Appliance Ratings and 13A plug tops

I was surprised however to find a wide range of steam irons rated at 3000W and even 3100W.  (that's almost 13.5A at 230V).

But what voltage is that rating for? Simple resistive heating elements will draw less power at lower voltages. A lot electric showers still claim their 240V rating - presumably because larger numbers sound better to marketing departments. If it is a 240V rating then it's still within 13A.


As for the CE marking it might be possible - most continental sockets have both 16A and 10A variants.


   - Andy.

Whilst the exact current of appliances may seem very important, it is as usual not quite as it appears. There is zero chance of the 13A fuse blowing on a 3.1kW appliance because of two factors:

1: the fusing factor which is not instant at 13A but much more like 17 or 18 A in one hour, there are examples in BS7671 graphs or manufacturers sites.

2: Irons are not continuously heated, the appliance only draws 3.1 kW when the thermostat is closed, at other times it draws zero. These two times are averaged to give the fusing time of the fuse, and at maybe 14 or 15A that time is very long, more likely never unless you remove heat a lot quicker than ironing your shirt! There is also the point made by Andy above about supply voltage and the actual power used.


I see no particular reason why the device should not be CE marked, it is obviously suitable to use at this rating. BS1363 does not say that the peak current for a plug may only be 13A, it gives this as the nominal rating. Unfortunately all of this is very badly understood by the trade, and it causes confusion in many discussions here.

statter:

Hello

I looked last year for portable heating and not surprisingly there was little above 2.5kW rating.  All the old 3kW stuff has long gone.

I was surprised however to find a wide range of steam irons rated at 3000W and even 3100W.  (that's almost 13.5A at 230V).  I realise of course that irons are likely to be much more intermittent than heating and that 13.5A is very unlikely to blow a 13A fuse but it cant be good practice to overload plug/ socket like this.  A decent ironing session can still be hours ……   OFC at 240v its under 13A but 230v has been the standard for ages now.   In my experience many socket/ plug combinations are marginal approaching their rating so deliberate overloading will only make matters worse over time.  I would have thought that these appliances wouldn't qualify for CE marking and couldn't be sold legally?

The power range is stated (according to the standards) at the nominal voltage, or highest nominal voltage, stated on the rating plate. In the UK, appliances are typically rated at either 230 V (so the maximum appliance rating is 3 kW ... 230 x 13 = 2990 W, the element resistance at operating temperature being 230/13 = 17.69 Ohms) or either 240 V or 220-240 V (so the maximum appliance rating is 3100 W ... 240 x 13 = 3120, the element resistance at operating temperature being 240/13 = 18.46 Ohms)


In use, the actual current of the appliance may go up or down.


Our calculations for BS 7671 (WIring Regulations) are usually done at 230 V.


The actual mains voltage can be higher, though. WIth supplies according to the ESQCR, the voltage may be as much as 253 V. At this voltage, in appliances with resistive loads, the actual load currents will be higher, so will the power output will also increase, with the two examples above:

• 3 kW, 230 V iron: Current = 253/17.69 = 14.3 A, power = 3618 W

• 3.1 kW 240 V iron: Current = 253/18.46 = 13.7 A, power = 3467 W

What might be surprising when you see this, is the 3 kW iron actually uses more power than the 240 V 3.1 kW iron !!!


Is this a problem? Definitely not, we do our calculations at 230 V, but the actual currents may be higher if the voltage is higher (or lower if the voltage is lower) The cable and protective device ratings can manage this tolerance.


Similarly, the voltage may be below the nominal rating. At the lower voltage (the utilization voltage in the UK, for ESQCR supplies, after voltage drop in the electrical installation, can be as low as 204.7 V), the power used by the irons, and hence their currents, also come down - in this case again, the 3 kW 230 V iron will use more current and power!

Does anyone know if there is a BS or EN for small appliances?

 

Very interesting comments. However, since domestic irons (or even industrial ones) are not within my expertise I would like to throw the following into the mix.


Irons are resistive loads, but we all know that as the temperature increases, so does the resistance. Is the power rating of an iron measured for when it is first switched on or is it the continuous rating (and if so at what temperature setting)?


Alasdair

Maximum power expected under "operating conditions" is usually quoted in standards where necessary.


Therefore I think it's always safe to assume the power is at the operating temperature (hence my addition of this in my previous post).

However, being very very pedantic, the temperature coefficient of resistance for heating element wire is usually orders of magnitude lower than the temperature coefficient of resistance for copper.


For example, constantan has a temperature coefficient of resistance at 20 ºC of -0.000074 whereas copper has a temperature coefficient of resistance of 0.004041


Hence, there would be very little noticeable change in resistance of constantan wire in the range 20 to 120 ºC (120C is often the rating of the thermal fuse in an iron).


I can confirm this is the case, because I have checked heating element resistances in many types of equipment at 10-20 º C when the operating temperature of the element is around 100 to 130 ºC, using a rough power calculation. You could also check this for yourself, say with a tumble dryer or simple convector heater.


Oven elements are subject to higher temperature changes, so you will see some difference between the resistance at 250 ºC , say, than 20 ºC ... and some ovens with self-cleaning capability may perhaps get up to 400 ºC

Old hat now, I know, but incandescent lamps are a different story.


First, tungsten has a higher temperature coefficient than heater element wire. Second, the temperature difference between "cold" and "hot" is much greater (operating at a filament temperature of > 3000 K, rather than the 400-650 K we've been talking about up to now).


Hence, incandescent lamps may well have a significant inrush current, but the power is still quoted under operating conditions.

Graham,

Many thanks for the clarification. While I suspected this would be the answer I was nowhere near fully confident so the confirmation is appreciated..

Alasdair

Thank you all for the above information it is interesting to see how the various standards fit together.   I suspect the formal answer is down to the rating being stated at the higher voltage (i.e. 240 rather than 230). The 3000W may even be rounded. Its all gone some way to satisfy my curiosity.  I suspect that the dearth of 3kW space heaters is because the plugs are problematic at 230V whilst even the latest steam generating irons have duty cycles that are low enough to get away with it.

There is a general acceptance that 13 amp plugs and sockets can not reliably and continually carry more than about 10 amps. Therefore domestic appliances are in practice limited to about 10 amps, presumably to avoid warranty claims about melted plugs.

The main exception is electric kettles, these are used so briefly that a full 13 amps is fine. Table top electric cookers are sometimes a full 13 amps, under worst case conditions, but much less most of the time and again are fine in practice.


Large steam clothes irons are not common domestically, but should also be fine as continual full load operation is unlikely. I have seen steam irons in a tailors workshop that require two 13 amp supplies. One for the iron itself, actual loading about 5 amps, and a second supply for the remote steam generator, actual loading about 12 amps. Often both plugged into the same socket with a death cube.


Domestic appliances likely to run continually are unlikely to exceed 10 amps these days.


3Kw heaters are still available but are marketed as being for industrial use and are therefore supplied without a plug. No warranty claims about melted plugs if plug not supplied.