lower voltage; it's not as easy to kill someone with 120V as 240V.

So there's a tradeoff. There is no best one size fits all solution.

Your deaths claim surprised me. AFAICT England has ~10 deaths by electrocution per year. The US has ~100 domestic electrocutions and even more occupational electrocutions.

How many of those deaths are attributable to electrical plugs, though? Given the US CPSC report at [1], I suspect that most of them aren't - in fact, one of the leading causes of electrocution deaths is "fractal wood burning".

[1]: https://www.cpsc.gov/s3fs-public/Electrocutions-2011-to-2020...

Hard to compare. Does not US allow people to work on their own outlets etc in their own house while in UK you need to hire an electrian.

In 2017, there were 13 electrocution-related deaths in the UK. In the US, there are between 500 and 1,000 electrocution deaths per year. This translates to 0.019 deaths per 100,000 inhabitants in the UK and between 0.149 and 0.298 deaths per 100,000 inhabitants in the US.

Note that there is 240V in every US home. Low power loads run from a center tap 120V circuit. Also I wonder if people manage to electrocute themselves on "high voltage" circuits (typically 7.5KV) which due to the 120V thing have to be run everywhere so are more available to the casual electrocutee. In the UK although there are high voltage transmission lines, they terminate at substations, are often buried at that point, and there are high fences make it hard to electrocute ones self. E.g. a guy around here managed to electrocute himself very badly (but still survived) out in the woods by touching a bear that had itself become electrocuted by a live unprotected 7.5KV line.

This is a very well written comment overall, but the energy loss in the wire is even worse than stated!

By modelling the wire as an (ideal) resistor and applying Ohm's law, you can get P = I^2*R. the power lost in the wire is actually proportional to the square of current through it!

Therefore, if you double the current, the heat quadruples instead of doubling! You actually have to use four times the copper (to decrease resistance by 4x and get heat under control), or the wasted energy quadruples too.

Crucially, voltage is not in the equation, so high voltages - tens or hundreds of kilovolts - are used for long distance power transmission to maximise efficiency (and other impedance-related reasons).

I was also surprised to read that heat is proportional to the current. In addition, increasing the temperature also increases the resistance in the conductor (Cu). It's around 0.4% per 1C for Cu, around 20% more heat at 70C.

Not sure about US, yet some high current lanes (thinks of threephase ~400V x 36A; IEC 60502-1) in the households are actually made of Al, not Cu. They tend to be underground though, the wires in the walls are still Cu.

Al wire is used a lot more than most people think. Here's the big differences between Al and Cu wire.

Cu is more conductive than Al so an Al wire has to have a cross section area about 1.56 times that of a Cu with the same current capacity.

But Cu is also denser than Al so the Al wire is only about 0.47 times the weight of the Cu wire.

Al is is much cheaper than Cu so the Al wire is only about 13% the cost of the Cu wire.

Al wire is prone to oxidation and thus needs an antioxidant paste applied at connection points.

Hasn't anyone tried using silver wires?

Silver wire is used for some things, but it is a lot more expensive than copper which rules it out for most applications.

Here is a table of the conductivity (in units of 10^7 S/m), the density, and the cost of copper (Cu), aluminum (Al), silver (Ag), and gold (Au).

               Cu    Al      Ag       Au
  Conductivity 5.96  3.5     6.3      4.1
  g/cm^3       8.96  2.6    10.5     19.3
  $/kg         9.03  1.2  1030    92100

If we had a copper wire with a specified capacity in amps, here is what aluminum, silver, and gold wires of the same length and capacity would weigh and cost as a percentage of the weight and cost of the copper wire, and what their diameter would be as a percentage of the diameter of the copper wire.

      Weight  Cost  Diameter
  Al    49       7    139
  Ag   110   12646     97
  Au   310 3190000    121

Suppose Ag was as cheap as, or even cheaper than Al, would it be more useful?

Sure, we could alloy and/or plate it. You have a source on Ag cheaper than Al you'd like to disclose to the class?

Silver and gold could be a lot more useful to society if they weren't so expensive. Plenty of asteroids out there to mine.

I don’t know is toaster are close to max power draw, but kettles certainly are.

Most places with 240V regularly have 16A sockets, allowing a maximum draw of 3840W of power. That’s the limit. Cheap fast kettles will often draw 3000W and boil 250ml of water at room tempature in 30s.

Kettles in the US are often limited to 15A and thus max 1800W (usually 1500W) and take twice as long (60s)

Technology Connections has a great video on this: https://youtu.be/_yMMTVVJI4c

I mention Impulse Labs and their battery-assisted 120V high power induction range in the comments elsewhere. Seems like a similar concept could be used to make an incredibly powerful kettle; throw in a battery that charges from the mains, and when you ask to boil, send in 20kW and boil the 250ml in 4 seconds.

    4.18 J/g/C * 250g * (1/ 20,000 kJ/s) * 75C = 3.918s

For that order of magnitude to work, in practice, the most challenging aspect will be getting enough surface area between the water and heater.

Otherwise, you will very quickly vaporize the water near the heater and the resulting lack of contact will inhibit heating the rest of the water volume.

Yeah that's a great way to start a fire with a lot of steam first :)

Microwave radiation could work to transfer heat in even as boiling initiates in spots. All the best kettles are BIS dual use items.

If you can transmit that amount of heat that quickly, I think it'd be much more convenient and feasible to have it in the form factor of an instant-boiling-water spout next to the sink, rather than a kettle. Then, rather than having to fill the kettle first, you directly get the amount of hot water you need into the vessel you want it in, and you can put a precise amount of heat into a precise amount of water flowing through a pipe to emit it at the right temperature.

By the way, you can already have a boiling water tap today, you just buy a device that uses hot water tank to store the energy you rather than the battery. Insinkerator sells these. It might not be as energy efficient as the hypothetical tankless water boiler as described by you, because you have some losses from the heat slowly leaking away from the tank, but given the battery costs, I suspect that over the lifetime of the device, these losses add up to less than what battery costs.

Yes — in fact this is a problem with the high-power setting on our induction hob (which I think is something like 5kW). Liquids bubble vigorously before they're even warm.

Houses are wired for 16A per circuit on both sides of the pond, with high-power appliances typically pulling around 10A to avoid tripping the fuse when something else is turned on at the same time. It's just a nice point where wires are easy to handle, plugs are compact, and everything is relatively cheap.

The US could have toasters and hair dryers that work as well as European ones if everything was wired for 32A, but you only do that for porch heaters or electric vehicle chargers.

No, the standard in the US is 15 or 20A. 20 is more popular nowadays.

240V appliances typically get a 35 or 50A circuit.

But then you also have to deal with the fact that a lot of homes have wiring that can only handle 10A, but someone has replaced the glass fuse with a 20A breaker. Fun stuff.

I still haven't seen a single 20A domestic appliance though

I have, though it's semi-prosumer equipment. The largest UPS systems for standard systems, like someone might want for a small home-lab rack, can be found with 20A 120V plugs that work in normal outlets; if they're on a 20A rated circuit like a kitchen refrigerator or bathroom sink outlet (the two most common sorts in rental units).

I suspect some beauty products might also use 20A, or in combination easily reach that.

Very common in light commercial applications but almost unheard of in residential because nobody installs nema 20a sockets in houses even if the circuit can handle it

More current needs thicker wires. The average US outlet is wired for 120v15amp. 20 amp circuits are somewhat common, though 20amp receptacles are not. Certainly not enough for commodity appliances to rely on.

Going to more than 20amp requires a multiphase circuit which are much more expensive and the plugs are unwieldy and not designed to be plugged and unplugged frequently.

> Going to more than 20amp requires a multiphase circuit

There is no multi-phase power available in the vast majority of US houses. A typical residence has a 120/240 split-phase service, which is single-phase only. A service drop is two hot conductors from two of the three transformer phases and a center-tapped (between the two hot legs) neutral conductor. Either hot leg is 120v to ground and line to line is 240V.

> https://en.m.wikipedia.org/wiki/Split-phase_electric_power

Single-phase breakers are also available in sizes larger than 20A, usually all the way up to 125A.

Having more current running through a wire means thicker wires. Higher voltage means less current to achieve the same power, so thinner wires for the same power. The tradeoff for higher voltage is it's more dangerous (higher chance of arcing etc).

You need thicker wires for the same power. Which is why Americans live in constant fear of power extension cords, and people in EU just daisy-chain them with abandon.

If you're in a country that uses type-G plugs, almost* all of those extension cords have fuses that break well below the current that will cause a problem.

* Currently using a cable spool which will have problems before blowing the fuse if it's wound up and I draw too much current. It has a thermal cutoff, but I still unspool some extra wire on the floor.

You need the same thickness of wire for 10A regardless of which voltage you have. So with 230V, your 10A wire will let you draw 2.3 kW while someone with 120V and 15A wire would only get 1.8 kW and pay more for the wiring.

You have a 240v toaster?

Well, closer to 230.

goes to check real quick between 238V and 245V at my outlets.

Living in an a country with 240v mains, yep.

Reply on news.ycombinator.com