Can I simply change this over.

Mcwill · Oct 23, 2023

Hi, I am changing the electric shower in a bathroom refurbishment (my own) and just wanted to ask about the mcb rating. The existing shower was an 8.5kw, I would like to install a 9.5kw... the cable to the shower is 10mm, it is only around four meters from the consumer unit, and the 10mm cable runs through the small loft space and on top of any insulation. Currently there is a 40a mcb for the shower circuit in the consumer unit... and this is where my question lies. According to instructions I have downloaded from various showers they all seem to state: 9.5kw shower requires a 40/45a breaker. When doing the calculation (9500 / 230 = 41.3) clearly that is over the 40a that is currently there. Would the 40a be sufficient in this case and would a 50a (they dont do a 45 for my consumer unit) be 'too' big and potentially dangerous..?
Many Thanks in advance for any info.

Dave OCD · Oct 23, 2023

Assuming there is also 30ma RCD protection in place for this circuit and the cable run is exactly as described I'd upgrade the MCB to a 50a if 45a isn't available. it's a bad idea for any overcurrent device to run at a slight overload constantly and although it probably wouldn't 'trip' an upgrade is much better.

Mcwill · Oct 24, 2023

Dave OCD said:
Assuming there is also 30ma RCD protection in place for this circuit and the cable run is exactly as described I'd upgrade the MCB to a 50a if 45a isn't available. it's a bad idea for any overcurrent device to run at a slight overload constantly and although it probably wouldn't 'trip' an upgrade is much better.

Ok, thanks Dave.