Thank you for your replies and guidance. All responses are very useful and I think going forward the best practice is to clarify this with the Client as it is opened to interpretations.
Yes, that is the question, as others often say. I say this is probably inadequate for a real office that is fully staffed, and working days are 8 or 9 hours, so diversity by both time and load is not applicable. Mike is quite right in that all the real offices I have ever worked in were pretty power-hungry, often inadequately lit, and some too cold to work without extra clothes, all of which hit productivity so are not cost-effective. 27W per square metre is probably enough for just lighting, perhaps all these people use pens and not computers, but I doubt it. An office which is 1/6 occupied is not a real figure, it is taking the maximum occupancy allowed per sq metre, and allowing for desks, cabinets, walkways and all the plants and dividers of course. A fairly full office probably has one person every 3 or 4 square metres, so the 54W figure probably could just about be met. Please do ask the other questions I suggested, they commonly come up in designs. In my home office, about 7 sq metres I have 3 computers, a laser printer, shredder, 2 desk lights, 2 chairs and about 5kW of other equipment, bits of which I use occasionally but rarely all of it, fed from 2 13A sockets to the supply. The total LED lighting is about 20W per sq m, until I use a 500W tungsten spotlight for videography. That could be replaced with a 100W LED I suppose, but you can see how it all adds up. If people are going to use skype and zoom, getting a decent picture does need light in the right direction, and sufficient to fade the rest of the office somewhat so that it is not distracting. BTW, I hate bookshelves as a background, they often give far too much of an impression of the speaker, particularly if the book titles can be read! There was a recent incident which caused severe embarrassment to someone to a lady, I wonder if you saw it on the BBC?
My office is a bit full, but you will get the idea!
No diversity should be applied to lighting in my view. It is reasonable to suppose that every light will be turned on if the office is used during the hours of darkness.
I would in fact add a small margin for manufacturing tolerances and and mains voltage variations. If the calculated load is say 60 amps, then allow for 66 amps. Add 5% for manufacturing tolerances and another 5% for mains voltage variations.
For general purpose small power circuits, a lot of diversity can usually be applied to the circuit ratings, 50 circuits each of 20 amps does not normally mean 1,000 amps in total.
If however the load to be supplied is specified or calculated in watts per square meter, then in my view this presumably includes diversity. (the exact loads are unknown and wont all draw the maximum power at the same time) If the specification states say 40 watts per square meter , then in my view it means 40 watts per square meter, and not some lower figure.
Such estimates work well as an average for LARGE office areas, provided that a reasonable figure of average watts per square meter is allowed.
For small offices it can end very badly. Consider an office five meters by five meters, an area of 25 square meters . At say 40 watts a square meter for general purpose power, that is only 1,000 wats. What if someone uses a 3Kw kettle, or a large vaccuum cleaner or a power tool ?
For 100 such offices, a total of 100 KW might be reasonable, they wont all use kettles or power tools at the same time ! but for a single small office no way.
A good approach for small offices in my view is to allow for the actual connected load for lighting, and say 40 watts per square meter for general purpose small power, plus 3KW, no matter how small be the office.
How many watts per square meter to allow calls for engineering judgement and experience.
Yes, this 54W/sqm is for lighting and power, the mechanical load was 50W/sqm. The 54W/sqm is actually an enhancement to BCO which states 27W/sqm for power for 1/6 occupancy.
It seem very low, real consumption depends what is being done in that office - maybe if the desks are big, and each one has nothing more than one chrome book style laptop and a very small reading lamp, and telephones are powered separately, or of it is a hot desk facility where half of them are unoccupied.
Even if that is true, if every third desk needs a printer copier or shredder and a few folk want twin monitors you could easily exceed that. Sitting here at home. I have two of these dell monitors at 75 watts each, and about 100W of dell laptop and its powered docking station, a dect phone and cradle at 12W or so and my installation is very modest compared to my wife as an senior accountant who has more screens, a dedicated laser printer, and a network hub.
Do not allow any further diversity, and in your shoes, I'd query if it is really correct.
Lighting on the desk surface will be perhaps 300 lux if work is only done on computer, rising to more like 500 where there are paper documents to be read as well. Relating that to circuits and watts will depend on lighting fitting types and locations.
As BOD says just confirm this with the specify in writing just in case your design is found wanting and you are not the client wants to claim on your PI.
It sound like this has come from what are known as "Industry Norms" published by various bodies. They are always conditional depending on the type of premises and what is installed.
Oh I never considered that people actually soak up lighting so 2 people in a room require more than 1 person or 100 people require a lot more. Simple me just chastised meeself about that.
Seriously though, the specifier should have made it clear or even left the term out if the figure includes it. I`m not surprised you thought to ask the question.
BODs remark is not a bad idea because of other questions.
These may be:
Is it a specifically low energy "Green" building, because certain permissions and Grants may depend on this number?
Are you expected to allow for any extra load at all in the design, it only takes a new vending machine and a water chiller to cause trouble?
How are personal appliances to be banned? Underdesk heaters, chargers for personal appliances etc are all very likely to "appear" which will upset your magic number. Diversity depends on having less load somewhere else!
Are there air conditioning and heating loads, you need full details, these may make other loads look small.
If the mains supply is limited to this number, it is likely to cause problems later, you need to be very sure that this specification is NOT your problem, whatever happens later.
If the electrical supply is expected to be the building heating, if it is very well insulated, what happens if there are unseasonably low temperatures? Where does extra heat come from, because it may be expected to be electrical? They may have included this in the "diversity" figure because it is only occasional, but needs to be considered.
