Demand Factor 'v' Diversity Factor - which is your preferred method

[SC-S]

Gents,

I'm applying for CEng through the CIBSE TRR route, my chosen subject determining maximum demands within commercial properties. I've been reading through some of the previous threads concerning manximum demand calculations and note almost everybody has a completely different approach, most of which are not actual recognised methods of determining maximum loads. These include applying factors such as 0.41 to a total load or taking a percentage of the breaker size and simply adding them all together. There is a general acknowledgement that the maximum estimated loads are a 'best guess' however there are wild differences in estimates between using data such as appendix 1 of the OSG or CIBSE Guide K. I also believe that electricians trained under C&G have a completely different approach to those engineers trained at universities.


I would therefore like to add some feedback from you guys within my report so that I can identify your preferred method of calculating maximum demands. All i'm looking for is the method you use on a day-to-day basis and how you were initially trained. As an example I was initially trained using C&G 2361 therefore used OSG appendix 1 however many of my colleagues studied their BEng at Southbank and used CIBSE Guide K or BSRIA rule of thumb.

Any and all comments woudl be greatly appreciated.

Thanks

Steve:mad2:

 

[malcolmsanford]

Add up the protection device values and then

domestic x 0.4
commercial x 0.6
industrial x 0.8

And as a rule of thumb as worked quite well for me over the last 40 yrs.

Bottom line for you obtaining CEng is that you will have to take each installation at face value and decide what you feel is the best way to design the installation, using your experience and knowledge as the real guide in applying diversity.

 

[SC-S]

Malcolm,

Thanks for being the first response. Appreciate each building is taken a face value however are your figures based on your own 'rule of thumb' or were thay an adaption of figures used by other electricians you worked with in the past? I would also be interested to know if you have ever checked your estimates against the actual loads either during commissioning or some period after PC? Thanks again.

 

[johnboy6083]

i think as an engineer at the design stage you will have an understanding of how the installation is going to be used. For example with commercial properties, you can be fairly certain that the lights will be on for the whole time that the premises is open. Air conditioning will be on for most of the time during the summer months, and heating during the winter. Its your experience, coupled with common sense that i believe will give the most accurate figure.

Some situations will not allow you to provide much diversity at all, for example pumping stations. This is because there are times, when every pump is expected to runs, during times of high water demand.

Please share your paper with us if possible, as i will be very interested to read it

Good luck,

John

 

[Welchyboy]

I recently calculated a maximum demand current for a bowls club new build, no gas all electric, amounst other loads it had an air scource heat pump /8.5 kw water heater/8kw cooker and various other hand driers/small water heaters and about 30x twin 5ft fluro fittings, i calculated the max load as per BS7671, it came to around 155A total,
when the job was complete we run just about everything at full throttle! plus all lights on, kettle on, etc and it struggled to get past 92A!

 

[netblindpaul]

Welchyboy,
Was that with a "normal" clamp meter or a power analyser?
I am guessing you did not allow for leading and lagging power factors in the kit when you estimated at 155A, thus there may have been a "cancellation factor" in the real install, it would have been interesting to see the changes in the PF as you loaded up the install.
That would have given you an idea of what was happening.

 

[Guest123]

I think a certain amount of a designers own 'wiliness' comes into play when calculating the maximum demand of an installation.

When attempting a calc for the first time, most will follow the guides blindly until experience is attained and confidence in your own ability & judgement comes.

I would say most use the OSG when first venturing into the world of max demand & diversity although the electrical installation design guide is also a good reference tool.

 

[beatboaby924y]

wondering if anyone could help me with this?

Was at a job today in a hub site to test a new radial circuit thats been put in for a new fire alarm panel, now even though I am fairly confident in testing I haven't filled in that many full test certificates (something I think is harder than testing the job). I would in the past fill in the test results and the gaffer would fill out the certificate. Today though I had to fill in the whole certificate and came to a couple of parts I was a little unsure of.

Supply characteristics

Supply protective device characteristics- Is this the main fuses at the bulkhead? eg BS88 160A not the sub db supply?

Fault current- The PFC at the main switch after the bulkhead? eg 1.22ka

External loop impedance- At the main switch after the bulkhead? eg 0.22 ohms

Max demand- There are no old test sheets with the infomation on them. So how would I work it out without adding all the circuits up and applying diversity because the place has db's scattered about all of the place. Can I work it out by the size of meter,incoming supply cable or bulkhead fuses?

Main switch or circuit breaker details

Is this at the db the circuit is supplied from? eg db supplied from a BS88 60 fuse?


Sorry if I'm being thick but don't want to find it out wrong.

cheers any help is welcomed

 

[brucelee]

OSG but find its a bit of a hit and miss as seems with max damand after diversity

I have been told
all cbs X 0.4
all loads x 0.4
or largest load +40% the rest

I would be very interested in what comes out the best way
from now on i am going with Malcolm as he knows better than I

and as lenny says the Iee elecys guide to design is on my hit list for books from amazon lol if thats the book he is referring to

by the way whats a BRSIA rule of thumb and the CIBSE guide

 

[SC-S]

BSRIA stands for 'Building Services Research And Information Association' (origional spelling incorrect as I have fat fingers and a small keypad). BSRIA is a test, instrumentation, research and consultancy organisation, providing specialist services in construction and building services engineering. You can hire test equipment from them or have items independantly tested. I've used them before when a light caught fire to have the fitting tested for safety. They produce a rule of thumb book which provide base level design information from electrical & mechanical loadings (w/m2) to structural allowances, costs, etc. Follow link to site www.bsria.co.uk/


CIBSE is the Chartered Institute of Building Services Engineering. They produce design guides for mechaical, electrical and public health services; esentially the Bible for services consultants. Guide K is for electricity in buildings and there are a number of Lightign Guide documents covering internal and external lighting. Follow thsi link and have a look at their bookshop. www.cibse.org/.

Hope this helps.

 

[Welchyboy]

Welchyboy,
Was that with a "normal" clamp meter or a power analyser?
I am guessing you did not allow for leading and lagging power factors in the kit when you estimated at 155A, thus there may have been a "cancellation factor" in the real install, it would have been interesting to see the changes in the PF as you loaded up the install.
That would have given you an idea of what was happening.

Hey paul

i used a standard clamp meter, if i had taken leading lagging currents into account as you said, would my calculated demand have been less or more?, i must be researching more into my current demand calcs by the sound of it

 

[netblindpaul]

Welchy,
Leading and lagging currents would "basically" cancel out in a standard clamp meter.

 

[Welchyboy]

That would explain things, i am sharpening up on my design lately so please bear with me,
i added a multiplier of 1.8 to all discharge lighting which was around 30x 58w twin fittings

so (2x58w) x 30 x 1.8 = 6264w

Allowable diversity at 90% = 6364 x 0.9 = 5637.6w

= 24.5A

would it make any difference in the 1.8 multiplier if i am using HF fluroescent fittings?

 

[netblindpaul]

All fittings would be different, check manuf info mate.

 

[esparks]

That would explain things, i am sharpening up on my design lately so please bear with me,
i added a multiplier of 1.8 to all discharge lighting which was around 30x 58w twin fittings

so (2x58w) x 30 x 1.8 = 6264w

Allowable diversity at 90% = 6364 x 0.9 = 5637.6w

= 24.5A

would it make any difference in the 1.8 multiplier if i am using HF fluroescent fittings?

I know I am a bit late commenting but for information only.

1/. I would not put 30x58W Twin Fluorescents on one circuit. The maximum circuit protection for lighting would normally be 16 Amp or 10 Amps.

2./ Therefore the normal lighting connected load for a 16 Amp circuit would be around 8 Amps, for a 10 Amp circuit the connected load would be 5 to 6 Amps.

3/. The 1.8 multiplier is to account for the start (switch-on) current and is recommended by most lighting manufacturers.

4/. In respect to demand factor / diversity, they are different and it depends at which stage of the design it is calculated. The best guides for an initial load assessment is the BSRIA Rule of Thumb (now in the 5th Edition) and / or the CIBSE Guide K, which I think is still in the 2005 version.

5/. The BSRIA Rule of Thumb generally gives loads in watts/sq.mtr for various building types.

6/. If the design is complete then it should be calculated as total connected load multiplied by diversity, which will be different for each type of load. This is applied to each individual distribution board and calculates the rating of the supply to the board.

7/. I would then apply a demand factor at the main switch panel, which basically is a diversity factor of all loads connected to the panel. This will give an estimated 'maximum demand' value. And I mean estimated not actual.

8/. To get any actual maximum demand value requires a line load analyser to be connected to the incoming supply. This needs to be left connected for a period of time to see when the peak demand periods are. It will also be different at different times of the year.