UK Can't get within maximum load when applying diversity

[ThatMatt72]

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.....

 

[davesparks]

Cheers again all. It's always good to get your input.
Dave, I appreciate your concern. I am actually trying to save the customer money, mate. A changeover switch is about £50. A new / additional supply, if they're gonna go over on demand would be way way more, right?
Assuming your assumption that the existing installation is in fact well designed and that I am a cowboy messing with God's own installation then all your points are in fact correct and I should go back shovelling poop for a better qualified man than I. However, lets for a second assume that I undertake work in compliance with the wiring regs and that I do care that the clients needs are being met, while at the same time I won't be overloading any circuits. Bearing that in mind, and working out diversity on the the set up of the circuits as they are, because neither you nor I have the slightest clue what is going to plugged in to the sockets when we are not looking, so that's the only way I can see that I can work it out staying in line with the regs. I understand your point about not bunging everything on one mcb / rcbo so I can get the measurement down to 32 amp. Yes you are correct there, but there is room for me to make changes that do not turn the installation into a horror show. Decisions sometimes have to be made to make something slightly less that ideal (not dangerous or prone to overloading in anyway), to accommodate some form of addition
They wan't heaters dotted around so that there is one at every desk point, so the staff don't argue over who gets to be near a heater and who doesn't. I can't see a problem with this, if that's what they want, mate. It can be done, so why not do it?
There's no main controller for the heating system. Its a simple case of dotting PLXE panel heaters around the room. They want to keep costs as low as possible, and I am trying to install something safe and effective that complies with regs and wont make anything go pop. In my experience, and I have installed quite a few of these type of units in the past, they do a pretty good job.
The AC units are already on an RCD so taking them off would render the fault protection worse than when I started, so thats why they are going back on RCD. I have obviously considered putting the heaters in RCBOs and may opt to consolidate a few of them onto each circuit, thereby cutting down on the amount of ways needed. I am happy to ditch the RCD main switch/ RCD set up and replace with BS60947/3 / RCBO set ups, but I may have to offer the client both options as the price will be different and that will be a factor for them. To be honest though, unless there is very good reason, I don't install anything without RCD in domestic or commercial setting. There is no "competent person" employed at the office and I have no idea about their PAT routine, if in fact they have one at all, so I hope that answers the RCD questions?
Those that might pick me up on it are my annual inspection guys, who are not averse to asking about diversity calcs in situations like this. Seems quite reasonable to me, to be honest.
Sorry, if I am coming across a bit tetchy guys, Dave in particular. I do appreciate everyone's input and ideas, but I am sure the purpose of most people posting on here is not to be roasted for ideas based on information that none of you have. If that's the case, then whats the point of this forum. Can't we share knowledge without trying to belittle each other?
Bottom line is this.....if I just add the heaters and they fire up the heaters and the aircon units and a kettle / toaster at the same time, and turn on the hot tap (firing the ariston under the counter) then there will most probably be a pop, so I go back to my £50 changover switch solution...have I sold anyone on the idea yet? ;-)

I'm not trying to belittle you, I'm just trying to put my thoughts across and ask questions about the reasoning behind your design.

The only positive point I can see for the changeover switch idea is that it will provide a crude interlock to prevent simultaneous heating and cooling.

No matter how you rearrange or combine the existing circuits you won't change the average or peak load, you will only increase the disruption caused by a fault.

Yes a group of people could all switch on a bunch of appliances at the same time, and all 8 heaters might all cut in on their thermostats at that exact same time, but that's pretty unlikely. And even then the load will settle down again before it reaches the tripping point of the OCPD for the submain.

Yes an assessor might ask you about this, but you just need to give a sensible answer. I suspect they would be more concerned about the blanket unnecessary application of RCDs demonstrating a lack of knowledge along with the odd combining of circuits.

 

[naylorpd]

Diversity is an excuse to downrate a calculated cable size. Fundamentally it ----u-me-s that you don't use all of the appliances all of the time. But when it gets cold everyone will switch on their personal heater.

IMO for your risk assessment you need to do a realistic load calculation based on what equipment is actually there, and which items are probably going to be operating simultaneously.

Perhaps you could find a way to split the landlord's under-utilised two phases.

 

[davesparks]

Diversity is an excuse to downrate a calculated cable size. Fundamentally it ----u-me-s that you don't use all of the appliances all of the time. But when it gets cold everyone will switch on their personal heater.

IMO for your risk assessment you need to do a realistic load calculation based on what equipment is actually there, and which items are probably going to be operating simultaneously.

Perhaps you could find a way to split the landlord's under-utilised two phases.

Diversity is a well established principle that does work and is not just an excuse to downrate anything.
It doesn't just assume that not all appliances are on at once, it takes account of the nature of the loads and how they behave. For example these electric heaters will be cycling on and off at different rates regardless of whether they are switched on all at the same time or not.

 

[Toneyz]

I asked a question in post 9 the reason behind it was if the three phases could be installed in a three-phase D.B. and the loads distributed across phases.