Safe cable size for 3kw immersion heater.

Hi, all. A friend of mine has just had a new boiler and cntral heating system to replace his original one. For some reason, the old immersion heater was connected to 1.5mmT&E. The electrician that came to wire up the new system including the new immersion heater, said that the 1.5mm T&E cable was ok to leave in and wire up to.!. Is this the case or was he just being lazy not wanting to rewire with 2.5mm T&E. The cable is on its own circuit and the length is aprox 25metres. Also, I was wondering if the fuse fitted is ok at 16amps, i'm just a little concerned that the safety device i.e the circuit breaker can stand more than the cable?
Any advice would be appreciated.
Andy Stu

how is the cable run? In particular does it run through any insulation?

I believe it runs from the consumer unit which is located in the single storey garage, up through the garage lof space which has a small amount of insulation, maybe 50mm then under the floor boards and into the airing cupboard. If the route is vital for the calculation then I can find out exactly in more detail.

Thanks,

Andy Stu

The route isn't vital - 1.5mm² T&E can safely carry more than 13A even if insulated and in conduit.

A 16A breaker is also fine, but the outlet in the airing cupboard should be fused at 13A.