maximum demand and diversity

[MWE]

hi,
Ive recently fitted a new CU to my house ready for assesment (Elecsa 14th dec).
Just wanted to know if I had worked out MD + D correctly cheers.

Hi integrity dual RCD board. LAYOUT AS FOLLOWS.

100A MAIN INCOMER
6A RCBO SMOKE ALARM MINUS 40% = 2.4A
6A RCBO SECURITY ALARM MINUS 40% = 2.4A
80A RCD 30mA
32A SHOWER (7.5KW 100% = 32A
32A GROUND 1ST FLOOR SOCKETS 100% = 32A
20A OUTSIDE SOCKETS MINUS 40% = 8A
6A LOFT LIGHTS MINUS 66% = 3.96A
80A RCD 30mA
32A COOKER (7KW) 100% = 32A
32A LOFT SOCKETS MINUS 40% = 12.8A
20A GARAGE SOCKETS/LIGHTS MINUS 40% = 8A
6A OUTSIDE LIGHTS MINUS 66% = 3.96A
6A GROUND 1ST FLOOR LIGHTS MINUS 66% = 3.96A

Total after diversity =131.08A

Maximum demand = 131.08A x rule of thumb = 52.4A entered onto test results

Am i correct ????????

 

[telectrix]

ask 100 electricians the same question and you'll get 100 different answers. there are so many different ideas on how to estimate total demand and i'll be wiiling to bet most of us just make an educated guess. my own method is to turn everything on, clamp meter one of the tails, then x 60%

 

[MWE]

Hi telectrix

Where does the 60% come from

 

[telectrix]

Hi telectrix

Where does the 60% come from

it's a guesstimate between the 100% and the 40%, with a bit thrown in for the 10A+30% for the cooker.

 

[malcolmsanford]

Well be amazed if the assessor will ask you how you came up with the loading for the CU but if he does then your not far out.

What I think your asking is the part of the EIC that deals with maximum demand. This is really guess work as no one knows what the maximum demand really is. You may use your ring final circuits for just small appliances, while next door plugs 5x 3kw heaters into theirs.

you may live alone and only use 1 ring and the oven on your cooker, next door as 6 kids and uses all the rings and oven, all day nearly so again what is diversity.

What you done is not wrong, but quite a few sparks use the DNO fuse rating as the maximum demand for the installation.

Your calculations are good but you mention rule of thumb. An easier way to do this would be in fact add all your protection devices together and multiply by 0.4 and you get 52.43 amps. The 0.4 rule of thumb is normally done by the DNO to calculate fuse sizes in domestic installations, for commercial/light industry it is 0.6 and industrial 0.8

 

[MWE]

Hi telectrix
I see, could you not do me a rough guestimate then on the figures I have stated,or like you say you only do the clamp method???

 

[telectrix]

i suggest you read malcolm's post. it's more in depth and more accurate than mine. like he says though, you can just enter the value of the DNO fuse.

 

[MWE]

Hi
Malcolmsandford never really looked at it from that perspective,as for the assesment then am i worring abit too much take it they just want to see safety,correct isolation precedure etc??

 

[MWE]

hi Malcolmsandford

If i were to add up all the breakers as suggested prior to diversity this would give me 79.2A maximum demand see now how the figures can vary so much. So which is correct i suppose as long as it is below the DNO'S cut-out it doesn't really matter???

 

[malcolmsanford]

If you did put the DNO fuse rating down as your maximum demand then again i'd be amazed if he siad anything, and if he did you just say that as it is a DNO fuse of 60amp that is all the demand it can give so it's the maximum.

He will ask things like safe isolation, he will ask you to do a few tests, Ze is a favourite for ELECSA. He will though chat to you and see if you know what your talking about, if your not sure say your not sure, and look it up, after all unless your really non competant he wants to pass you. He knows you will be nervous and he also knows that even the most experienced sparks can't know it all, just be relaxed, friendly and above all look and feel competant.

 

[telectrix]

100% agree with malcolm. priorities are safe working and an understaqnding of installation and testing. not just being able to take readings, but to understand why these readings are important. e.g. low value of Zs ( < max. for a particular fuse/mcb) means the fuse/mcb will break within the max. permissable time in the event of a fault current. as malcolm says, he's not going to try and trip you up, he just needs to know you are competent and understand what you are doing.

 

[baza]

when I asked my Elecsa assessor at my assesssment for advice on max demand calcs, he told me about the book - The IEE Electrical Installation Design Guide, which is quite a helpful book, but he also told me that most sparks just add the breakers up and times by 0.4

 

[SimpleSimon]

Hi MWE
What you've done is correct but they wont ask you about it, they just look at it on the certain n move on.
Just to be on the safe side keep your calculations in easy reach and you'll than demonstrate a knowledge on the subject.
In the box I would write the original figure before applying rule of thumb, that's the way I do it.

 

[SDELEC]

Hi MWE, good post as i am going through same process at the moment.i am trying to work out ratings for the 30ma rcd, what calc/table did you use to select 80a instead of 63amp rcd? I have split up into 2 groups the breakers I intend to use , applied diversity and am sure2 x 63a will be ok for mine. would 80a be better incase of future additions?

 

[malcolmsanford]

If your fitting a new split or high integrity board didn't they supply 1x 63amp and 1x 80 amp RCD with it?

 

[SDELEC]

If your fitting a new split or high integrity board didn't they supply 1x 63amp and 1x 80 amp RCD with it?

Have not bought one yet, i am a bit limited as to which board I choose due to the space available, The wylex 17th ed fits the bill. it comes with 2 x 63a rcd. would you recommend swapping one for 80a?

 

[malcolmsanford]

Really in all honesty current rating of the RCD is not that important as it is providing fault protection and not overcurrent protection. So really if you wanted you could fit a 125amp one as long as the important part which is the 30mA in a domestic situation is there.

IMO I would fit the 80amp as now installations are becoming larger with showers and cookers etc, and whatever else is going to hit us in the future.

 

[Sparky Ninja]

when I asked my Elecsa assessor at my assesssment for advice on max demand calcs, he told me about the book - The IEE Electrical Installation Design Guide, which is quite a helpful book, but he also told me that most sparks just add the breakers up and times by 0.4

That's the book I use and stick with.



here is a calculation I did last week.

 

[gpz900jim]

I asked my assessor about whether I would have been better fitting the 80a rcds instead of the 63s . He said that the rating on an rcd is its service rating and not its maximum load capacity,