Why kettles boil slowly in the US

I saw a tweet recently that intrigued me:

The voltage of mains electricity varies from country to country: the majority of countries use between 200 and 240 volts, but a small minority (most notably the US, Canada and Japan) use between 100 and 127 volts.

Countries using 100-127 volts are shown in red; countries using 200-240 volts are shown in blue. Countries with a mixture of the two systems are shown in purple.

The voltage* of an electrical supply is what pushes electrons around in a circuit. The higher the voltage, the faster the electrons move and thus the higher the current (one amp is equivalent to about six billion billion electrons flowing past a point per second). With a low voltage the rate of transfer of electrical energy is therefore much slower. In the UK, with a mains voltage of 230 V and a limit of 13 A per socket the maximum possible power to one appliance is 2990 watts (2990 joules per second). In the USA, with a mains voltage of 120 V and a limit of 15 A per outlet the maximum possible power is reduced to only 1800 watts, which is why in the US many large appliances (e.g. washing machines, tumble dryers) have to be connected to a separate high-voltage circuit.

To raise the temperature of one litre of water from 15°C to boiling at 100°C requires a little bit over 355 kilojoules of energy. An “average” kettle in the UK runs at about 2800 W and in the US at about 1500 W; if we assume that both kettles are 100% efficient† then a UK kettle supplying 2800 joules per second will take 127 seconds to boil and a US kettle supplying 1500 J/s will take 237 seconds, more than a minute and a half longer. This is such a problem that many households in the US still use an old-fashioned stove-top kettle.

* As a physicist I would normally use the term “potential difference” in place of “voltage” but voltage is better understood by the general public. Looks like the engineers (who prefer “voltage”) won that battle.

† As electric kettles actually use the joule heating effect that is responsible for most of the energy wasted in other electrical devices this isn’t a terribly unfair assumption.

17 thoughts on “Why kettles boil slowly in the US

  1. That’s scratched an itch I didn’t know I had. Just last night I noticed a character on Homeland put an old-fashioned kettle on the gas stove in their sleek modern kitchen and mentally filed it away as a clue to something tea-related that might be picked up later. Thank you!

  2. It might be worth mentioning altitude as well. In Johannesburg, water boils at 98°C (208°F) rather than the sea-level 100°C (212°F). I believe water boils at 88°C (190°F) in La Rinconada, Peru which is 5100 metres (16728′) above sea level according to Wikipedia.

  3. In countries like the US where more current is needed per Watt, I guess the wire conductors must be thicker and more expensive (copper is pricey these days!) than they would be elsewhere where voltage is higher. The upside for people in the US might be that you have a better chance of getting away alive from an electric shock.

  4. Yes. For two devices, one in the US and one elsewhere with 230 V supply the US device would require more current and thicker connectors. It would certainly be interesting to look at rates of death due to electrocution in US v 230 V supply countries.

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  6. I’m not sure about death rates, but on 240V we have a very smart feature in Australia… individual on/off switches for each plug. I’m moving back to the US after 12 years here and cannot think of a day without my electric kettle… I guess I’ll just have to wait a bit longer for that boil.

  7. We have the same system in the UK. I find it really weird that the US doesn’t have individual on/off switches.

  8. I thought about this problem after visiting the UK for the first time 10 years ago. I was so addicted to the fast boil, I wondered if a US electrician would be able to wire up a UK plug so that we could use your kettles? Would that be possible?

  9. There’s a saying in the electrical engineering community: Volts jolts, but mills (milliamps) kills. That is, a high voltage will shock you (static shocks when you rub your feet over carpet tend to be a few thousand volts), but what will kill you is a high current. For two equal-power appliances, the higher voltage, lower current form used in Europe is probably marginally safer (electrical power is the voltage multiplied by the current).

    More importantly, you CAN run a European kettle off a North American mains line with some effort. The NA mains system uses a split-phase scheme, with two live wires each at 120 V but 180 degrees out of phase, and then a neutral lead at 0 V. You can rewire things to use the two live wires instead of live and neutral, allowing you to produce a 240 V socket. Electric cookers, boilers and other high power devices are usually connected this way as standard – I believe this is how the high voltage circuit mentioned in the article is achieved, leaving the neutral to serve as ground.

    Since Europe uses a 230 V three-phase scheme (and a different frequency), this is almost certainly an inadvisable hack for most appliances, but a kettle circuit is sufficiently simple and robust that it is safe provided you use a suitably fused plug.

    a quick google found this: http://wiki.answers.com/Q/Can_you_plug_a_230V_50Hz_appliance_into_a_240V_60Hz_outlet

    as Martinel writes, it is essential that you include a 13 A fuse somewhere along the kettle lead as the house circuit breakers will be for much higher current. Since the current draw is controlled by the resistive load of the appliance itself, it should function perfectly unless the grid supply has a power surge.

  10. * As an engineer I have to point out that the speed of electrons in a circuit (close to the speed of light) are independent of voltage. Increased voltage increases the number of electrons not the speed. For a given heating element that added number of electrons would “speed up” the thermal vibrations of your kettle heating element (it gets hotter) which in turn speeds up the time you have a cup of tea in your hand. It is more likely though that the 220v heating element is twice as long (or wide or high) and there is no speeding up of the thermal vibration at all, there is just more surface area being used to transfer the thermal energy to the water (which speeds up it’s thermal vibration which us humans measure as Ouch! That’s Hot!)

  11. As a physicist, I have to point out that electrons travel in a circuit at nowhere even close to the speed of light. Drift velocity is usually of the order of a millimetre per second. Increasing voltage increases current, and increases the amount of energy possessed by each charge. I’m not really sure what the point you’re getting at is.

  12. The point I was making was that your statement “The higher the voltage, the faster the electrons move” is inaccurate. Yes, the drift velocity is really slow but it increases with current not voltage. The speed of the electrons (Fermi speed) is dependent only on the material (copper) and that is really fast. Your statement might lead some one to believe that when they hit the circuit with a higher voltage the speed of the circuit increases. It does not, the speed of the circuit is about 50% of the speed of light and does not change. It is as you last stated, “Increasing voltage increases current, and increases the amount of energy possessed by each charge”. The problem is that your example is using similar currents (13amps @ 220v) and (15 amps @ 110v). There is no increase in current, in fact there is a decrease in current going from 110v to 220v thus you actually have a slower drift velocity.
    The correct wording would be something like “The higher the voltage the faster the transfer of electrical energy (not faster electrons)

  13. Actually, I’m pretty happy with it as it is.

    At constant resistance an increase in voltage causes an increase in current and an increase in current is an increase in the rate of flow of charge (i.e. the drift velocity). I don’t care about the actual speed of the electrons within the wire, but only the speed at which they move down the wire.

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