Maximum Demand Installation

[dilog]

Evening all. At the risk of being ridiculed I wonder if someone can clarify something for me!

On the initial inspection forms for installations there is a box for "maximum Demand". This can be in KVA or amps. I've been told that you add all the breakers on the board up then it is 40% of this total.

I can't recall why its 40% and looking through all the books cannot ascertain why. I appreciate diversity and the like but in the books it can vary depending on the circuit, so why is it an overall 40% regardless of what's on the board?

Any help appreciated. Domestic installation by the way.

 

[SparkyChick]

The way I was advised by my scheme is ignore the largest OCPD, sum the rest and then use 40% of that total and add the largest.

This was described as 'a quick method'

 

[Ian1981]

The 40% way if you like is usually done by the highest rated circuit so this could be a 9kw shower at 39 amps then 40% of the other circuits after diversity has been Applied to them also by which I mean not the OCPD ratings, the actual circuit loads themselves.

 

[Octopus]

Add them up and multiply by 0.25 - then think again..............

It all depends on the number of circuits and how the building is heated .......

 

[dilog]

Mixture of answers then! I think sparkychick was told the same as me then. But it's never been mentioned about the largest circuit. But sort of clarifies it now.

Many thanks for the replies, and appreciate the lack of Mickey taking!

 

[Ian1981]

Why would you use the rating of the OCPD?
They rarely draw their exact current rating so your final diversity figure would be very pessimistic.

 

[Octopus]

Mixture of answers then! I think sparkychick was told the same as me then. But it's never been mentioned about the largest circuit. But sort of clarifies it now.

Many thanks for the replies, and appreciate the lack of Mickey taking!

I think you need to take care adopting ideas off the internet. BS 7671 is way behind the times when it comes to advising on such matters.....

So it's a free for all

No longer do houses have 6 way boards.

 

[Baker1988]

The way i was taught at college was to add all the breakers up and multiply by 0.4 (so 40 percent) thats what i did for my final exam at college and it was marked as correct so that is what i have done since.

 

[SJD]

For example, I've recently completed a refurb of a 4-bed house and separated the sockets to several circuits. Before all the sockets were on one RFC on a 32A MCB, now there are 3 off RFCs, each with their own 32A MCB. With more or less the same appliances, is the maximum demand any more than before?

 

[NDG Elecs]

I take a different approach. I think is there an electric shower, or hob, or other appliance with a lot of chew. Then I take the people who live at the property into consideration and there likely patterns of behaviour. In summary I write something I think is appropriate in the box, for domestic properties it is unlikely to ever matter and would never get called into question anyway.
As SJD says really.
Imagine a 90 year old widow living in a five bed detached house...kids long since flown the nest etc etc... Ocpds could add up to a silly figure, whereas in reality the TV will be on, the kettle occasionally and a few lights etc..

 

[Octopus]

The way i was taught at college was to add all the breakers up and multiply by 0.4 (so 40 percent) thats what i did for my final exam at college and it was marked as correct so that is what i have done since.

Just tried that on our place and got 102 amps.

Using my 0.25 drops it to 62 amps which is still way ott as we have gas heating

 

[Baker1988]

Just tried that on our place and got 102 amps.

Using my 0.25 drops it to 62 amps which is still way ott as we have gas heating

I have never really got that high result always lower than the main dno fuse so all good.

if it was excessivly high then i would look into it a bit deeper and try to get a better figure.

also mine comes to 52A which is a hell of a lot more than we use i recon we would be hard pushed to use more than 20 amps at any one time

 

[Octopus]

The main reason I completely disagree with the diversity calcs in the OSG is that most modern builds have more circuits ....

We have 13 , all on rcbos BUT that was me splitting up the circuits to minimise any disruption.

Eg 2 outside sockets on a 16a radial.... Main uses, lawnmower and led Xmas lights....

 

[Wilko]

I was in a big detached house the other day and it had 2 big CUs full of circuits. House had gas fired HW, CH and hob and a normal single phase cutout of 100A or less. I don't think the load has not doubled when they had it rewired with 30 odd circuits over the two boards .

 

[Midwest]

What does it matter, just another box to fill in. As those guides in the OSG are way behind the times, think of a figure, double it, square root it, if there's an R in the month halve it. Or just put the size of the main fuse.