Maximum Demand

[Nedd]

Hi all

I was told by a lecturer an acceptable method of calculating maximum

demand as required on EIC. I think it was, add all fuses or mcbs at the

db together then divide the total by 40 the remainder is the demand

is this right or not. If not which is the best way to calculate the load

Thank you .

 

[Sidewinder]

No not quite, though it "may" work for a small domestic, never tried it.

I usually use my engineering judgement

 

[Doc Hudson]

With modern multi circuit CU's that method invariable gives a ridiculously high value. It may be acceptable for a traditional 4 way CU, e.g. 1xSocket 1xLight 1xCooker 1xImm heater or shower. Try the logic for yourself.  calc a figure for 30A socket, 5A light 30A cooker, 15A Imm heater. Then recalc if property rewired & CU replaced (max load remains the same) but circuits now divided 2x 20A sockets 1x32A kitchen/utility 2x10A lights 1x16A Imm heater 1x16A Garden socket & pond feature. 1x10A loft lights & TV amp. Max demand has to be calculated on realistic simultaneous circuit loads during normal use, not multiple circuit loads that in real world would never occur together.

Doc H. 

 

[Doc Hudson]

I had answered this in your duplicate thread which is now deleted. Please only post one copy of each question as it only clutters the forum. Thank you.

Doc H.

 

[Manator]

The method you describe would only be suitable for small installations though I would calculate on all circuits multiplied by 0.42.

Example:

Upstairs ring 32amp

Downstairs ring 32amp

Kitchen 32amp

Upstairs lights 6amp

Downstairs lights 6amp= 32+32+32+6+6 = 108

108x 0.42=45.36 rounded up to the highest round number = 46amp.

Your method = 108 divided by 40 =2.7amp

This method can only be used as an estimate, proper calculation can never be neglegted. In mathamatics engineers and mathmaticians use all kinds of tricks to estimate an answer which is what you have been instructed to do, and I have done in my example, this is only the first stage in proper calculation, it must be backed up by the proper calculation and the estimate is used to verify the calculation.

Designers of installation often use the data to design within the limits of the supply and demand.

Doc Hudson has raised a very good point and raised a flaw in calculating maximum demand using the above described methods.

Lets take an old installation (domestic) that has four circuits, 30 amp for sockets, 40 amp for shower, 6 amp for lights and 6 amp for alarm.

Based on estimated calculation described above

30+40+6+6=82 =82x0,42 = 34.44 rounded to higher full number = 35amp.

Now lets split the circuits and install a new consumer unit.

Up and down ring circuits, Kitchen circuit,up and down lighting circuits, alarm.

Thus

32+32+32+6+6+6 = 114

114x0.42= 47.88 rounded to full number 48amp.

So just by installing a consumer unit we have added to the maximum demand of the property? The maximum demand on our example has increased by 13amp by the simple addition of a new board.

Maximum demand and diversity has been given as a guide to those who are unable to design properly, it sets out the basics without the requirement to possess a degree in engineering, it is based on safe restrictions and is very often over estimated compared to an electrical engineers design.

For an experiment put everything you can on, and use a clamp meter to register the total demand in your home, I think you will be surprised by the results, better still if you have test equipment that records demand take results through the day and just see how low the readings are.

 

[SPECIAL LOCATION]

Not sure if its clear from what the guys have already said above.. 

But max demand has to also take into account duration and times that equipment is in use...

Remember most cables and fuses can take in excess of their design current for a short period without an major issues..

So if an installation consisted of  the following high power items...

and we ignore all other items for the sake of this illustration...

a cooker running @ max 25A

a shower running @ max 40A

a tumble drier running at max @ 12A

a washing machine running @ max 10A

a rapid boil kettle running @ max 12A

and a dish washer also running @ max 10A 

If none of these items are on at the same time then the max demand is only 40A (the biggest item)!

If the kettle & the shower were both on at the same time, as the kettle will automatically turn off after a couple of mins,

a short peak of 52A may be there for 2mins but for 95%+ of the shower cycle the max is still only 40A

If the washing machine and the dishwasher are on at the same time although they may both max @ 10A..

this is only during the heating cycles for the water.. for the bulk of the running time it is just a few motors and pumps working...

So unless the two heat cycles exactly coincide the total load of the dishwasher and the washing machine may still be less than say 15A..

The tumble drier will be a more constant 12A as the heating elements are on for most of the tumble cycle!

But if its a single person living in the property and they never do cooking while in the shower..

and never run the tumble drier during the day time...

The max demand may still be less than 50A even if all appliances are used at some time during every 24hr period...

But if you just added them all together thats 109A  which would need a 46% calc to get to realistic figure...

If the same property had a family of seven with 3 lads who play football every other day and 2 daughters who are helping on the local farm so they can ride the ponies..

and mum is always washing dirty clothes and the tumble drier is going for 60%+ of the day..

and there is always one of the kids in the shower for half and hour when mum is cooking....

Max demand could easily be 90A+  which is a 83% adjustment!!!!

Hopefully you can see the simple add MCB's and dived total is a bit hit & miss 

 

[SLIPSHOD & SLAPDASH]

Well.................... Over the years i have been taught, & have read up on various ways to calculate this but has any one considered the fact that the size of the main fuse dictates the most current you can draw ?

I am now going to hide behind the sofa till the storm passes.

 

[SLIPSHOD & SLAPDASH]

The heating has gone off and its dusty down here so I am going to bed, someone cover me as I make a run for it.