# Minimal fill levels on electric kettles **URL:** \ **Category:** Factual Questions\ **Created:** [June 1, 2022, 7:36pm UTC](https://boards.straightdope.com/t/minimal-fill-levels-on-electric-kettles/965518 "2022-06-01T19:36:55Z")\ **Posts on this page:** 20\ **Page:** 3
**Author:** ![DPRK](https://avatars.discourse-cdn.com/v4/letter/d/4491bb/32.png) [@DPRK](https://boards.straightdope.com/u/DPRK)\ **Post date:** [June 3, 2022, 4:35pm UTC](https://boards.straightdope.com/t/minimal-fill-levels-on-electric-kettles/965518/41 "2022-06-03T16:35:20Z")
I am thinking that for a measly 1500W, you might as well use the gas burner. ---
**Author:** ![psychonaut](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/psychonaut/32/4655_2.png) [@psychonaut](https://boards.straightdope.com/u/psychonaut)\ **Post date:** [June 3, 2022, 4:45pm UTC](https://boards.straightdope.com/t/minimal-fill-levels-on-electric-kettles/965518/42 "2022-06-03T16:45:42Z")
> [@Martin\_Hyde](#): > > At least the kettle he is presenting, has a little bimetallic switch in the base, the different metals expand at different rates as the temperature changes, causing the disc in the switch to buckle, which will close/open the switch. This of course does respond to changes in temperature, but it isn’t quite based on 212F/100C–as his testing shows. In fact, it makes perfect sense that it **wouldn’t** be based on 100C–100C is the boiling temperature at sea level, but not everyone lives at sea level. Instead, the switch is designed to fire at a temperature significantly lower than the boiling point of water. Actually, this doesn’t make sense at all, because one of the most common use cases for kettles is to actually boil water. (A proper cup of tea, for example, requires boiling, not merely hot, water.) If the kettle shuts off before the water reaches 100 degrees Celsius, then it’s going to be useless for the majority of people living near sea level. Every kettle I’ve ever used, including in coastal cities, has actually brought the water to boil. I was under the impression that all of these kettles used a switch mechanism similar to the one you describe. ---
**Author:** ![Martin\_Hyde](https://avatars.discourse-cdn.com/v4/letter/m/47e85d/32.png) [@Martin\_Hyde](https://boards.straightdope.com/u/Martin_Hyde)\ **Post date:** [June 3, 2022, 4:47pm UTC](https://boards.straightdope.com/t/minimal-fill-levels-on-electric-kettles/965518/43 "2022-06-03T16:47:20Z")
The kettle does not switch off before the water reaches 100C. It shuts off when **boiling has occurred** , this is measured by hot vapor pushed through a tube passing over a bimetallic switch–the temperature sensitivity of the switch is much lower than 100C, but that doesn’t relate to the water temperature. Additionally, it is designed such that enough boiling must have occurred that sufficient pressure exists to push enough of the hot vapor into the tube to trigger the switch, which is why it doesn’t switch off the instant even the smallest bit has boiled, but typically at a healthy rolling boil. ---
**Author:** ![Teuton](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/teuton/32/3864_2.png) [@Teuton](https://boards.straightdope.com/u/Teuton)\ **Post date:** [June 3, 2022, 4:48pm UTC](https://boards.straightdope.com/t/minimal-fill-levels-on-electric-kettles/965518/44 "2022-06-03T16:48:27Z")
> [@psychonaut](#): > > Every kettle I’ve ever used, including in coastal cities, has actually brought the water to boil. Yes, they need boiling water (whatever temperature water boils at where you are) to shut off. If water boils at 80C where you are up in the mountains, it will cut off earlier not because it’s designed to cut off at a lower temperature, but because the steam generation activates the pressure switch. ---
**Author:** ![psychonaut](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/psychonaut/32/4655_2.png) [@psychonaut](https://boards.straightdope.com/u/psychonaut)\ **Post date:** [June 3, 2022, 4:48pm UTC](https://boards.straightdope.com/t/minimal-fill-levels-on-electric-kettles/965518/45 "2022-06-03T16:48:34Z")
Oh, I see what you mean, then! So the switch is inaccessible to the liquid; it’s only the vapour that can activate it. ---
**Author:** ![Martin\_Hyde](https://avatars.discourse-cdn.com/v4/letter/m/47e85d/32.png) [@Martin\_Hyde](https://boards.straightdope.com/u/Martin_Hyde)\ **Post date:** [June 3, 2022, 4:49pm UTC](https://boards.straightdope.com/t/minimal-fill-levels-on-electric-kettles/965518/46 "2022-06-03T16:49:56Z")
Right, they’ve essentially designed a kettle to detect boiling and respond to that, instead of worrying so much about temperature. But of course, as the video shows…the design isn’t universal, another kettle he looks at appear to have an actual thermal switch attached to the metal of the base right where the element is **and a pressure switch** , the first kettle doesn’t have that. He speculates the thermal switch may be an emergency kill switch if it detects the heating element itself is hitting some temperature threshold that might cause it to burn up. ---
**Author:** ![Folacin](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/folacin/32/3195_2.png) [@Folacin](https://boards.straightdope.com/u/Folacin)\ **Post date:** [June 3, 2022, 9:11pm UTC](https://boards.straightdope.com/t/minimal-fill-levels-on-electric-kettles/965518/47 "2022-06-03T21:11:50Z")
> [@DPRK](#): > > I am thinking that for a measly 1500W, you might as well use the gas burner. Per the video, you’re wasting a lot of energy with the burner (air gap, heat escaping around the kettle) and of course polluting your indoor air. And short of a range with a super BTU burner, even a 850W electric kettle is faster. ---
**Author:** ![Dr.Strangelove](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dr.strangelove/32/6613_2.png) [@Dr.Strangelove](https://boards.straightdope.com/u/Dr.Strangelove)\ **Post date:** [June 3, 2022, 9:30pm UTC](https://boards.straightdope.com/t/minimal-fill-levels-on-electric-kettles/965518/48 "2022-06-03T21:30:41Z")
> [@md-2000](#): > > Code says a circuit should not deliver more than 80% of it’s rated power continuously, so 80% of 15A is 12A A continuous load is defined by the NEC as one that goes for three hours or more. An electric kettle in typical use clearly isn’t running for that long, and should be able to run at the full 15 A. And indeed, you can find 1800 W electric kettles, though for some reason the majority seem to be \<=1500 W. ---
**Author:** ![crowmanyclouds](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/crowmanyclouds/32/19884_2.png) [@crowmanyclouds](https://boards.straightdope.com/u/crowmanyclouds)\ **Post date:** [June 3, 2022, 9:56pm UTC](https://boards.straightdope.com/t/minimal-fill-levels-on-electric-kettles/965518/49 "2022-06-03T21:56:33Z")
> [@Dr.Strangelove](#): > > {…} though for some reason the majority seem to be \<=1500 W. ~~The Simpsons~~ Technology Connections already did it, [![](https://img.youtube.com/vi/V-jmSjy2ArM/maxresdefault.jpg "Space Heater Nonsense") ](https://www.youtube.com/watch?v=V-jmSjy2ArM&t=544s) ---
**Author:** ![Dr.Strangelove](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dr.strangelove/32/6613_2.png) [@Dr.Strangelove](https://boards.straightdope.com/u/Dr.Strangelove)\ **Post date:** [June 3, 2022, 9:58pm UTC](https://boards.straightdope.com/t/minimal-fill-levels-on-electric-kettles/965518/50 "2022-06-03T21:58:30Z")
> [@crowmanyclouds](#): > > ~~The Simpsons~~ Technology Connections already did it, Again, the limit is 3 hours. An electric kettle is not expected to run for 3+ hours straight. A space heater is. ---
**Author:** ![crowmanyclouds](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/crowmanyclouds/32/19884_2.png) [@crowmanyclouds](https://boards.straightdope.com/u/crowmanyclouds)\ **Post date:** [June 3, 2022, 10:02pm UTC](https://boards.straightdope.com/t/minimal-fill-levels-on-electric-kettles/965518/51 "2022-06-03T22:02:08Z")
I’d assume they’re using that limit despite the fact that it doesn’t actually apply. One might imagine there’s a Chinese factory that just makes 1500W heating elements that get used everywhere. ---
**Author:** ![Dr.Strangelove](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dr.strangelove/32/6613_2.png) [@Dr.Strangelove](https://boards.straightdope.com/u/Dr.Strangelove)\ **Post date:** [June 3, 2022, 10:14pm UTC](https://boards.straightdope.com/t/minimal-fill-levels-on-electric-kettles/965518/52 "2022-06-03T22:14:33Z")
One somewhat amusing fix to the problem comes to mind: battery powered kettles. Now, anyone with some intuition about the heat capacity of water is probably thinking this is ridiculous, but battery cost has come down tremendously and the situation isn’t as bad as you think. Bringing 1 l of 10 C water to a boil requires 105 W-h of energy. A lithium-polymer battery with 110 W-h of energy (5 A-h capacity, 6 cells in series, 20C discharge) costs around $50 retail. That 20C discharge rate is equivalent to around 2.2 kW. That’s in addition to the 1.8 kW from the wall, making for a decent 4 kW total and bringing a liter of water to a boil in only 94 s. In fact, I’ve overspecced the battery capacity. Since it’s only supplying about half the energy, we can also boil 2 l of water in 210 s, with 1.8 kW coming from both the wall and the battery. Of course it takes a while to recharge, but presumably it can do so while you’re drinking your tea. ---
**Author:** ![Dr.Strangelove](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dr.strangelove/32/6613_2.png) [@Dr.Strangelove](https://boards.straightdope.com/u/Dr.Strangelove)\ **Post date:** [June 3, 2022, 10:29pm UTC](https://boards.straightdope.com/t/minimal-fill-levels-on-electric-kettles/965518/53 "2022-06-03T22:29:30Z")
> [@crowmanyclouds](#): > > I’d assume they’re using that limit despite the fact that it doesn’t actually apply. Probably. Never ascribe to malice what can be adequately explained by laziness. Maybe it takes more certification effort to demonstrate that the expected load is non-continuous, and so mostly they just use the lower limit to keep things easy. Though I wouldn’t be surprised about your theory of a factory pumping out 1500 W elements, either, ---
**Author:** ![bob\_2](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bob_2/32/3341_2.png) [@bob\_2](https://boards.straightdope.com/u/bob_2)\ **Post date:** [June 3, 2022, 11:03pm UTC](https://boards.straightdope.com/t/minimal-fill-levels-on-electric-kettles/965518/54 "2022-06-03T23:03:43Z")
> [@md-2000](#): > > Using the same logic for UK, as mentioned above, 13A\*80%\*240V = 2496W. I assume 13A/2500W is for special applications like our 20A? What’s a standard outlet? (10A ===\> 2000W ?) In the UK all outlets are three-pin 13 amps and are normally switched. Some appliances do not need to be grounded, so the earth pin is simply not used. Appliances (like my electric cooker) are hard-wired with suitable (usually 20amp) twin and earth, directly to the main fuse board. As well as that, all plugs are fitted with a suitable fuse. This may be 3amps for a lamp or a charger, up to 13amp for a 3Kw heater or an electric kettle. All new appliances are ready fitted with a plug. ---
**Author:** ![hibernicus](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/hibernicus/32/2802_2.png) [@hibernicus](https://boards.straightdope.com/u/hibernicus)\ **Post date:** [June 4, 2022, 10:29pm UTC](https://boards.straightdope.com/t/minimal-fill-levels-on-electric-kettles/965518/55 "2022-06-04T22:29:19Z")
> [@SciFiSam](#): > > As the above cite says, it was standardised to be 220v to comply with most of Europe, but in practice that made no difference (it became 220-240 volts rather than 220 volts only). Not 220V, 230V (with a tolerance of +10% and -6%, to give a range of 216V to 253V). The DNOs didn’t need to change the nominal supply voltage to remain within tolerance but the nominal standard voltage for _appliances_ did change from 240V to 230V. A kettle rated for 3 kW at 230V will deliver 3.3 kW at 253V and will boil the water 10% faster. ---
**Author:** ![hibernicus](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/hibernicus/32/2802_2.png) [@hibernicus](https://boards.straightdope.com/u/hibernicus)\ **Post date:** [June 4, 2022, 10:38pm UTC](https://boards.straightdope.com/t/minimal-fill-levels-on-electric-kettles/965518/56 "2022-06-04T22:38:05Z")
> [@bob\_2](#): > > Some appliances do not need to be grounded, so the earth pin is simply not used. Normally that applies to low-current double-insulated appliances like a yogurt maker or something. But an interesting example of an appliance that is not grounded in some countries is an electric toaster, despite presenting a high risk of electrocution if the chassis becomes live. The reason is to protect someone who is poking around inside with a metal knife or fork while holding the metal exterior with the other hand. ---
**Author:** ![puzzlegal](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/puzzlegal/32/3827_2.png) [@puzzlegal](https://boards.straightdope.com/u/puzzlegal)\ **Post date:** [June 4, 2022, 11:07pm UTC](https://boards.straightdope.com/t/minimal-fill-levels-on-electric-kettles/965518/57 "2022-06-04T23:07:03Z")
This thread is fascinating. My current electric kettle can be set to any temp from “hot” to “boiling” in either F or C, and it’s happy to hold water at 180F for several minutes. The electric kettle i had in college just heated and stayed “on” until you turned it off. A similar one caught a desk on fire over a holiday. I had no idea there was a whole class of electric kettle that turned itself off when the water boils. How clever. ---
**Author:** ![kenobi\_65](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/kenobi_65/32/13178_2.png) [@kenobi\_65](https://boards.straightdope.com/u/kenobi_65)\ **Post date:** [June 4, 2022, 11:25pm UTC](https://boards.straightdope.com/t/minimal-fill-levels-on-electric-kettles/965518/58 "2022-06-04T23:25:07Z")
> [@puzzlegal](#): > > I had no idea there was a whole class of electric kettle that turned itself off when the water boils. How clever. I have two kettles, made by different manufacturers. The less-expensive one of the two just has a simple switch which you depress to start it, and which automatically flips off once it reaches boiling. The more-expensive one also has a “warmer” button, which will keep the element on, at a lower level, after boiling, to keep the water warm. I’ve never experimented to see if that warmer button has a timer, to keep it from being on for an extended period (the longest I think I’ve had it on is maybe 2 hours); as the water is kept at a sub-boiling temperature, having the kettle run dry is probably not very likely. ---
**Author:** ![SciFiSam](https://avatars.discourse-cdn.com/v4/letter/s/47e85d/32.png) [@SciFiSam](https://boards.straightdope.com/u/SciFiSam)\ **Post date:** [June 6, 2022, 9:39am UTC](https://boards.straightdope.com/t/minimal-fill-levels-on-electric-kettles/965518/59 "2022-06-06T09:39:04Z")
> [@hibernicus](#): > > Not 220V, 230V (with a tolerance of +10% and -6%, to give a range of 216V to 253V). In practice it’s 220-240v though, because of the 13a sockets pretty much everyone has. I’ve never seen a kettle over 3000amps (which seems to be the standard except for mini kettles and really cheap £12 ones - which still switch themselves off). ---
**Author:** ![bob\_2](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bob_2/32/3341_2.png) [@bob\_2](https://boards.straightdope.com/u/bob_2)\ **Post date:** [June 6, 2022, 10:27am UTC](https://boards.straightdope.com/t/minimal-fill-levels-on-electric-kettles/965518/60 "2022-06-06T10:27:07Z")
There are small (one litre) and low power (\<1000W) kettles available, but far from being cheaper, they are usually more expensive than their bigger brethren. These kettles are to be found in camping and caravanning shops because the power outlets on most campsites will trip if the load exceeds 10amps. [Previous page](https://boards.straightdope.com/t/minimal-fill-levels-on-electric-kettles/965518.md?page=2) [Next page](https://boards.straightdope.com/t/minimal-fill-levels-on-electric-kettles/965518.md?page=4)