2KW Fan heaters wired with 2 core 0.75mm flex!?

[JohnW2]

An appliance rated as 2 kW when supplied with 230 volts will have a resistance of 26.45 Ohms and a current draw of 8.7 Amps ... The same appliance when supplied with 253 volts ( 230 + 10% ) will draw 9.5 Amps and be dissipating 2.4 kW ... Since 253 volts is possible in the UK then cables to a 2 kW at 230 volt appliance should be able to safely carry 9.5 amps.

Your arithmetic is obviously correct. However:

1...Since BS7671 requires design calculations to be undertaken at nominal voltage (230V), one can but presume that the CCCs tabulated in BS7671 have built-in 'margins' to allow for the possibility that the voltage may be as much as 10% above nominal.

2...The vast majority of manufacturers still quote powers of most heat-producing devices (heaters, cookers, showers etc.) at 240V. If the fan heater in question is specified as 2 kW at 240V, then it's resistance would be 28.8 Ω, hence a current at 253V of about 8.8A.

3...More generally, one also needs to remember that, in the case of equipment involving SMPSs, current will decrease with increasing supply voltage and, more importantly, increase with reducing supply voltage.

Kind Regards, John

 

[JohnW2]

According to IEC 60335-1, which a fan heater should be tested to, 0.75mm2 supply cords are allowed for appliances with a current rating less than or equal to 6A. However, a portable appliance (like a fan heater), is allowed to use a 0.75mm2 supply cord for a current rating up up 10A, provided the length of the cord does not exceed 2m. ... So the supply cord complies with the standard.

That's interesting, and obviously answers the OP's question.

However, is there any indication/explanation/justification as to why this is allowed - in particular, why there is a difference between 'portable' and 'non-portable' appliances, and why there is a difference dependent upon the length of the cable?

Kind Regards, John

 

[Chris61]

If there is one this that IEC standards are sorely lacking that is rationales and justifications. I suspect it was lobbying by manufacturers but I can't be sure. Maybe some manufacturers of some portable appliances didn't like the less flexible 1.0 or 1.5mm2 cables on their appliances so the 2m length was agreed as a compromise...

 

[OwainDIYer]

However, is there any indication/explanation/justification as to why this is allowed - in particular, why there is a difference between 'portable' and 'non-portable' appliances, and why there is a difference dependent upon the length of the cable?

Portable appliances in the EU will be connected to a socket circuit protected at a maximum of 20 A (might even be 16 A?) (except in the UK where they will be protected by a 13 A max fuse in the plug) and that will adequately protect the cable.

The length of the cable is because its resistance has to be low enough for the protective device to trip quickly enough so that the total energy let through is below that required to melt the cable, which is calculated as I²t or current squared times seconds. A longer cable with higher resistance would allow a lower fault current to flow for longer, causing damage.

Short skinny cables are also cheaper for manufacturers.

 

[JohnW2]

However, is there any indication/explanation/justification as to why this is allowed - in particular, why there is a difference between 'portable' and 'non-portable' appliances, and why there is a difference dependent upon the length of the cable?

Portable appliances in the EU will be connected to a socket circuit protected at a maximum of 20 A (might even be 16 A?) (except in the UK where they will be protected by a 13 A max fuse in the plug) and that will adequately protect the cable.

That's clearly someone's view - but it isn't clear (at least to me) how/why a 20A, 16A or 13A protective device would "adequately protect" a cable which BS7671 believes has a maximum CCC of 6A. In the case of, say, portable hand tools, toasters, kettles etc., in 'pragmatic common sense' terms I could understand it - even if they drew 10A, it would only be for short periods, during which I'm sure that '6A rated' cable would come to no harm. However, a 2kW fan heater could be on for hours, so I find that much more difficult to understand.

The length of the cable is because its resistance has to be low enough for the protective device to trip quickly enough so that the total energy let through is below that required to melt the cable, which is calculated as I²t or current squared times seconds. A longer cable with higher resistance would allow a lower fault current to flow for longer, causing damage.

Sure, we know all that - but it's a totally different issue from that of the CCC of the cable - i.e. it does not,in itself, explain why a '6A rated' (per BS7671) cable is allowed to carry 10A.

More generally, you do raise an interesting point. We design sockets circuits such that the Zs, as measured at the furthest socket on a circuit, is low enough to result in acceptable disconnection times, and to ensure that conductors have an adequate CSA to cope with the 'let through' (not strictly 'energy') of the protective device - but if someone then comes along and plugs in a portable appliance (e.g. garden tool) with 20m or 50m of flex (its own, or an extension), then all of those design calculations could go out of the window. I suppose it's just another illustration of the fact that sockets circuits are a designer's nightmare - in the absence of knowledge of what will/may be plugged into what socket, it is totally impossible to undertake 'proper design'.

Kind Regards, John

 

[JohnW2]

If there is one this that IEC standards are sorely lacking that is rationales and justifications. I suspect it was lobbying by manufacturers but I can't be sure. Maybe some manufacturers of some portable appliances didn't like the less flexible 1.0 or 1.5mm2 cables on their appliances so the 2m length was agreed as a compromise...

That's all probably true, but it's a bit strange that it was considered acceptable to adopt a 'compromise' which appears to fly in the face of BS7671!

Kind Regards, John

 

[JohnW2]

... and I probably should have added ... it seems particularly odd that 0.75mm² should be allowed to supply 10A to a fan heater (provided it was no longer than 2m), but only allowed to supply 6A to a 'fixed appliance' (whose flex would probably usually be no more tha 2m). The logic for any difference escapes me, but if there is to be a difference, I would almost have expected it to be the other way around!

Kind Regards, John

 

[plugwash]

The current ratings of flex are weird anyway. 0.75mm is considered to be 6A but 1.0mm is considered to be 10A. A 1/3 increase in CSA leads to a two-thirds increase in nominal CCC.

This is the opposite of what one would expect from a thermal argument where CCC scales sublinarly with CSA.

 

[OwainDIYer]

but if someone then comes along and plugs in a portable appliance (e.g. garden tool) with 20m or 50m of flex (its own, or an extension), then all of those design calculations could go out of the window.

along with the extension lead ... and the exported earth ...

Maybe in 10 years time all wall sockets will have to be USB and anything over 5V 2A will have to be hardwired in by an electrician.

 

[JohnW2]

The current ratings of flex are weird anyway. 0.75mm is considered to be 6A but 1.0mm is considered to be 10A. A 1/3 increase in CSA leads to a two-thirds increase in nominal CCC. This is the opposite of what one would expect from a thermal argument where CCC scales sublinarly with CSA.

Indeed so - and I also suspect that the flex CCCs are extremely conservative. I can't see that the difference between PVC/PVC flex and PVC/PVC T+E is anything like enough to explain why the CCC of 1.0mm² flex is only 10A, whilst the CCC of 1.0mm² T+E in free air (Ref Method E) is 17A.

The BS7671 CCCs in general (i.e. most most/all other than flex) do show (in most cases fairly dramatically) the 'sublinear' relationship between CSA and CCC to which you refer.

Kind Regards, John

 

[JohnW2]

but if someone then comes along and plugs in a portable appliance (e.g. garden tool) with 20m or 50m of flex (its own, or an extension), then all of those design calculations could go out of the window.

along with the extension lead ... and the exported earth ... Maybe in 10 years time all wall sockets will have to be USB and anything over 5V 2A will have to be hardwired in by an electrician.

Who knows - but what you 'suggest' would obviously not be possible unless one also banned truly 'portable' LV electric equipment, which would be a very far-reaching step!

So long as the number of deaths and serious injuries due to electricity remain much lower than those due the the use of ladders, tools etc., one cannot help but suggest that 'Nanny'probably ought to have a rest, or at least move her attentions to other areas of risk!

Kind Regards, John

 

[ban-all-sheds]

Garden ponds, for example

 

[OwainDIYer]

Garden ponds, for example

Or baths, especially for children under 3.

Safer to wash the baby in the water-butt.

 

[endecotp]

is there any indication/explanation/justification as to why this is allowed - in particular, why there is a difference between 'portable' and 'non-portable' appliances, and why there is a difference dependent upon the length of the cable?

Including rationale in specs is something that doesn't happen enough.

My guess, for what it's worth, is that shorter flexes on portable devices are less likely to be hidden under rugs or coiled up than longer ones.

 

[JohnW2]

is there any indication/explanation/justification as to why this is allowed - in particular, why there is a difference between 'portable' and 'non-portable' appliances, and why there is a difference dependent upon the length of the cable?

Including rationale in specs is something that doesn't happen enough.

Indeed, but I would say that was probably a serious understatement - it seems pretty rare for it to happen at all!

My guess, for what it's worth, is that shorter flexes on portable devices are less likely to be hidden under rugs or coiled up than longer ones.

Who knows. However, even if that were the case it still doesn't explain why they allow a cable 'rated' (per BS7671) at 6A to carry 10A, even if its <2m long and not under a rug or coiled up! As I've said, in comparison with the BS7671 rating for the same CSA of almost anything other than flex, the flex 'CCCs' do seem extremely 'conservative' - but I'm not sure that is an acceptable excuse for exceeding them!

Kind Regards, John