7,300 Watts supplied on a 2.5mm2 T&E.

Graham, you have far too much time on your hands on what is a lovely weekend summer's afternoon!

I should have learned by now not to challenge Graham's wisdom!

Sadly, Hampshire Libraries cancelled their subscription to OED this year so I have to make do with my Concise version. Nonetheless, “pedancy” does not appear. “Pedantry”, we are told, comes from the French, “pédanterie” or Italian “pedanteria”.

The French seem to have a subtly different meaning. According to l'Académie Française, “pédenterie” is (my translation) “An affectation or pretension of a person who wants to show the superiority of his mind or knowledge.” Then there is “Pédentisme".

I have to admit that I can find nothing wrong etymologically with “pedancy” in the same way that “idiocy” denotes the quality of being an idiot. ?

 

For me it's a C2 because the cable could be overloaded if, e.g. the heater were inadvertently left running with the plug out. No fault to danger! My recommended remedy would be to downsize the MCB to 20 A. There should be no nuisance tripping given the intended use. (Or 25 A if RM C.)

Chris, the heater is rated at 7.3kW 240 Volts and draws just over 30 Amps.

433.1

433.1.1.

Z.

gkenyon: 
 

whjohnson: 
 

Go on Dave, tell us what current a 2.5mm cable can carry at -273K. I'm intrigued.

Since 2.5mm at 70' will carry something like 21A in a best case clipped direct scenario, and given that -273K is absolute zero in degrees C terms, I would imagine that the current carrying capacity would be much much higher. However, we would need length of run and duration of load to get a bit nearer.

very very roughly, with ambient temp 30 degrees, with a 32 A load after a few minutes use (up to 5 mins but may be less):

• Ref method C (clipped direct) get to about 80 deg C
• Ref method 102# gets to about 90 deg C
• Ref method 103# gets to well over 100 degrees C

 

I think in this particular case, the installation may have been lucky based on duration of use and ambient temperature, and I'd guess Ref method C ???

If for any reason the cable is accidentally further thermally insulated, or the ambient temp gets above 25 degrees, things could change quite quickly.

Indeed.  The exact run is unknown, but the chalet is like an old  wooden shed. If the cable runs in the roof void with the sun beating down on the roof outside, and the cable is covered by thermal insulation I suspect a danger of fire.

Z.

AJJewsbury: 
 

Zoomup: 
 

Perhaps the pointed headed men in white coats at the I.E.T. could consider a Cfut correction factor. We already have Ca, Cc, Cd, Cf, Cg, Ci, Cs to name but a few.

Cfut cold refer to the functional utilisation time conditions. So, if applied, it would allow a large load to be supplied by an otherwise inadequately sized cable, but only for a short period of time, before the cable becomes dangerously hot and starts smoking. This approach will save materials and money.

 

Z.

No need - the concept is already in BS 7671 - see 533.2.1 3rd paragraph (although done by adjusting the design current rather than by a correction factor).

   - Andy.

533.2.1 3rd paragraph is concerned with choosing the values of In and I2 surely. I was concerned with the duty cycle of a cable. Say it is overloaded by 30 percent for short periods, say 10 percent of its operational time, would it really suffer?

Z.

Some years ago I found 9.5 kW shower on a 2.5 mm T&E hanging loose in an airing cupboard, I let the shower run for several minutes then told the customer to feel the cable, she said “If I’d know I could have dried my knickers on it”.

 Take an electric motor of 3kW. What is the wire size in the windings? Is it around 2.5mm2 or is it considerably less? Why is this satisfactory? 

Some motors are rated for less than continuous duty. Also many motors are fan cooled. An inductive load opposes the supply current. Then there's back E.M.F. Oh, and the current is shared over several windings.

Here is a nice little 3kW geared motor…….

Z.

533.2.1 3rd paragraph is concerned with choosing the values of In and I2 surely.

Read it again… ".. based on values of Ib and Iz for the thermally equivalent constant load".

   - Andy.

very very roughly, with ambient temp 30 degrees, with a 32 A load after a few minutes use (up to 5 mins but may be less):

• Ref method C (clipped direct) get to about 80 deg C
• Ref method 102# gets to about 90 deg C
• Ref method 103# gets to well over 100 degrees C

What's the assumed initial conductor temperature?

GN 6 (at least my ancient copy) suggests 15min for 2.5mm2 when considering 'duration of load current for equivalent constant load' - suggesting to me at least that a cable would take around that long to get to 70 degrees from cold (ambient) - so I'd expected significantly longer again to 80/90/100 degrees from cold.

   - Andy.

gkenyon: 
 

whjohnson: 
 

Go on Dave, tell us what current a 2.5mm cable can carry at -273K. I'm intrigued.

 

0 K surely? It's not called absolute zero for nothing!

-273C yes …


(Apologies for pedancy)

-273C is substantially above absolute zero in thermodynamic terms.

On the substantive topic why not fit a time switch to limit the run time?