Inspection Job & Diversity HELP Please

[Newboy2013]

Hi, clearly a new boy with a diversity question to ask. I need to install two new circuits for NAPIT to inspect and am running into a bit of trouble at the chosen site. The issue is one of diversity with two Consumer Units, as firstly there appears to be no guidance on the loading of Distribution Circuits (I have assumed breaker rating value), and secondly before I have even started work things look overloaded with the way that I have worked things out.

The house has two CU's, teh second one installed recently to provide power and lighting to the extension, and is itself fed from a 40A breaker in CU1 via 10mm2 cable.

Simpley doing a calc on info below gets me to a point where with diversity I have a calling for over 120A through a 100A switch. Have I done the calcs right, and would a better solution be to install a Henley block and feed both CU's from this.

Consumer Unit	Breaker	Rating (A)	Points	Type	Purpose / Notes	Diversity (W)	Diversity (A)
CU1	MCB 01	6	9	Radial	Tungsten Lights Up Right	900	3.9
CU1	MCB 02	6	5	Radial	Tungsten Lights Down Right	500	2.2
CU1	MCB 03	6	4	Radial	Tungsten Lights Up Left	400	1.7
CU1	MCB 04	6	6	Radial	Tungsten Lights Down Left	600	2.6
CU1	MCB 05	6	7	Radial	Tungsten Lights Utility	700	3.0
CU1	MCB 06	Blank	0
CU1	MCB 07	20	3	Radial	Power Utility		8
CU1	MCB 08	Blank
CU1	MCB 09	30	11	Ring	Power Left		30
CU1	MCB 10	30	8	Ring	Power Right		12
CU1	MCB 11	40	1	Radial	Distribution Circuit CU 2		40.0
CU1	MCB 12	30	1	Radial	Cooker (11.1kW)		21.5
		180					125.0

CU2	MCB 01	32	8	Ring	Extension Power		32
CU2	MCB 02	32	3	Ring	Garage Power		12.8
CU2	MCB 03	6	9	Radial	Entension Lights Tungsten	900	3.9
CU2	MCB 04	16	1	Radial	Central Heating (112 W)		0.0
							48.7

 

[Pete999]

Appendix A page 109 OSG

 

[Newboy2013]

Appendix A page 109 OSG

Thats not an answer as thats where I got the info from! It is a question of interpretation that I am after!

 

[telectrix]

take 100% of heaviest loaded circuit, then 40% of all other circuits added togetrher. i get than about 80A. but i'll bet a clamp meter will show a lot less in normal use.

 

[Octopus]

Add up all the MCB values and multiply by 0.25

 

[Newboy2013]

Add up all the MCB values and multiply by 0.25

From a practical perspective I think that you (and previous poster) are right, the total current is likely to be well less that 80A. However as this is my 'inspection' job I feel (perhaps unnecessarily) that I need to be spot on with the regs / OSG. If this were a normal job I would merge the two CU's into one with the appropriate switch, but as its an inspection I need to be able to prove the design, and that's where I am struggling a bit. The OSG suggests values for lights, power and cookers (all of which I have included in the far right column) and this shows a theoretical overload. So should I split the feeds and have 2x 25mm tails to each CU?

 

[Engineer54]

take 100% of heaviest loaded circuit, then 40% of all other circuits added togetrher. i get than about 80A. but i'll bet a clamp meter will show a lot less in normal use.

Id just X 0.4 over the whole thing, and you would still be well inside the real max demand of any domestic installation!!

 

[Engineer54]

From a practical perspective I think that you (and previous poster) are right, the total current is likely to be well less that 80A. However as this is my 'inspection' job I feel (perhaps unnecessarily) that I need to be spot on with the regs / OSG. If this were a normal job I would merge the two CU's into one with the appropriate switch, but as its an inspection I need to be able to prove the design, and that's where I am struggling a bit. The OSG suggests values for lights, power and cookers (all of which I have included in the far right column) and this shows a theoretical overload. So should I split the feeds and have 2x 25mm tails to each CU?

Do you honestly believe that this NAPIT inspector is going to start asking questions on diversity of this installation?? lol!! Even if he did, he should be more than aware of how max demand/diversity is calculated/assessed in the real world...

The BS7671 and OSG formulas do not work, ...end of, as you have found out for yourself. No-one uses them for that very reason. The only exception is the time proven cooker diversity calculation formula.

 

[Silly Sausage]

You can see one obvious flaw in your calculations.
CU1/11 Feeding CU2 - 40A
Your calc for CU2 is 48.7A

 

[Richard Burns]

As said above the OSG calculations would only really work on a very old house with about 4 circuits, once you get into modern CUs the number of circuits escalates without changing the actual demand significantly.

You have a lot of light points there, perhaps some are downlights so limited to 50W, but I do not know the size of the house.
Total of 31 lights half of which may be on at any time so 7A.
As CU2 is fed from CU1 then you can calculate say 50% of the breaker (20A) for CU2 as the total load from CU2 and then ignore the rest of your calculation for CU2 as it is already included in the total.
Leave the cooker calculation as you have it and assume about 30A for power usage in total for running kettle, toaster, and fan heater (for example since kettle and toaster will be on for such a short time they will be irrelevant.)

Total 78A, hey hang on this is the same as the 80A approximations above!

 

[telectrix]

calculating diversity and max. load is about an exact a science as guessing the weight of an elephant's turd in a bath of liquid aluminium.

 

[Engineer54]

As said above the OSG calculations would only really work on a very old house with about 4 circuits, once you get into modern CUs the number of circuits escalates without changing the actual demand significantly.

Total 78A, hey hang on this is the same as the 80A approximations above!

Yep and as Tel rightly stated, will still be well over anything that will be seen at the tails!!
If this installation ever sees 50A, it'll be for a few minutes only and as rare as a blue moon!! lol!!