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Problem Working Out Maximum Demand And Divercity


Clifford Hopcroft

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I have aProblem working out maximum demand and diversity. I have 8 circuits

20x 10w down lights

1x 5 kW heating ring

1x 4.5 kW heating ring

2 x socket ring

1x 7.2kw hob

1x 3 kW oven

1x 10.5 kW shower.

The problem is using the on site guide I'm getting 154.3 a!!!! What am I doing wrong. This is a 2 bed flat with everything electric

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How did you achieve that figure?

We cannot really see where you have gone wrong without seeing HOW you got to that figure

.....and welcome!

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Add up all the breakers and multiply by 0.25

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you're suffering from 'the more circuits the higher the load' syndrome. Unless aall the circuits are going to be used at full power all the time, there isn't an issue.

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So how is the best way to work it out. Is murdoch's x0.25 of of the breakers the best way. Thanks

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Kerching I

Took 66% of 20 x 100 for the lights

The 2 heating circuits I took the first 10a + 50% of any remaining amps

Hob and oven first 10a plus 30% of remaining amps

Ring 1 100%

Ring 2 40%

Shower 100%

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Why do you have the downlights at 100W (I'm guessing there as you haven't shown which units you're using)

 

Can they take 100W lamps??

 

Other than that you've used the OSG method which is total rubbish,,, i.e. more circuits=higher current ,,,, this just doesn't work in real life

 

 

 

AND.... what are the heating circuits supplying?

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Kerching I

Took 66% of 20 x 100 for the lights

The 2 heating circuits I took the first 10a + 50% of any remaining amps

Hob and oven first 10a plus 30% of remaining amps

Ring 1 100%

Ring 2 40%

Shower 100%

 

 

So you are saying in a real world application ALL of the items you listed above will be on running at the rated values you have listed SIMULTANEOUSLY for a reasonable duration each day....?

I don't think so !!

 

 

The first mistake you have made is to go to table A1 & A2 of appendix A..

without taking note of the second paragraph on the opening page of appendix A...

 

 

Which basically says it is IMPOSSIBLE to give tables for calculations for each and every type of installtion... 

Some tips and guidance are given..

BUT they can be increased / decreased / ignored as it is the person who is designing the installation who has the specialist knowledge relating to what they have installed... 

 

 

So you have got to think of which of your loads are going to be running at what capacity for significant durations simultaneously ???

 

We have said before loads of times on the forum..

converting an old 4 way Wylex CU with 1x 30A socket, 1x 30A cooker, 1x 5A light 1x 15A immersion heater..

to a bigger board with 1x 32A cooker 1x 32A socket, 2x 20A socket, 3x 6A lights, 1x 16A immersion heat..

 

doesn't suddenly make the max demand any different just because there are twice as many circuits..

 

 

 

Think of the types of appliances NOT the number of circuits...

 

e.g.   a 3kw electric kettle would only be on for a few minutes while the water boils..

a 2k toaster will only be on for a short time while the toast cooks....

a 2.5kw hair dryer would only be on for a few minutes while hair is being dried...

The shower is probably on for 10 or 15mins in a single use....

 

 

Try to think of your installation and what appliances may realistically be on for half and hour or more together...

 

(remember a washing matching on a 60 min cycle is only heating water for a short duration..

the bulk of the time it is no where near max power..)

 

 

So your first ring circuit running at 100% for an hour in one go is very unlikly...

 

Use your expertise on installation knowledge not tables in the book!

 

 

Guinness

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On Site Guide

 

Appendix A - Maximum demand and diversity.

 

Read the 1st page.   and then work out how to use the tables.

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