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I'm having an unvented hot water cylinder installed which has two immersion elements (no central heating). The installation means that we can change an existing feeble electric shower into normal shower with some decent power.
The new tank will not fit where the existing tank is so it's going to be located just behind the electric shower. Is it OK to use the dedicated electrical circuit that used to feed the shower (so would become redundant) to feed the two immersion elements?
Would the MCB need to be changed? Can't remember what it is now but it's higher than the existing 16 amp immersion circuit.
A 3kw immersion heater is 13 amps, so two is 26amps.
A shower circuit is probably going to be 6mm2 with either a 32A or 40amp mcb so the cable could carry both safely.
The cable to each element is probably going to be 1.5mm heat resistant flex; maybe 2.5mm. The MCBs are overrated for the current carrying capacity of the flex so ideally you should add fused switches (FCUs) for each circuit. I'd need to check the regs. but I think that if the length of the flex is less than 3m and the cable can carry the throughput of the prospective fault current (adiabatic equation) then you don't need to provide overload protection so you could just join the flex to the 6mm without fusing it down.
The problem will be getting 2x 6mm cables into an FCU (you'll daisy chain the FCUs so the first has 2 cables). You might get them to fit but it will be a PITA to do.
Anyway, to get back to basics - yes the cable will take it. Check they're not 6kW immersions.
It is also more complex if you want to only have one on at a time. Let me know if this is the requirement as I have a circuit diagram for a relay based solution that solves this.
Rich.
50A junction box on the end of the shower feed, 2 x 2.5mm.flex to 2 x fused switches, into 2 x digital timers, into your elements.
If you can get one, swap the mcb to 32a, might even be a spare in the board. Your plumber should be part p etc
Cheers guys.
Both def 3Kw immersions. There is a master switch like this [img] 
[/img]
so I'll come out of the back of this into a fused switch for each immersion element - sound right?
I've got a proportional immersion controller to make use of surplus solar production so that will be trickle feeding any spare power throughout the day with a timed boost period from about 5 in the afternoon to ensure we've got enough hot water for the evening.
Sounds right but the cable between the fused switch (photo)and the FCUs should have a current carrying capacity greater than the rating of the MCB in the consumer unit otherwise they become the weak link in the chain so to speak.
Rich.
I'm guessing here but I presume that the cable currently running from the existing switch to the shower must have that capacity, so I could just this to connect to the FCUs