Diversity and multiple consumer units

[paulbristow46]

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

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[littlespark]

Imagine all those circuits were in one db…. Then do your calcs…. Might knock a few amps off.

Really, it’ll be lucky if you pull 32A total at any moment. Unless you’ve got an EV, electric shower or weed farm in the attic.

 

[paulbristow46]

Imagine all those circuits were in one db…. Then do your calcs…. Might knock a few amps off.

Really, it’ll be lucky if you pull 32A total at any moment. Unless you’ve got an EV, electric shower or weed farm in the attic.

wish I could put all the circuits in the one CU, NAPIT say I'm non compliant due to diversity over the 100amp manufacturers guidance on the main switch even though i've split the tails

 

[MJPD29]

Realistically you’ll never be anywhere near 100amps. You’d be hard pushed unless you were charging a car with 2 electric showers running

 

[pc1966]

Diversity calculations are really a bit of a fuzzy area, there is no accurate formulae, just examples that are typically seen in real life (like domestic cooker case):

• As @littlespark points out, if they were in the same CU would it be compliant?
• Does having two CU change how the users of the installation would behave?
• What is in the garage that would draw 40.8A ever?

If the garage has just a handful of sockets off a 32A RFC, and nothing like a hot-tub or EV charger that really would draw that for a prolonged period, then drop it to 20A and the imaginary problem has gone away...

If it is a EV charger then get one that has demand management via a CT on the incoming supply.

 

[nicebutdim]

This is a question I've posed previously on this forum as modern installations are inclined to split the same load over a greater number of circuits, thus increasing the likelihood of exceeding maximum demand.

Does the IET need to catch up with its own changing recommendations?

 

[pc1966]

This is a question I've posed previously on this forum as modern installations are inclined to split the same load over a greater number of circuits, thus increasing the likelihood of exceeding maximum demand.

Does the IET need to catch up with its own changing recommendations?

A good point.

In other countries they adopt a more direct approach, I think ROI now has a 63A MCB as incomer to the installation, presumably still with a HRC fuse somewhere upstream as ultimate fault limiter, but that way you have real difficulties blowing the DNO side. Not impossible, depending on C/D curve and PFC, but not most cases and if too greedy for power you have to go and rest it.

I think in the EU, certainly France and Italy, you pay for connection capacity and the DNO provides a RCBO-like device that limits you to what you pay for in the same way.

I can't say I'm totally happy with those approaches, as loss of all power at night can be a danger of course in finding and resetting the device in the dark.

 

[Risteard]

A good point.

In other countries they adopt a more direct approach, I think ROI now has a 63A MCB as incomer to the installation, presumably still with a HRC fuse somewhere upstream as ultimate fault limiter, but that way you have real difficulties blowing the DNO side. Not impossible, depending on C/D curve and PFC, but not most cases and if too greedy for power you have to go and rest it.

I think in the EU, certainly France and Italy, you pay for connection capacity and the DNO provides a RCBO-like device that limits you to what you pay for in the same way.

I can't say I'm totally happy with those approaches, as loss of all power at night can be a danger of course in finding and resetting the device in the dark.

Yes an MCB is used as the main overcurrent device. However it's not a new concept - used to be done with Neozed switch fuses at the main distribution board, and Diazeds before that.

 

[pc1966]

Yes an MCB is used as the main overcurrent device. However it's not a new concept - used to be done with Neozed switch fuses at the main distribution board, and Diazeds before that.

Is there still a supply company fuse as the ultimate isolation point / high PFC limits, etc, that is separate from the consumer's installation?

 

[Risteard]

Is there still a supply company fuse as the ultimate isolation point / high PFC limits, etc, that is separate from the consumer's installation?

The norm is that it goes from the DSO (ESB) fuse to a DSO isolator with a contractor's side, and then to the main overcurrent device. The ESB don't always fit the isolator anymore though, particularly with the smart meter rollout. Not sure why as it creates issues, but perhaps it's partly to do with space in the Permali Box.