In article ,
Charles Middleton wrote:

Is there a way by looking at the cable breaker etc I can determine the
maximum rating possible?

P = I * V

So if it's a 30A breaker, max. power would be 30 * 230 or 6.9KW.

But thats assuming 100% efficiency - you'll lose some to the cable at
those amps which I think was mentioned earlier in the thread..

If it were me, I'd uprate the wiring and breaker to handle the biggest
wattage shower I could get my hands on, having been fed-up in the past
with dribbly little 8KW showers in the middle of winter )-:

Gordon