Diversity

[top banana]

Hi everyone,

A diversity question concerning 2 consumer units fed from 100amp DNO supply

Background, So I have an 18th Ed compliant consumer unit (DB1) fed by a 25m 2 core SWA into a IP rated box, the SWA is then split into 2*25mm tails feeding DB1 and a small 3 module enclosure incorporating an MCB and 100amp isolator, the MCB feeds a 10mm twin and earth submain to DB2., small 5 module garage type C/U

DB1 comprises SPD and RCBOs due to a TT earth system, there are 8 RCBOs highest rating 32 amps applying NAPIT diversity I have 100% of the highest rating 32amps, then 40% of all other circuits 16+16+20+20+16+6+6=100amps*40% = 40a+32a=72amps which doesn't exceed 100 amp main switch.

DB2 small garage consumer unit is fed by a 10mm twin and earth cable which is terminated into an MCB (32A) and 100amp isolator from the 25mm split tails, the garage consumer unit has 3 RCBOs (32,16,6 amps) 100% of 32amps, the 40% of other circuits = 16+6=22amps *40% =8.8A+32=40.8A

So as both DB1 and DB2 are fed by split 25m tails ultimately both are fed from a 100amp main DNO supply fuse and after diversity I have both DB1 and DB2, 72A+40.8A = 112A which is in excess of the main 100amp supply fuse is this correct? or do I apply diversity separately to each consumer unit as its split tails?

Many thanks in advance

 

[Chewbacca]

Diversity is a guideline only. The more circuits you have the higher the figure will be, and as it's good practice to have more circuits for ease of maintenance and reduction of nuisance in the case of a fault, then the diversity calculations are about as much use as Darth Vader.

 

[Andy™]

DB1 comprises SPD and RCBOs due to a TT earth system, there are 8 RCBOs highest rating 32 amps applying NAPIT diversity I have 100% of the highest rating 32amps, then 40% of all other circuits 16+16+20+20+16+6+6=100amps*40% = 40a+32a=72amps which doesn't exceed 100 amp main switch.

adding numbers and dividing by a magical number isn't really any use

lets say an installation has a 30a for sockets and 5a for lights. total 35a, divide by your magical 40% and youre at 14a

another installation has 32a up sockets, 32a down sockets, 32a kit sockets, 32a cooker, 6a up lights, 6a down lights, 6a alarm, 6a heating. total 146a. magical 40% and your demand is 58a.

difference between them? nothing. same house, same loads, same use, just it got rewired and a new consumer put in, yet your calculations now say something completely different

in reality, for a house, unless there's multiple showers, EV points etc then you don't really need to worry about max demand

 

[top banana]

Diversity is a guideline only. The more circuits you have the higher the figure will be, and as it's good practice to have more circuits for ease of maintenance and reduction of nuisance in the case of a fault, then the diversity calculations are about as much use as Darth Vader.

Totally mate however, NAPIT have got me on non compliance in terms of diversity and manufacturers guidance on 100amp main fuse on CU

 

[Murdoch]

Really all this demonstrates how out of touch NAPIT guidance is.

the bottom line is that the typical home now has more circuits, but less load unless the home has an EV, a hot tub and an electric shower

 

[Murdoch]

Totally mate however, NAPIT have got me on non compliance in terms of diversity and manufacturers guidance on 100amp main fuse on CU

ouch. That’s not only harsh but dumb.

I would challenge that immediately

 

[top banana]

Really all this demonstrates how out of touch NAPIT guidance is.

the bottom line is that the typical home now has more circuits, but less load unless the home has an EV, a hot tub and an electric shower

agreed mate especially as the Gov wants to do away with Gas in order to move forward with total electric houses it baffles me this diversity

 

[top banana]

adding numbers and dividing by a magical number isn't really any use

lets say an installation has a 30a for sockets and 5a for lights. total 35a, divide by your magical 40% and youre at 14a

another installation has 32a up sockets, 32a down sockets, 32a kit sockets, 32a cooker, 6a up lights, 6a down lights, 6a alarm, 6a heating. total 146a. magical 40% and your demand is 58a.

difference between them? nothing. same house, same loads, same use, just it got rewired and a new consumer put in, yet your calculations now say something completely different

in reality, for a house, unless there's multiple showers, EV points etc then you don't really need to worry about max demand

totally mate its a bungalow with an attic conversion and no gas so all electric, 1 shower and electric intelligent heating, no EV etc

 

[top banana]

DB1 is under the 100amp, but adding the other DB2 that supplies a small ring, LED lights and an electric shower pushes the total over 100amps, can I treat the consumer units separately in terms of diversity

 

[binky]

DB1 is under the 100amp, but adding the other DB2 that supplies a small ring, LED lights and an electric shower pushes the total over 100amps, can I treat the consumer units separately in terms of diversity

It's been so long since I calculated diversity on anything, I'm not sure what to suggest. However DNOs used to use 40% of the total figure, which would just about bring the numbers in. Funny really as DNO allows 16amps per house...

 

[mort2376]

The main switch is rated at 100a. The dno fuse is 100a.

You cannot then "in theory" draw more than 100a.

While we all know this is not exactly true there are plenty of installations like the one you described.

This is going to become more and more of an issue as I find I'm putting in more 20way boards than ever now.

I sometimes think they live in a perfect world the people who makes these rules. Bit like the Microsoft world lol

 

[GM at CA UK]

Instead of using diversity calculations you could perhaps try logging the installation for a couple of weeks and see what sort of current demand actually occurs. You may be surprised how low it really is. We were involved in a case recently where 5 flats were fed from a 100A fuse, with no known issues over the last 50 years, but the DNO were suggesting an upgraded supply was required. Logging for a couple of weeks showed the total current demand for all 5 flats never exceeded 49A and that peak only lasted for a couple of minutes. The average current demand over the 2 week period was only 8.85A. A case study on it was printed in the Professional Electrician July/Aug 23 edition Click here to read PE article

 

[mort2376]

If you ever get to look at the main cable coming in to a property it normally looks like around 10mm/16mm for a domestic property, yet its got a 100a fuse on it.

Also don't forget the dno fuse is there to protect their network not the property !

With regard to maximum demand and such I put 20kva as that is what is supplied to domestic properties in the UK.

If it comes to a legal argument over it then a logger is the easiest way forward either that or go round and record the maximum current for each current using appliance and use that.

It's an impossible situation as if you log for a week then the customer buys an air fryer or new high current appliance the figures are out of the window.

We install so each circuit is protected correctly. If we then have to start demanding the dno provides infrastructure to cope with 150a per house installations as that's what they could on paper draw then the county will fall apart faster than it already is.

Let's put 3 phase into everyone's house just incase they get an eV charger and an air source pump, air conditioning and put 4 electric showers in so each bedroom has their own !

Just had an email saying a leading manufacturer has released a 39 useable way dual stack board. How is that ever going to meet any diversity max demand calculation ????????

(Can you tell today isn't a good day by any chance !!!!!)

 

[top banana]

Thanks everyone much appreciated, the only reason for the Diversity question is that NAPIT have slapped a non compliance on me for exceeding manufacturers recommendations on the 100amp main fuse LOL

 

[UNG]

The move to all electric living is fraught with problems not least the local cabling infrastructure. Chatting to a number of DNO guys over the last few years they have all said that the local cable networks will need major reinforcement to cope, as an example about 3 years back a cable blew up down the road from a job I was on as it happened I had the DNO coming out to change the service head on the job I was on the next day so got the lowdown on what had happened evidently one phase was very overloaded so when the evening meal was being cooked that day it tipped the balance so one core of the 16mm² 4 core PILC gave up taking out the whole cable and shutting down a large area of the network
The 2 DNO areas I work across both have a lot of local network cabling that is mostly 16mm² 4 core PILC with some limited parts in 25mm² while network diversity has worked for many decades the problem will be how the network will stand up to the nightly charging of EV's at most households or will the "smart" EV chargepoints throttle back to a minimum charge or no charge so in the morning your EV hasn't got the charge / range expected.

I think the green revolution need to be more open and honest about what is actually achieveable, I can see a time when cheap off peak electricity is a thing of the past and peak loading will more likely be overnight. A recent BBC programme on EV's showed a fancy EV charging station which had a 5Mw incoming supply it was suggested that it may have been better sited at the motorway services a short distance up the road and the response was the supply capacity we needed wasn't available so how much diversity is allowed in that supply

 

[UNG]

Thanks everyone much appreciated, the only reason for the Diversity question is that NAPIT have slapped a non compliance on me for exceeding manufacturers recommendations on the 100amp main fuse LOL

Is it the 100A DNO fuse / service head they are putting as a non compliance or the 100A main switch in the CU

 

[Murdoch]

Thanks everyone much appreciated, the only reason for the Diversity question is that NAPIT have slapped a non compliance on me for exceeding manufacturers recommendations on the 100amp main fuse LOL

I still think this is complete nonsense and you should ask them to justify this reasoning

If you total up all our breakers it comes to 280 amps

The DNO fuse is 80 A

Our average consumption per day is 7 KWh

 

[UNG]

I still think this is complete nonsense and you should ask them to justify this reasoning

If you total up all our breakers it comes to 280 amps

The DNO fuse is 80 A

Our average consumption per day is 7 KWh

Maybe we've all been doing wrong for years

 

[Murdoch]

Maybe we've all been doing wrong for years

definitely not

the guidance dates back to the days of 4 - 6 fuses …..

I would add them up and multiply by 0.25

 

[UNG]

definitely not

the guidance dates back to the days of 4 - 6 fuses …..

I would add them up and multiply by 0.25

I agree with you, but there seems to be a different agenda and method of working these days

When you look at the OP's figures they look to be very generous if not optimistic and probably the installation will never come close to the overall level indicated

This is more likely the NAPIT inspector has to find something and he was struggling so got a bit petty