I have an electric shower on a 10mm cable direct from the board with its own MCB and it has worked perfectly since installation. I would now like to fit a 9.6kW instant water heater to supply one tap in the bathroom and one in the kitchen. It would be difficult to run another 10mm cable back to the board, so, can I fit a 45amp junction box from the existing 10mm feed to the shower, to power the instant water heater? Distance from the JB to the water heater would not be more than 3 metres.