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

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.

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.