Load/Cable Sizing

British Standard 7671.

Have a look at your cable calculations again. How much expansion do you envisage? 400A seems excessive.

<...400A seems excessive....>

What voltage, though? The OP hasn't shared that titbit.

in addition to your voltage . . . please also share your location? ambient temperature?variation between day and night temperature and seasonal variation? whether your cable will be armored? whether your cable will be laid underground or in air? whether you will use aluminum or copper cable? what will the length of the run be? what type of end connections will you use? do you expect any harmonics to be present? what do you expect the power factor to be?

once we have all the titbits in place then we shall be glad to offer a whole heap of solutions . . .

Quote "My total load after diversity and all is 146KW,3 phase supply"

That's impossible. Please list the load in the apartments.

Load assessment depends on the type of loads each apartment have-AC(room/central), electric cooker,lights,socket connected appliances,elevators,water pump etc. You didn't give connected load,diversity assumed etc to enable us to comment. Visit a similar apartment and check the kW or Amps drawn on a typical day/night to get realistic figure. For example in my(???) country in an 8 storeyed apartment Condo with 36 apartments of 3 bedrooms and 4 bigger apartments including an elevator and 2 water pumps and without central AC the main 3pole MCCB installed by the utility is rated at 160A 3phase meaning an average load of approximately 10A single phase for each apartment including lift and pumps and few room AC units in some apartments. Similarly if you have experience you can estimate without calculations. Also measure the area(sq ft) of apartments you visit to get an idea of unit rate (watts/sqft) and have it as a reference.

first of all, you need to state if you are in the UK and following the 17th Ed because of that.

For those that don't know, if you are following the 17th Ed IEE electrical regulations, then it is accepted that you are conforming to BS7671.

You must remember that if you are either installing OR designing an electrical system for a building, the regulations IEE 17th Ed, must be followed, if not then the Building Inspector/City Planning Officer to which you will present your design as part of the overall project package, will reject your application for approval and permission to begin construction.

If the regulations, and "on site guide" instruct you to do something, then the planning officers will be looking for that compliance.

I strongly suggest that as you hesitate to accept what is given by the 17th, you contact a NICEIC electrical contractor, a larger company would be better, as they will have a design department.

Asking this forum to explain your doubts and worries will give you answers from all over the world, and with no disrespect to our overseas members, they are not familiar with the UK regulations and more importantly, the implementation thereof.

Rules and regulations are for guidance of inexperienced people. If anyone without proven experience design or estimate load in an electrical installation he might end up in trouble. A CEO of a utility once told many engineers claim to have a degree or charter(C.Eng/P.Eng) but they over-estimate loads in buildings and ask for higher capacity(A/kVA) power connections but after energising they will find sometimes half the recommended capacity is sufficient. Many engineers do not visit completed installations once they design to verify their estimated/calculated load. Another issue in this forum is many members do not provide basic information. In this thread they never informed the country,the type of apartment(for high class or luxury,lower/upper middle class,low class,shanty dwellers etc) what are the equipments (washing m/c,AC,clothes dryer,dishwasher,electric range,fridge,microwave iron,space heaters etc) used in the apartments etc. Without knowing these if anyone estimate the load by considering only ratings of sockets installed he may not get good results.

Pnaban, with respect, you are in Srilanka, and I cannot agree with your comments. The IEE Elect regulations are there for the professional to KEEP them in line with current legislation in the UK.

The regulation are there for a purpose, unless you have worked in the UK, designed systems using the regs, then you are speaking with no understanding or comprehension of OUR regulations in the UK.

Do they over design...."A CEO of a utility once told many engineers claim to have a degree or charter(C.Eng/P.Eng) but they over-estimate loads in buildings and ask for higher capacity(A/kVA) power connections but after energising they will find sometimes half the recommended capacity is sufficient."

Possibly... but that is NOT applicable here if using the UK regulation correctly......

In the UK, to be able to work properly, one requires to pass an examination in the UK electrical regulations, so it keeps EVERYONE in line and on track.

In UK are engineers required by law or by their boss/organisation to visit energised buildings/projects and measure the load on typical day/night to confirm/justify their calculations/estimates of load/main incomer?. If they do not do it engineering becomes a theory not a practical applied technology.

i have worked as a consultant to consultants for projects for firms based in UK and other countries.

the terms of reference for appointment of consultants included penalties for failure of design, energy losses and other aspects of non performance traceable to defective work by the consultants.

when designers become responsible for work done by them and legally accountable and liable, then the concerns about visiting the site and auditing automatically deemed part of the larger responsibilities.

I must ask have you EVER read the UK IEE 17th Ed electrical regulations?

Still I didn't read the 17th edition. Could you give a link to read the pages relevant to this thread for all participants who are interested?. Thanks.

As I though you are speaking out of turn with NO knowledge or experience of the subject matter.

Post a link you ask..... compounding your lack of knowledge of the UK Regulations.

There is no link....

Again, as I suggested in many posts and to you in this thread, if you don't know what you are talking about.. say nothing.

My father has a saying..." Open it and prove it.. keep it shut and let everyone think it!

brich . .

i suppose you must also explain that codes and standards are priced publications with copyrights in place . . . whereas it may be necessary and allowable to quote a clause or a part, it may never be correct to link to a site displaying what may be an un-authorized copy . . .

maybe you may help with a link pointing to where a copy may be available for purchase . . .

Thought of it..but as you have explained it well enough I have nothing to nor want to add to your comments.

However as our friend is in Srilanka and has NOT read the 17th.. what is the point in a fruitless continuance, the OP has not replied, and those that have studied and PASSED the required examination in the 17th Ed IEE Regulations for the UK, know and conform to the requirements of the regulations.

Need I say more? No!

bingo!

This forum is not a quiz or competition.Members sometime quote from codes or at least the ref nr of the relevant clause. Should the C.Eng who gave test report prior to energising the installation take measurements or record Amp,kVA,KW in each circuit and maximum demand to justify the design and if varies widely to report to utility as well as to design engineer's boss that the design engineer didn't do a good job and as a result they have applied for a larger kVA from utility?. Or does the C.Eng his own calculation or ask for calculation sheet prior to testing?.

My dear brich and kebian, I referred a book, which I misplaced somewhere,written by Electrical Contractors association(UK) on IEE wiring regulation(17th edition)which says in clause 311.1-diversity could be applied a)for items on a final circuit(except socket outlets-assume W/sq m),b)between final circuits,c)between sub DBs or sub main cables

d)at each MDB. Table C3.2 gives diversity factors for a)lights-0.7,0.8,0.85 and 0.9, b)heating-0.6,0.7,0.8 and c)sockets-W/m^2 or table C3.1 gives an assessment of load equal to estimating MD for commercial offices- a)without AC-55 or 60 or 70%,b)including AC-100,110 and 120%. Also it says "it should be noted that many engineers,technicians and electricians overestimate loadings-perhaps to play safe and so on. At least now why don't you give your calculations-step by step for forum members to see?.

Please read posts 12 and 16,Is there a requirement for design engineer/organisation to take measurements after energisation and confirm accuracy of design?. Do consultants take measurements and feed back to designer to make improvements in future projects?.

Just had a look back at your past posts kebian, have you bought any books on this subject? I would doubt it very much, I think you are wanting other people to do you work for you as usual.

Dear Members,

I very much apprecaite your comments,suggestions etc. Some more details on my design.

there are 4No 2bedroom apartments and 3No 4bedroom, the former has 4 ACs per apartment and the latter 5ACs per apartment. there are cookers and washing machines. no lift, no dishwashers etc. this is West Africa.

my total overall load like i said is about 146KW, so my main incomer was 250A mccb but allow for say 20% future capacity - 300A . but here 300A doesnt come as a standard, that is why i chose 400A adjustable incomer

of course am not that experinced as an engineer, and that why i want your help especially w.r.t. economization. is it am oversizing and not applying enough overall diversity?

TonyS, i have IEE reg 17th edi i bought for alil over 50 pounds. is it wrong for me as a not so experienced young engineer to seek help as i climb up???????, i find some of your responses too personal and abit unfair.

You surprise me.. trying to adopt the UK regulations to West Africa, as for your statement....

"of course am not that experinced as an engineer, and that why i want your help......."

...we already worked that bit out..... As to why you want the use the 17th with no practical experience is way beyond me, did you buy the "on-site guides" to go with the regs?

I strongly suggest that you look very hard for someone near you that has the practical & design experience you need and let the experts do it, as you my friend are dangerous on your own.

Could you please give connected appliances and light/socket(amp & no of phases) in each area(bed room,kitchen,sitting room,bathroom etc)?.

you miss my point.

that i am not that experience does not mean i dont know much to do this.

i am confident of my design calc's - the 4 bedroom apatments DD is about 22KW(*3), the 2bedroom DD is about 19KW(*4) giving a total of around 146KW. generally its 3 10A light ccts, 3 30A ring ccts, 4/5 AC 32A ccts, 32A ccu, allow for washing machine (2500W) for some aprtmnts.

am confident of my calculations and ok with my application of the iee reg.

the help i need is on the my main incomer/ cable size, thats all. i need to knw if its ok or i have oversized and cost my client more than is due.

i have this difficulty because its my first domestic design, industry where i started and gained my pratical experience- Nestle Gh IC and Mill Plants, are quite straight forward, in terms of loading and diversity as compared to domestic as far as my experience is concerned. so pls lets concentrate on my incomer and supply cable size, okay

thanks

Please give a sample calculation for an apartment as given below:

Item Watts Quantity connected load(W) DF Load(W)

lights ....... ......... .................... .... ...........

AC ............ ......... ..................... ....... ..........

washing m/c ............... ........... .................... ............ ...........

water heater .............. ............. .................. ............ ...........

electric iron........

rice cooker.........

etc etc

Total.......................................................................................................

Overall diversity ............................

Calculated load ..........................

Kebian

At least after reading post 26 please ask Mr. brich(honorable britisher) to send relevant pages from 17th edition and his calculation as per IEE wiring regulation (17th edition). You can calculate now in three different ways(BS,NEC,my method) and tell forum members which is the most accurate, after energising the installation. In fact I was a member of IEE from 1965 and realised IEE and BS are very useful guide to electrical professionals except the diversity factors used for load estimates as equipments in homes vary from region to region in the planet. In fact IEE wiring regulation was known as "electrician's bible" but it covers only wiring lights/sockets but not covering industrial installations. They have different BS for each kind of work say lighting/illumination,earthing,lightning protection.motors etc but NEC is a complete book with many chapters each for a particular area. In fact I have suggested to prepare a BEC(british electric code) with many chapters each devoted to a certain aspect of work similar to NEC for easy reference instead of having a library of all BS/CP etc which only rich people/engineers can afford. IET should be aware of engineer's income in poor countries.

Good try but no cigar for you.. If you think I'm doing your work for you..think again!

In this thread there are three ways(BS/NEC/my method or proven experience) to calculate the load. If experienced members give proper guidance to inexperienced engineers they will improve their knowledge. A very good example is the ECM Web workings based on NEC and also my method. We didn't get only the BS7671 method.

I gave guidance..get someone who knows what they are doing. Furthermore as you seem to have covered all points and have declared that you have the proven experience & expertise, why ask me to contribute?

And for the final time BS7671 ONLY applies to the UK!

If BS method also is published in this forum members can judge which is more accurate-BS or NEC or proven experience

Kebian

Below are two links that guide you througt apartment calculations. It is based on the NEC. Take a look and see if is any help. We are asking you to list the apartment loads. Your calculation appear way too high.

http://ecmweb.com/nec/code-basics/electric_clearing_multifamily_dwelling/index.html

http://ecmweb.com/nec/code-basics/multifamily-dwelling-unit-service-feeder-calculations-20091201/index.html

Pw and Tony

Is this anything like British Standard 7671?

I actually found a copy on line. It was hard to read. It was in English.

For AC circuits why did you apply a DF of 70%?. For lights and sockets why you did not apply DF?. Quote from some code/regulations. What are the loads connected in sockets and their rating(W)?. Finally if you apply an overall diversity factor(simultaneous loading of all apartments) you will get more accurate results. Why don't you ask the british engineer to calculate and show the final results?. When do you think the installation will be energised?. Please measure/record at MSB the total current drawn at peak load as well as at the CB feeding each apartment and let us know.

Wareagle,

Thank you very much, it was more than helpful, exactly what i needed.

All other replies, i appreciate your comments, suggestions and "attacks"

I just needed the experienced Engrs input/advise/correction so i wud not cost my client so much in terms of load/sizing calculations, especially as this is my first domestic design

Kebian

You still didn't reply to post 30. What is the code applicable in your country(BS/NEC/VDE etc)?. What kind of climate is in your country-is it like USA(winter very cold/snowy summer hot)or UK(generally cold and snowy) or Saudi arabia(summer very hot and winter wind is very cold,no rain or snow)) or India (north/south,high altitude or low altitude). This thread looks incomplete without vital information from you.

Pnaban,

we use BS, its normally hot here in West Africa

Why don't you answer my post nr 30 and fill the form given in post 25,we can calculate and see how accurate the results will be compared to NEC and BS?. Also don't forget to inform us the max demand when the installation is energised.

dear pnaban,

i simply marvel at your perseverance! admire it too.

in lighter vein - reminds me of the anecdote of the pack of four boy scouts who helped the hollering old lady cross the street by carrying her over . . . hitch was that she was actually looking for a cab on this side of the road and they did not understand her language when she asked for help to get a cab.

on a serious note now . . . i recall reading in post number 34 that OP has said that he has received "EXACTLY" what he wanted.

however your happiness to serve is commendable and i am sure i will befriend you to seek your expert guidance whenever i am stuck with an insurmountable problem in electrical engineering.

God bless!

- Soeb

He want to finish this thread incompletely. For the benefit of all forum members he should calculate using the three methods(BS/NEC/mine or experience)

Please give a sample calculation for an apartment as given below:

Item Watts Quantity connected load(W) DF Load(W)

lights .20W...... .12 No per circuit (*3)........ .................... .... 80%...........

AC ....2500W........ ..5 circuits....... ..................... ....... ...70% ???.......

washing m/c ...700W............ ........... .................... ............ ...........

water heater .............. ............. .................. ............ ...........

electric iron.1000W.......

cooker Unit 3000W........

Ring ciruit 20A breaker 4600W *3 circuits....DF..100& 1st cct + 40% of remainder=appx 8000W ????

Note: per my calc, washing machine, iron etc are under ring ccts,except cooker unit

Total.....appx 30KW per 4 BR apartment so20KW*3 + 18KW*4=....appx 198KW..........................................

Overall diversity ............................

Calculated load ......appx 132KW....................

__________________
pnaban

This too is high.Please calculate according to my post.We will see the results.

Hi friends,

The 4*150mm sq copper cable can carry 400A current.To be safe use 250A MCCB.

4*185mm sq copper cable can carry 460A current. Here use 400A MCCB .

My suggestion is to go for 2nd option. Your country is a high temperature area . The A/C units may work together at full load for a long time in some season.

Hope this is a simple answer to the Tread.

Did you check the load calculation,after that only we should think about cable sizing?.

With an assumption that it is already done by him.

How do you know whether it is correct?. Like ECM web why don't he and the britisher give step-by-step calculation procedure so that all forum members can judge it?.

Arifkunnah,

from table 4D4A of IEE reg, BS 7671, 70 degree celcius multicore armoured cable, thermoplastic insulation. underground/ around building:

120mmsq-192A

150mmsq-217A

185mmsq-243A

240mmsq-280A

????????

Don't jump the gun. Before sizing main cable why you didn't apply DF for lighting circuits and for sockets and ACs which code specified the DF you applied?. Also ask the british engineer to give his step by step calculation. My estimate is less than 100kVA. Now at least give your and britisher's assumed DF.

Pnaban

Light_80%

sockets( ring circuits) 20A breaker, 3 circuits.

100% for 1st ring cct, 30% for remaining two ccts this totals appx 7.4kW for all 3 ccts

again this takes care of all electrical appliances, except the cooker control unit. there are no water heaters

Please mention the relevant clauses of the 17th edition you considered for diversity for lights,sockets,AC,cooker etc. In my post 48 by mistake I have mentioned some values of DF as % but it should read as W/m^2. Also in my book"Guide to the wiring regulations-17th edition-2008)"it is mentioned under diversity to consider a)connected load,b)duty cycle(cyclic load)-starting,operating and stoppage times(operating time/total time),c)crest or load factor and Max demand=connected load x diversity or compare with table of norms for similar installations. They use for major plant loads diversity and added to w/m^2 of general areas of installation. Also while assessing individual loads there are differences between rated current given by manufacturer and actual current consumed. They have found this to be 50 to 70% of quoted value. The watts per sq m method depends on size and type of installation. This value could be obtained from BSRIA,CIBSE or consulting design firms or large contractors. For diversity loads are 2 types-due to law of averages will not be on at the same time and loads due to fact will not be on at the same time. Diversity can be applied a)for items on a final circuit b)between similar final circuits c)between SDB/sub main cables d)at each MDB. DF can only be used by those with suitable experience on the type of installation.

I used the On site guide of the IEE reg, specifically guidance note 1.

Note also that wrt diversity the reg also allows the engineer to use his discretion to some extent, depending on some factors.

As i have done, given our domestic load levels, appliances etc and also taking guidance from some snr engineers' designs.

After a review, my total load is about 132KW, overall diversity 75% gives appx 100KW

total current becomes 174A, 125% for future additions gives 218A

so the 250A mccb incomer will do.

Any comment on my 75% overall diveristy????

in one of the above replies, 185mmsq can take 400A?????

but table 4D4A of iee reg BS 7671 does not agree. but in his favour, i have seen some snr designers, specify 4*185mmsq for 400A panel boards???? doesnt match table 4D4A, any comments???

Remember I told it to be less than 100kW. Now please measure the maximum demand(Amps/KA by load analyser) and compare with the utility's MD and let us know to confirm whether our calculation(diversity)is correct.