fused spurs / double Pole Switches

Tell me how hot the heating elements will be 'after the initial few minutes' and I'll do the approximate sums for you :)
I'm not sure how hot the actual filament will be surrounded by insulation and water presumably at 100°C.
It could be much more than 100°C.

The other crucial question is, of course, whether you are right in saying that a '3kW appliance' is 3kW 'when cold' - or whether the 3kW actually relates to 'after the first few minutes'.
They are according to the measurements, e.g. immersions are 3kW and around 19Ω when cold.

After all, with filament lamps, we quote the wattage when hot, not cold (the latter of which would be extremely high, for at least a few milliseconds).
Lamps are different :)
 
Sponsored Links
Tell me how hot the heating elements will be 'after the initial few minutes' and I'll do the approximate sums for you :)
I'm not sure how hot the actual filament will be surrounded by insulation and water presumably at 100°C. It could be much more than 100°C.
You're obviously thinking of elements immersed in water (kettles, immersions etc.) and, as we've often discussed, goodness knows what temp the actual element runs at - obviously at least 100°, probably a lot higher. In the absence of cooling by water, elements coud get much hotter. For example, fan heater and hair dryer elements start glowing red hot (several hundred degrees) almost immediately if the fan slows or stops, so I suspect usually runs very hot. Similarly, toasters run red hot all the time.
The other crucial question is, of course, whether you are right in saying that a '3kW appliance' is 3kW 'when cold' - or whether the 3kW actually relates to 'after the first few minutes'.
They are according to the measurements, e.g. immersions are 3kW and around 19Ω when cold.
Fair enough, even if a bit tangential to this thread (one doesn't usually plug immersions into BS1363 sockets!). If that's the case then, as you're implying, 19Ω equates to 3.03kW (12.63A) at 240V (seemingly to voltage at which most ratings are quoted) - so 3kW (at 240V) does, indeed, appear to be the cold rating. If we assume that's at 20°C, and that the absolute minimum 'running' temp is 100°C, the resistance would then have risen to abt least 25Ω - about 2.3kW (9.6A) at 240V.

If (I would suspect far from impossible) the element temp rose to 200°C, then resistance would be about 32.7Ω - about 1.76kW (7.3A) at 240V

Kind Regards, John
 
Fair enough, even if a bit tangential to this thread (one doesn't usually plug immersions into BS1363 sockets!).
I was just using that as an example.
Washing machine 1700W elements are 34Ωish.

elements are If that's the case then, as you're implying, 19Ω equates to 3.03kW (12.63A) at 240V (seemingly to voltage at which most ratings are quoted) - so 3kW (at 240V) does, indeed, appear to be the cold rating.
They must be (worst case) otherwise design considerations would be inadequate.

If we assume that's at 20°C, and that the absolute minimum 'running' temp is 100°C, the resistance would then have risen to abt least 25Ω - about 2.3kW (9.6A) at 240V.

If (I would suspect far from impossible) the element temp rose to 200°C, then resistance would be about 32.7Ω - about 1.76kW (7.3A) at 240V
Thank you.


So - two 3kW loads will not overload a double socket.
 
I was just using that as an example. Washing machine 1700W elements are 34Ωish.
Fair enough - the pattern certainly seems consistent.
... so 3kW (at 240V) does, indeed, appear to be the cold rating.
They must be (worst case) otherwise design considerations would be inadequate.
On the basis of the cold resistance figures you're quoting, it does seem to be the cold rating - but I don't think your 'must be' necessarily works. If it's only for the first few seconds, or even a minute or two, we are happy to design (cable CCC-wise) without taking the initial current of, say, a filament lamp or motor into account.
If we assume that's at 20°C, and that the absolute minimum 'running' temp is 100°C, the resistance would then have risen to abt least 25Ω - about 2.3kW (9.6A) at 240V. ...
Thank you. So - two 3kW loads will not overload a double socket.
That statement needs a fair bit of qualification :) IF the loads are both 3kW resistive at 240V when element temperature is 20° and IF the normal running temp of the element is at least 100°C, then the total load when running (on 240V) would be less than 20A.

Kind Regards, John
 
Sponsored Links
IF the loads are both 3kW resistive at 240V when element temperature is 20° and IF the normal running temp of the element is at least 100°C, then the total load when running (on 240V) would be less than 20A.
Why would it be?

Nichrome, for example, has a temperature coefficient of resistance of .0004, so if an element using that had a resistance of 19Ω at 20°C it would be 19.61Ω at 100°C, and would be dissipating 2937W at 240V, so 2 of them would be considerably more than 20A, and you could not supply them from a double socket.
 
If that's the case then, as you're implying, 19Ω equates to 3.03kW (12.63A) at 240V (seemingly to voltage at which most ratings are quoted) - so 3kW (at 240V) does, indeed, appear to be the cold rating. If we assume that's at 20°C, and that the absolute minimum 'running' temp is 100°C, the resistance would then have risen to abt least 25Ω - about 2.3kW (9.6A) at 240V.

If (I would suspect far from impossible) the element temp rose to 200°C, then resistance would be about 32.7Ω - about 1.76kW (7.3A) at 240V
You appear to be assuming that the elements are made of copper.

I don't think they are.
 
IF the loads are both 3kW resistive at 240V when element temperature is 20° and IF the normal running temp of the element is at least 100°C, then the total load when running (on 240V) would be less than 20A.
Why would it be? ... Nichrome, for example, has a temperature coefficient of resistance of .0004 ....
In that case, it obviously wouldn't be. As in the other thread to which EFLI referred, in the absence of knowing what metal(s) we are dealing with, I used a figure of 0.004 which is the rough ballpark of coefficients for many metals (including copper, aluminium, lead, gold, silver, platinum, tungsten, even iron). If you know that the heating elements we're talking about use nichrome, then your calculation would obviously be appropriate.

Kind Regards, John
 
You appear to be assuming that the elements are made of copper. ....
... or, as I've just written, aluminium, lead, gold, silver, platinum, tungsten, or iron etc. etc. - in fact probably most 'common metals' other than nichrome (if you call nichrome common!).

Kind Regards, John
 
Nichrome is very commonly used for heating elements.

Copper might be, but I am dubious. Ditto tungsten.

Aluminium, lead, gold, silver, platinum, iron...? No - I really don't think so.

Do you, honestly?
 
Nichrome is very commonly used for heating elements.
Yes, come to think about it, I believe you're right - either that or various other alloys, the coefficients of resistivity of which i wouldn't know!
Copper might be, but I am dubious. Ditto tungsten.
I certainly doubt copper, but wouldn't be too surprised by tungsten.
Aluminium, lead, gold, silver, platinum, iron...? No - I really don't think so. Do you, honestly?
Of course not - I was merely illustrating that the coefficient is in the ballpark of 0.004 for 'many (elemental) metals'.

In any event, this is all an extreme tangent created by EFLI trying to find a way of arguing that two 'large loads' would not overload a double socket. If it really worked out as EFLI would like, then we would probably be able to run his "3kW loads" on 10A circuits and cables as well!

Kind Regards, John
 

DIYnot Local

Staff member

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

Select the supplier or trade you require, enter your location to begin your search.


Are you a trade or supplier? You can create your listing free at DIYnot Local

 
Sponsored Links
Back
Top