Maximum demand & Diversity lesson required please

[Joe Hyde]

Hi,

I am looking for a little advise on how to apply diversity and work out maximum demand on a new installation I am designing in a Cabin.

Cabin CU has 3 circuits
Circuit 1
6 Socket outlets - to supply 2.7KW Floor standing Hair dryer
Electric Oil filled radiator
Hand held hair dryer
Kettle
radio

Using 2.5mm T&E and 16 or 20A MCB depending on advise regarding diversity

Circuit 2 Lights - 6 * Fluorescent 58W Lights plus an outdoor 300W floodlight
Using 1.5 T&E 3A MCB

Circuit 3 7.2KW Shower
Using 4mm Flex on 32A MCB

My Main Question is cable size back to the house 20m away
I will be using SWA Buried

Can I apply the 60% rule here ? should cable be rated min 52A ? Is it somewhere in between ?

Any guidance would be appreciated,

Thank you

 

[Guest55]

lets have a look ......

well all your plug in loads are going to exceed 16 / 20 A even with diversity so your gonna need a 32a ring or radial.

and your lighting comes to 4.1a so a 3a mcb is no good either

shower is bog standard 31a jobby.

ignoring diversity for a moment , the biggest limiting factor for cable selection will be volt drop for the lighting , not current capacity.

without doing a load of maths as im just home for work , you should get it all on a 50a type C mcb which will give you a bit of discrimination with the cabin mcb's but cablewise at least 10mm maybe 16mm as a ballpark guesstimate.
i'd also be checking the dno fuse size before commencing work if the cabin supply is being fed from a house.

 

[markythesparky]

Joe are you a Sparks?

This work is notifiable and like Biff said its not a case of just running a 50a supply to the cabin. you need to work out the diversity for the whole property from the intake.

 

[Joe Hyde]

Just Qualified , Trying to become competent ! hence Asking for advise

 

[markythesparky]

have you got an On Site Guide? from memory this has details on diversity.

 

[GLENNSPARK]

Hi,

I am looking for a little advise on how to apply diversity and work out maximum demand on a new installation I am designing in a Cabin.

Cabin CU has 3 circuits
Circuit 1
6 Socket outlets - to supply 2.7KW Floor standing Hair dryer
Electric Oil filled radiator
Hand held hair dryer
Kettle
radio

Using 2.5mm T&E and 16 or 20A MCB depending on advise regarding diversity

Circuit 2 Lights - 6 * Fluorescent 58W Lights plus an outdoor 300W floodlight
Using 1.5 T&E 3A MCB

Circuit 3 7.2KW Shower
Using 4mm Flex on 32A MCB

My Main Question is cable size back to the house 20m away
I will be using SWA Buried

Can I apply the 60% rule here ? should cable be rated min 52A ? Is it somewhere in between ?

Any guidance would be appreciated,

Thank you

instantanious water heater.....no diversity allowed here...

 

[Joe Hyde]

I do, Read it from cover to cover a few times, I know Im only learning and college cannot teach me half of what I need to know,
I was taught on the EIC form to put down 60% of the Total Breaker ratings as Maximum demand !
Is this just the lazy answer ?
OSG is not very clear on the subject ! I am going through 7671 now to try to answer my own questions !

 

[johnboy6083]

Max demand and diversity is a subject that has no definitive rule book. As far as I know, the only figures that are used regularly are the CIBSE ones.
they give W/m2 from what I'm aware.

as for us normal blokes on the tools, your knowledge of what your installing is the only real way of calculating diversity.
a rule of thumb for domestics is to take the highest rated MCB and add this value to the test of the MCB values x 0.4

 

[ackbarthestar]

Looks like your client is setting up a small hair salon, which takes it into the commercial world. Watch out for the second and third hair drier being moved in at a later date !

The hair drier is likely to be used to set purms and the like so will be running for up to 3/4 hour at a time. plus all the other appliances so you will not see much change, if any, using a B32A breaker.

The lights will be on for the duration so that's another 3-4A FLC plus starting so that might well be a C6A

The shower has no diversity, although, at a guess, it will be only on for 5 minutes at a time. Still you wouldn't want to run the circuit hot - not good for impending assessments ! So that's a B32A.

If the whole lot was running at the same time it, as Biff55 says, will affect the volt drop.

I am not going to advise on the size of sub-main since there are other factors to be considered.

 

[Joe Hyde]

Looks like your client is setting up a small hair salon,

Dog Grooming !

You may be right through , I have too see into her future as much as possible !

Looking like a 32A Ring for sockets
16mm 3 core SWA supply !

 

[Guest55]

Dog Grooming !

!

oh ! , in which case i'd be considering a 10ma rcd / rcbo for the shower.
30ma good for people , too high for animals imo.
wouldnt want to get sued because an expensive designer pooch got zapped in the dog wash lol.

 

[ackbarthestar]

What you want is one of my homemade patent designs :detective::4::54:unk: