Water heater - what size?

[highspark]

Went to job today and as I was pulling cables to one side I grabbed hold of a 1.5mm t&e and it was very very warm. Traced it back from unmarked fused board and it goes to a water heater. It was on a 15amp breaker which was also very warm. Could not identify what wattage the tank carries but it isn't a large one. What size breaker and cable are these usually on? Would a 20amp 2.5mm radial be sufficient or am I looking more towards a 4mm 30 amp?

 

[Marvo]

Couldn't you put a clamp meter on the supply either at the MCB or at the heater itself. It would be better than guessing.

If the 1.5mm cable was a short length and wasn't running in insulation then a 15A MCB would probably be about right. If the MCB had no history of tripping it might just be close to the 15A, they do get luke warm when they're running close to full load.