Yes a 2.5mm2 T&E cable can supply a 7,300 Watt instantaneous water heater at 240 Volts. I came across this today. The supply was from a B32 M.C.B. So why do we use bigger cable than 2.5mm2 if it will do the job?
The run was about 5m maximum.
Z.
Nowhere in this thread is there a discussion about conductor temperature. Basically, the values in Table 4D5 are for the conductor being at 70 degrees C.
If the currents are exceeded for the particular reference method, then the conductor can heat up to above 70 degrees C. It only takes a few minutes to do that.
What happens after that?
The possibility of burns aside, this can damage terminals and accessories. It can cause premature failure of protective devices if the cable is terminated to a protective device. Worse still, as I have seen on a shower (similar kind of thing) that should have had 10 sq mm cable because of the cable running for a few m in insulation at the shower end, but only had 6 sq mm (which was OK for the rest of the run) - because the shower is a heating appliance, the terminals were being heated from both sides, and unfortunately the terminals at the shower was the victim, not the cable. I'd been told this happened “every couple of years".
I guess the worst-case scenario is that something heats up behind covers and causes a fire.
So overall, from experience I would correct this issue. It's not OK just to let it be, even if the cable is “clipped direct” and you're only overloading it by a few A (well, that would be about 20 % in this case).
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.
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):
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.
Let's assume that normally the heater will be used to ⅔ fill a 15 l washing up bowl at 50 deg C with the incoming water at 15 deg C. How long will it take?
10,000 grams of water are to be heated by 35 deg C, which requires 10,000 x 35 x 4.2 joules = 1.47 MJ. The heater provides 7,300 joules/sec. So the time required is 200 seconds or nearly 3½ minutes.
The circuit could be made compliant by fitting a B20 MCB. Would that do?
Yes, according to Figure 3A4 - it would trip only after 500 seconds.
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 Pearson:
gkenyon:
(Apologies for pedancy)
Pedantry even! ?
? There are those pedants who can only see it as pedancy in English, and those who want to import (and mis-spell) an Italian word root ‘pedanter' rather than treat the word as “adopted into English” and follow the “ant” to “ancy" approach (although that usually applies to adverbial nouns).
Yet at the same time, some of those who support pedantry would be highly unlikely to use octopodes, favouring probably octopuses but perhaps octopi … pedantically the latter being wholly incorrect, because octopus is not Latin, but phonetically-spelled Greek οκτάπους (8-footed).
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