Maximum Demand Load (Certs)

[danny124]

Hi guys,

ready for it yet another blonde moment from me, trying to get my head round my training.


Regarding Maximum Demand load Amp/kA on the certificate

If you have an existing installation you can measure the maximum demand load by by switching almost everything on the circuits and measuring the load from the live tail into the consumer unit and either take the amps or kA.
Is this right ?


Now how do you work it out for a new installation. Im rewiring my shed for an assesment for testing and inspection. Do you add up all the amp of the breakers in the installation and then add or subrtract the factors from the on site guide, i.e lights you take 66% of the amperage of the lighting etc etc.
OR AM I WRONG

This is just another brain fart, i went through this somewhere in my training but having lots on my mind i think im missing the obvious here.

Could someone give me the low down of both methods please

Much appriciated

 

[SirKit Breaker]

Hi guys,

ready for it yet another blonde moment from me, trying to get my head round my training.


Regarding Maximum Demand load Amp/kA on the certificate

If you have an existing installation you can measure the maximum demand load by by switching almost everything on the circuits and measuring the load from the live tail into the consumer unit and either take the amps or kA.
Is this right ?


Now how do you work it out for a new installation. Im rewiring my shed for an assesment for testing and inspection. Do you add up all the amp of the breakers in the installation and then add or subrtract the factors from the on site guide, i.e lights you take 66% of the amperage of the lighting etc etc.
OR AM I WRONG

This is just another brain fart, i went through this somewhere in my training but having lots on my mind i think im missing the obvious here.

Could someone give me the low down of both methods please

Much appriciated

The formula i use is the following.

Add up the total amps of all the MCB's or fuses in the CU, apply the diversity calculations allowed in the BRB and times the result by 0.4. So 126a x 0.4 = maximum demand of 50.4a.
It is an old well worn formula but it is near enough accurate. I have used this method since time began and my last 4 Elecsa assessors have had no complaints.

Cheers............Howard

 

[mattyevz]

i use the same formula as above too.

 

[danny124]

Howard & mattyevz you are stars thank you so much for the reply so quickly.

That seems to be an easier method.

Howard im doing my assesment with Elecsa too, so i put it on the design info ready for the assesment guy.


Thanks

Danny

 

[acat]

That should be no problem but make sure you know the other method of calculating diversity as it may just rasie the question the way you have done it.

Good luck on your assessment and let us know how you get on


Chris

 

[danny124]

Thanks for heads up acat. i look out my notes for calculating diversity. I got them somewhere lol

 

[acat]

Dont need to know them off by heart but you do need to know where to find the information. Best to know safe isolation, inspection and testing expected results and what could be wrong if you get a different reading
Nothing to worry about they are not out to catch you out with stupid questions like city and guilds they are just making sure that you are safe and complying with BS7671.


Chris

 

[m4tty]

Hi im doing a board change for Elecsa assessment and am stuck on maximum demand. The old 8 way Wilex rewirable had 8 circuits and the new board has 11 circuits. Now the max demand when using the 0.4 calculation works out higher than the 60A mainfuse. What do you do in this situation?

Sorry to threadjack mate

 

[danny124]

course not m4tty thats a good question and like to hear the answer to that

 

[ezzzekiel]

you shoul
take rating of largest circuit
then 40% of remainder for rule of thumb

 

[m4tty]

you shoul
take rating of largest circuit
then 40% of remainder for rule of thumb

Cheers Ezzzekiel,

Isnt it take rating of largest circuit out of the calculation for now then 40% of the rest then add the largest one back on?

But this totalled about 88A and the mainfuse was 60A so what would you do in this situation.

Cheers

 

[Denatone]

There's a table in the OSG for percentages of each type of circuit for diversity purposes

 

[ezzzekiel]

yep rule of thumb uses more caution, you could apply diversity correctly as Denatone suggests and see what figure you come out with.
its not really set in stone science, a lot of guys would just quote the main fuse size as max. demand.

if youre close i wouldnt worry about it, or ask dno to uprate the fuse to 80a.

 

[SirKit Breaker]

Howard & mattyevz you are stars thank you so much for the reply so quickly.

That seems to be an easier method.

Howard im doing my assesment with Elecsa too, so i put it on the design info ready for the assesment guy.


Thanks

Danny

Which part of the Country are you? Who is doing your Elecsa assessment. I've had 3 different assessors in 4 assessments, they are all good honest blokes, dont worry about the odd mistake on job or paperwork they dont expect perfection. they want safety and competence....oh and your money

 

[ezzzekiel]

they want safety and competence....oh and your money

lol