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Hi all, I wonder if you might chuck in your 10 cents worth on this on?
I have been asked to install electric heating in an office up on the 4th floor of a building.
I've looked at the layout of the room, with regards of seating etc and after consulting with the occupiers have decided on the layout / positioning of heaters.
Assuming that this needs no further discussion, I have settled on 8x 750 watt Dimplex PLX units as I use them pretty regularly and they seem to do the job well enough.
I am planning to run individual 2.5mm radial circuits to an FCUs for each heater, with a 5amp fuse in each. Total draw would be 8x 750 watts = 6kW (26amps).
The problem is the existing circuits are arranged in such a way that, when diversity is applied, the total calculated demand is already higher than the 63amp main switch located 3 floors below allows for. That said, there is nothing plugged in that particularly uses any power (about 6 PCs in total and a few phones / printers / usual office crap.) There is an instantaneous water heater in the kitchenette, but that's on a 13amp FCU taken from one of the ring finals. The existing lighting is arranged in to 3x 10amp circuits (although the whole lot could easily have been on one 6amp RCBO.) Finally there are 3x aircon units each on a 10amp RCBO. There is no heating installed and the last tenants used to plug in bunch of floor mounted 2/3kW convector jobs, which never overloaded the main fuse. That said, however I'm not happy exceeding maximum demand on paper, even though I'm pretty sure it'll never be exceeded.
Here's what I thought might be my solution:
I take the 3x aircon circuits out of the existing CU and install a 32amp MCB for a submain.
The submain pops directly into a 32amp changeover switch with 3 positions (A, B and OFF).
The changeover switch supplies 2 lots of tails (A & B) going into a 15 way CU.
Tails A feed into a 32A/30mA RCD main switch (RCD A) protecting 8x 6amp MCBs (one for each panel heater)
Tails B feed into a 32A/30mA RCD main switch (RCD B) protecting 3x 10amp MCBs (one for each aircon unit)
Total ways required 15.
The changeover switch will ensure that the RCD A and RCD B can never be energised at the same time thereby keeping my maximum demand exactly where it already is during the summer months and lowering it during the months where the heating will be used.

Does that sound like a plan, or am I way off base here? I am also thinking of consolidating some of the existing circuits as there is no need for the 3 ring finals and 3 lighting radials.
OK....over to you, my learned chums.....
 
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