Additional Demand

[Z1gic89]

I am currently wiring an empty shell of an office which joins on to an existing premises.

The main supply for the building is an 80amp fuse, there is space for  20 people to work at the moment, of which about 15 do, and next door could house an additional 12 with a storage area bellow, whats steps do i take to work out maximum demand, to see if the 80 amp fuse needs to be upgraded or a new supply installed to the empty shell adjoining office.

i tried adding up all rcd values and multiplying by 0.4 but i got nearly 200 amps, where am i going wrong, or is the original installation too big for the supply fuse already

 

[Andy™]

there is a lot more to it than simply adding up everything and muliplying by 0.4.... you can easily have a 2 way board with a 6 & 32a mcb, re-wire the property and have 2x6 & 3x 32. your calculation says its different but in reality its still the same.

and if it was too big for the supply already, then it would probably have blown a fuse by now. you have tested the actual current being used now, havent you?

 

[Z1gic89]

just 30amps being used at the moment, that is with two floors of office space used at about 75% occupancy, so i could add another floor, which would make the total around 45amps? but i need to take into account that none of the aircon/heating units are on, which, when the new unit for next door is installed. could add up to 100amps!!!!!!

the electrician who installed the board next door, used a 50amp breaker from the main d/b, and i am currently wiring next door off the new board, i have no experience in this area (calculating maximum demand) and i am just after a bit of advice as i do not want to blow the main fuse when the office gets busy/cold/hot...

 FYI the office i am talking about is where i work, the comapny itself is not an electrical comapny, i just install small industrial installations to go with the ventilation contracts we get, so i am the only person in this company with electrical knowledge, and they have tasked me with the fit out of the office next door...

 

[Steve3948]

You also need to consider rest room/tea room/canteen.

 

[Blue Duck]

And when its cold in winter staff will bring their own 2kw heaters from home to keep warm and melt the neutral bar!

 

[Evans Electric]

Is that 80A  a single phase supply or 3 ph ? 

If its single phase its basically equal to a domestic supply for a house with ,say 4 people in it.

How much air-con is there ?  

Offices in winter can be quite high current users if the heating is electric , with lighting on constantly , IT equip , air con /heating but it never fails to suprise me when some places function happily away on a small supply .  

Look at your fixed loads first , the ones that will always be in use , not much natural light , lighting will be 100% ,    IT stuff is on all day, something like 2A per work station .   Don't worry about the office kettle.    

 

[Sidewinder]

Fixed electric heating?

 

[SPECIAL LOCATION]

I am currently wiring an empty shell of an office which joins on to an existing premises.

The main supply for the building is an 80amp fuse, there is space for  20 people to work at the moment, of which about 15 do, and next door could house an additional 12 with a storage area bellow, whats steps do i take to work out maximum demand, to see if the 80 amp fuse needs to be upgraded or a new supply installed to the empty shell adjoining office.

i tried adding up all rcd values and multiplying by 0.4 but i got nearly 200 amps, where am i going wrong, or is the original installation too big for the supply fuse already

so  RCD x 0.4 =>200Amps...

200Amps / 0.4 = 500Amps..

RCD's = 500Amps...

Typlcal RCD's  

30ma   ....    500A / 30ma = 16,667 RCD's fitted...

or

100ma   ....   500A / 100ma = 5000 RCD's fitted..

So you have somewhere between  5,000 & 16,000+  RCD's fitted....

YOU GOT TOOOO MANY RCD's  me thinks!!!!!

the electrician who installed the board next door, used a 50amp breaker from the main d/b, and i am currently wiring next door off the new board, i have no experience in this area (calculating maximum demand) and i am just after a bit of advice as i do not want to blow the main fuse when the office gets busy/cold/hot...

 FYI the office i am talking about is where i work, the comapny itself is not an electrical comapny, i just install small industrial installations to go with the ventilation contracts we get, so i am the only person in this company with electrical knowledge, and they have tasked me with the fit out of the office next door...

Electricity At Work Regulations 1989  .. Regulation 16...

"Persons to be competent to prevent danger and injury"  etc.. etc...

By your own admission its sounds to me as though you will be breaking the law if you continue to do this work without some other competent person supervising or guiding you...?

I would get a written declaration off you employer that they are insisting you breach EAWR 1989 before you proceed any further!

Asking on the internet will not be much defense if the place burns down due to you design calualtions..

:shakehead

 

[Z1gic89]

Ok thanks for your input, i feel i am competent to prevent danger, i will not do anything untill i am 100% sure that what i am doing is safe, and i will get it checked out before i start, I just thought i'd do as much as i can on my own before i start bothering my old colleges, so i thought i would see what this forum has to say on total demand/diversity.

The heating/cooling is ran off mitsubishi heat pump/air con systems, for each main office there is a larger unit, at maximum 20 amps.

For each meeting room the is a small one, running at 5 amps, there are 5 small ones and two large ones, plus a large and a small extra going next door. so approximately 90 amps at full capacity?

the 80 amp supply is single phase and seems very small to me,

Look at your fixed loads first , the ones that will always be in use , not much natural light , lighting will be 100% ,    IT stuff is on all day, something like 2A per work station .   Don't worry about the office kettle.    

In total there are going to be approximately 36 work stations, so if the office is full (which it wont be) that's aprox 72 amps.just for IT  

my feeling is to get the DNO to install a new feed for the office next door, but i am going to try to find out if i could upgrade the main switch to 125 amps and the incoming 80 amp fuse up to a 125 amp,

 

[Andy™]

my feeling is to get the DNO to install a new feed for the office next door, but i am going to try to find out if i could upgrade the main switch to 125 amps and the incoming 80 amp fuse up to a 125 amp,

unlikely. the cutout is usually rated to 100a max. if you want a bigger supply then you better have plenty £

 

[binky]

what's the chance of all air-cons running at same time?

 

[Z1gic89]

what's the chance of all air-cons running at same time?

 not really, but the larger 3 could all run at the same time, and the smaller ones just when meetings rooms are being used, plus comms room running all the time, i'm currently trying to find out all the nominal running currents for each air-con

 

[Steve3948]

The last AC units I connected had a delayed start-up setting, so you could stagger them on start-up so in the event of a power cut, they don't all start together.

 

[Z1gic89]

Description No. of points served Connected Load per point Total Connected Load Of All Points Allowance For Diversity (From Table 3) Resulting Contribution To Maximum Instalation Demand (in amps) LED Square Fittings 15 38w 2.64 amps 90% 2.4 2d stair fittings 2 28w 56/230= 0.2 amps 90% 0.18 LED spot lights 5 12.5w 62.5/230= 0.27 90% 0.24 32a ring (floor) 10 see note 1 32 A 100% of current demand of largest circuit plus 50% of current demand of every other circuit 32 32a ring (floor) 10 see note 1 32 A 17 323a ring (wall) 8 see note 1 32 A 17 16a radial (below D/B) 1 see note 1 16 A 8 Heating/air con 1 32 A 32 A 100% of largest circuit plus 75% of F.L of the remaining Circuits 32 Meeting room Heating/Air con 1 16 amps 16 amps 12           120.82

This is the total demand i worked out using guidance note 1. and NIC guidlines.  Obviously its high.. what obvious thing am i missing?

the electrician who wired the original office which has twice as many circuits and twice as many heaters/air con units, put the maximum demand as just 60amps what am i missing? 

is there leway for me to just decide how many amps are going to be used on each circuit? so instead of putting 32 amps id put say 15 amps for 6 work stations?

 

[Z1gic89]

office max demand calculation.xlsx OK that didn't make much sense this is what i meant

 

[SPECIAL LOCATION]

Description No. of points served Connected Load per point Total Connected Load Of All Points Allowance For Diversity (From Table 3) Resulting Contribution To Maximum Instalation Demand (in amps) LED Square Fittings 15 38w 2.64 amps 90% 2.4 2d stair fittings 2 28w 56/230= 0.2 amps 90% 0.18 LED spot lights 5 12.5w 62.5/230= 0.27 90% 0.24 32a ring (floor) 10 see note 1 32 A 100% of current demand of largest circuit plus 50% of current demand of every other circuit 32 32a ring (floor) 10 see note 1 32 A 17 323a ring (wall) 8 see note 1 32 A 17 16a radial (below D/B) 1 see note 1 16 A 8 Heating/air con 1 32 A 32 A 100% of largest circuit plus 75% of F.L of the remaining Circuits 32 Meeting room Heating/Air con 1 16 amps 16 amps 12           120.82

This is the total demand i worked out using guidance note 1. and NIC guidlines.  Obviously its high.. what obvious thing am i missing?

the electrician who wired the original office which has twice as many circuits and twice as many heaters/air con units, put the maximum demand as just 60amps what am i missing? 

is there leway for me to just decide how many amps are going to be used on each circuit? so instead of putting 32 amps id put say 15 amps for 6 work stations?

I would guess you have failed to fully ready paragraph 2 of appendix H guidance Note 1....

says something like information is intended as guidance because it is IMPOSSIBLE to specify diversity allowance for every type of installation...

Such allowance call for specialist knowledge & experience...

Figures in tables may therefor be increased or decreased as decide by competent person responsible for design... 

etc..

ect...

in a nut shell you cannot simply add up MCB ratings and/or use percentages of MCB ratings to calculate max demand...

It is knowledge of the installation and how it is going to be used that is needed...

e.g. 

I could split a 32A ring into 2x 20A radials..

that doesn't suddenly make the max demand go up by 8Amps...

The loading could well stay the same but it is just more convenient if one MCB trips!!!

IMHO the rule of thumb figures in the tables are only any use for the simplest of CU's..

such as 4 or 6way small domestic installation

Consider two identical properties...  same fuse box and final circuits..

one has two elderly persons age 80+ very little power useage..

The other has family of 8.. couple with 6 children aged between 5 and 19..

much more power used...

Calculating MCB ratings would be a waste of time and incorrect!!!

You are going to have to use your knowledge of the installation to make a judgment call and sign off for your design on the electrical certifiacte....

If you are not confident to do that best get some help..

it is almost impossible for anyone on here to accurately calculate anything as we don't know the site!

Guinness

 

[Sidewinder]

Just be careful with heating and cooling loads!

There are some rules of thumb that are used, and you can get a copy of:

"ELECTRICAL INSTALLATION DESIGN GUIDE, Calculations for Electricians and Designers, 2nd Edition" Paul Cooke/The IET
ISBN 978-1-84919-657-4
 
 
This is available from all good book shops & direct from the IET.
It does give a greater insight into diversity calculations and estimations.
 
HOWEVER, you could end up in serious bother IF you get the loading calcs wrong, please sort this out before you do the job, don;t do as one "spark" I ended up going behind who added 15kW of heating on top of the 12kW already there, and the couple of kW of lighting, and around 3kW of portable heating that was in use, and melted the pitch out of the DNO cut out onto the tinder dry 100+ year old timber floor of the Chapel, quite possibly causing a fire risk...

 

[binky]

give up on diversity calcs and focus on loadings

 

[Z1gic89]

Thanks for all your help, i guess i just didn't want to face the fact that i will have to have a separate supply installed or upgrade the existing one,  

 

[binky]

better than a melt down, and a lot less expensive to the business - think about wages, contracts/sales lost, peed off customers, very peed off managers (your job), etc etc. Put a figure to that lot, and the upgrade is cheap!