Diversity

In addition, what do people normally rate power circuits at?

Using the OPD device results in insanely high figures.

Diversity can be more of an art than a science - so much depends on individual circumstances and as a result the published circuit-based guidance tends to be very conservative.


There are a variety of different approaches - trying a few different ones in the same situation can sometimes be illuminating. Some observe that final circuits are designed to satisfy wide range of criteria - not just the size of loads - which typically results in a larger number and/or higher rated circuits that strictly necessary - so ignoring the circuits you actually have and basing diversity calculations on the minimum possible number/size of circuits from just the load point of view can give more realistic results. Going further you can just base calculations on typical Watts per square metre of floorspace and ignore the division into final circuits altogether.


Your example sounds a little unusual (rather a lot of showers for what seems (judging but the power and lighting circuits) quite small office areas). You'd normally have more diversity to apply at each DB/submain so things don't normally add up anything like as fast as your 900A suggests. If you have a three units per floor and three floors then you'd have a total of 18 showers - even applying the OSG tables you'd only take 100% of the first two and just 25% of the remaining 16 - which even doing likewise for the other loads will come out far lower then just adding up the per DB totals. Plus of course, for this sort of size installation, you'll probably spread your single phase loads over three phases - so reducing your numbers by two-thirds.


  - Andy.

Is the whole building supplied as single phase ?


As a sensible starting point, what's the gross internal area (GIA) - from there apply something like 35W/m2 to give you a total kW demand (everything credible in an office building)


If you then take a floor by floor approach and apply a diversity of something like 0.8 to the total figure above and pro rata that back to the total GIA you should then get a sensible value per floor


From there you should be able to apply at least a factor of 0.8 to the sum total of each floor

In summary, apply 35w/m2 to the GIA and diversify that by 0.8 per floor and a further 0.8 to collect each floor at the intake (or as a single step, GIA(m2) x 35 x 0.64 = facility W/m2


You can deduce amps from kW when you have the numbers (remembering if it is 3 phase, you would ideally like some balance across the phases, so assume 33% of demand is on each phase)


The above is conservative, and includes pretty well everything required like lifts, electrical input into ventilation and other mechanical plant, but doesn't include any electrically driven heating or cooling (VRF, Chillers and the like)


If you can find a copy, look at the British Council for Offices guidance which might suggest that you actually only need about 15m2 for the office space (power and lighting) - but that's diversified for a reasonably big building where you expect some people to be off site, on leave, in meetings etc)


Regards


OMS

I do not agree with the above assessment regarding the electric showers.

Whilst the accepted guide is indeed to take 100% of the largest two units plus 25% of the remainder, this should in my view be applied per phase and not to the total.

Presuming a three phase supply and that the 18 showers are connected 6 to each phase, then I would calculate on the basis of the first two showers at 100%=64 amps, and the remaining four showers at 25%=32 amps. That is 96 amps per phase for the showers.


The principle behind diversity calculations is of course the assumption that as the number of loads increases, that the chance of them being used at the same time declines. With single phase loads, the calculation should therefore be per phase, and not in my view by taking the total and then dividing by three.


Had the supply been single phase, then in my view, to take 100% of the first two showers, and 25% of the remaining sixteen showers would be correct. In practice we may assume three phase as single phase supplies of that capacity are almost unknown.

Whilst the accepted guide is indeed to take 100% of the largest two units plus 25% of the remainder, this should in my view be applied per phase and not to the total.

Agreed (I was trying to make two different point and had overlooked the combination of the two!)

  - Andy.

If these are offices with no parking spaces and where employees are encouraged to cycle to work, then there may be many sweaty personages all trying to use the showers at 08:50, and diversity might take a nosedive.

Thank you all so much for your replies.

Andy...there aren't that many showers. I just put a example together for brevity. :-) (No-one needs to be that clean).


The real life example I was working on was:

1 x 3Ph main Board;

feeding 3 x 3ph sub main boards (1 per floor);

Each feeding 7 x 1 Ph Dbs on the floor.


The main issue I had was that there were 33 general power circuits.

So I spoke to the client, and we agreed to assume 2 x 32A supplies per floor. (Approx 125m2 floor area per floor). So then applied diversity to that.


What I've done, in the end, is to take the building as a whole, apply diversity to that, to calculate the main supply.

Then, divided the building into floors, and carried out diversity to each floor, to give the sub main calculations to each floor's 3ph board.

I'm about to do the same for each 1ph DB to give me the supply to each of those.

Does this should reasonable?


Apologies for any spelling mistakes...spell uncheck is going crazy at the moment.


No.9 in Table A2 seems a really simple way of calculating..is it too simple?


Once again, I really appreciate your comments and advice.

wallywombat:

If these are offices with no parking spaces and where employees are encouraged to cycle to work, then there may be many sweaty personages all trying to use the showers at 08:50, and diversity might take a nosedive.

But equally, all those people in showers won't be at their desks etc, so there is some balancing going on here. Unless the whole workforce is made up of teenage girls, I would expect most morning showers by cyclists to be short sharp activities  - and there could easily be several consecutive showers that would actually only get you to appoint where the cable is at operating temperature (ie the cable constant)  


Whilst we should not be designing for small overloads of long duration, a bit of a flurry of activity with the showers would not neccesarily be of any note in terms of a sensible design


If there really are 18 showers, then I suspect a good engineering challenge would be to consider a calorifier and a 3 phase immersion input (so you can trade much lower input energy for time to achieve the same number of showers)


Regards


OMS