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.
There are a lot of unknowns there, but 2.5mm2 could be satisfactory, if the circumstances were right.
The biggest factor is how long the water heater is intended to be on for at a time - less than around 15min followed by a reasonable off time and it's quite likely that the design current could be based on a thermally equivalent current that's a fair bit lower than 7300/230.
Then the 2.5mm2 might have a reasonably high Iz - depending on how it's installed. Likewise if the ambient temperature is actually less than 30 degrees when the heater is in use, there might be an additional margin there.
There again, if a reduced lifespan was acceptable running the conductor a little higher than 70 degrees would be unlikely to cause any problems in practice - again if the duty cycle was low, the reduced lifespan would likely not be noticeable. Not to the letter of BS 7671, of course, but still.
Or do like the Germans do and use 2.5mm2 for everything (domestic) powerwise - and just add more phases when you want more than 3.6kW
- Andy.
Whilst the 2.5mm cable will indeed do the job, Use of that size can not be recommended under any ordinary circumstances.
Even if use is intended to be intermittent, consider mechanical failure or human error or misuse that runs it for hours, on a hot day.
Consider later enclosing or boxing in of the cable, for neatness.
Consider replacement of the water heater with an 8 kw version “that must be OK as it is only very slightly larger”
Consider a short circuit after the cable has already exceeded its intended operating temperature.
AJJewsbury:
There are a lot of unknowns there, but 2.5mm2 could be satisfactory, if the circumstances were right.
The biggest factor is how long the water heater is intended to be on for at a time - less than around 15min followed by a reasonable off time and it's quite likely that the design current could be based on a thermally equivalent current that's a fair bit lower than 7300/230.
Then the 2.5mm2 might have a reasonably high Iz - depending on how it's installed. Likewise if the ambient temperature is actually less than 30 degrees when the heater is in use, there might be an additional margin there.
There again, if a reduced lifespan was acceptable running the conductor a little higher than 70 degrees would be unlikely to cause any problems in practice - again if the duty cycle was low, the reduced lifespan would likely not be noticeable. Not to the letter of BS 7671, of course, but still.
Or do like the Germans do and use 2.5mm2 for everything (domestic) powerwise - and just add more phases when you want more than 3.6kW
- Andy.
The water heater is probably only used for a few minutes at a time to fill a washing up bowl at the kitchen sink. I believe that the original intent was to install a 3kW water heater, but the type was changed for the 7,300W instantaneous type water heater under the kitchen sink. A spare B32 M.C.B. was used for this in the consumer unit.
Personally I am not happy with this and will code it as unacceptable.
Z.
Today I found a holiday chalet protected by a B50 M.C.B. and 30mA R.C.D. It has a 30/35 Amp shower. A 1kW bathroom fan heater. Several plug in room heaters. A 30 Amp free standing cooker. A 7kW water heater. Plus lights and other odds and stuff. All supplied through short 6.0mm2 tails showing no signs of overheating.
Z.
I have been through this at length. Cable current ratings in BS7671 are for 24/7 use at the stated load. Once one does not do this the rating can be much higher, provided there is reasonable certainty that the current cannot be significantly more than the BS7671 value for long periods. Reduced ambient will give a higher rating even 24/7. Consider this, what is the rating of a copper cable 2.5mm2 at -270 degrees centigrade? 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? Z may code this, but what is the correct coding? In the “magic book” this is probably a C2, but this is clearly not correct. The potential danger is reasonably considered by me to be zero. Say the cable gets to 100C due to running the heater for a few hours by accident. The potential danger is certainly not electrical, although a burn could result to a person. Even this is fairly unlikely as we rarely grip cables tightly to see how hot they are. I note that any EICR codes issued by an NICEIC registered contractor must (their regulation) be as in the “magic book”. Guess who does not understand BS7671 or Electricity. 3 guesses, any takers?
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.
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.
Years ago I saw quite a few dolphin showers that had their supply taken directly from the back of a socket in a nearby bedroom, no signs of scorching and they were six KW min and been installed years previously.
Testimony to the resilience of the good old ring final circuit?
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