Consumer Unit circuits list and diversity.....

[Stephen H]

Hi, I want to fit a new consumer unit as old one only has 6 MCB's, upstairs lighting, downstairs lighting, cooker, immersion, upstairs ring main and downstairs ring main.

I have tried to maximise the diversity and to consider the circuit loadings so that a fault in one part does not take out too much of anything else and of course thought about H&S.

I have come up with the following list of circuits and have put the reason for it in brackets where necessary)

1. 6A ground floor lighting
2. 6A 1st Floor lighting
3. 6A Loft lighting (we plan to convert loft at later date)
4. 16A or 20A Outside lighting (so that if MCB trips when floodlight blows, it doesn't take out half the house lights and the halogens can be up to 500 watts each)

1. 32A Immersion

1. 32A 6 ring electric Hob (replacing old gas hob)
2. 32A double width oven

1. 32A Ground floor ring main (excluding kitchen and utility)

1. 32A Kitchen & Utility ring main (due to washing machine, dishwasher, toaster, kettle etc being high loads so I need to separate this from the original downstairs ring main)
2. 32A 1st Floor ring main
3. 32A Loft ring main (we plan to convert loft at later date)
4. 32A External sockets and Garage sockets (this will be an RCBO so it does not take out the main RCD)

1. 6A smoke detectors (need high integrity supply)

1. 6A house alarm system (need high integrity supply)
2. 6A combi boiler, thermal store and wood burner pump. The wood burner has a pump and must have a high integrity supply or wood burner will create steam in event of power loss so do not want anything else connected to this circuit.

1. A spare MCB way in case we decide to fit solar PV panels.

This is a 4 bedroom house with a 100A supply.

Now this means at least a 16 way CU. Now am I OK to do this configuration and I assume no issues with this on a 100A supply?

I am prepared to have 16 RCBOs rather than have a split load CU and 16 MCB's

Are there any other issues I need to consider?

Regards,

Stephen

[ARWadsworth]

Start here

16 or 20A for outside lighting? You need to reconsider your choice of outside lights.

[A.Lee]

I'd be tempted to put No. 3 on the downstairs lighting circuit.

You'll only need a 6A breaker. I've never had a problem with tripping when a bulb goes, so again, would put the outside lights on one of the lighting circuits, but spur them off with a 1 or 3 A fuse.

16A is plenty.

I'd be incline to put these on the same cable/circuit. a 10mm t+e should be good for most installations, though you need an idea of expected wattage of the oven/hob.

...

Again, I think No.11 should be on the upstairs circuit, as bedrooms are very rarely 'power suckers', and even less rarely cause tripping of MCBs

There is a good argument that smoke alarms should be on a lighting circuit, as if they cause a problem, you have the incentive to get them fixed, as the ligths are out too.

Is that necessary? The ones I've fitted dont like being fitted with flourescents on the same circuit. I'd put it on another circuit, the immersion would be ideal, if you spur off it.

What do you mean by an high integrity supply? I understood that to be having extra earth paths by making the earth of a radial circuit into a ring - is that what you mean, or, you want it to stay on if other circuits have tripped? If that is the case, you can put it on a non-rcd protected circuit, but I really dont think it is necessary, just use an RCBO for that circuit.

No real need, as it may be simpler in the future to have a separate box at the side of your CU to take in the PV supply.

No problem. You are very unlikely to draw even 80A at any one time.

RCBOs are the way to go. I recommend them to all the people I work for, as there is no chance of nuisance tripping taking out half of the circuits. Niglon RCBOs are £18, other place have them cheaper.

[Doctor Drivel]

Fit a double RCD consumer unit.

[Stephen H]

Well we have a detached house with land all round it.

We have the following to illuminate for both security and entertainment purposes:

Garage driveway Front door pavement Left garden Rear garden right garden front garden

We will be using three position switches, "Off", "On" and "auto via PIR".

Not only that there will be a selection switch for each side of the house, 500W floodlight or 3 x 18 watt CFL wall lights.

So lightest load would be 15 x 18 watts = 270 Watts or 5 x 500 watts, = 2.5kW, so by my reckoning, that's a 16A MCB.

I have found in my experience that just 2 x 500 Watt floods can trip a

6A MCB, but a 500 W and a 300 W together doesn't.

I suppose I could use 300 Watt tubes instead of 500 watt tubes, that will get the max load down to 1500 Watts which will still trip a 6A MCB, so would need a 10A MCB.

On a related note, is it OK to do a lighting ring main in the same way as a wall sockets ring main, so that for the outside lighting I use

1.5mm2 lighting ring on a 10A MCB? This is analogous to using 2.5mm" T&E on a ring main with a 32A MCB.

Regards

Stephen.

[ARWadsworth]

A 10A MCB will power 2300W of lighting. A 5A MCB will power 1380W of lighting. Google for "pir low energy lights" for something better than 500W halogens (especially for the driveway and by the front door) where 500W halogens are overkill.

NO. And 1.5T&E can, depending on the installation method, carry 20A. Even when covered in insulation it can carry 10A.

I suggest that you read

?title=Calculating_A_Cable_Sizeand make a plan before starting the rewire.

[Stephen H]

OK so how come its "OK" to do a ring main for wall sockets on a 32A MCB but Not OK for the same for the lighting circuit?

[ARWadsworth]

You can, if you capable of designing the circuit. It would not be one of the "standard" circuits that you would expect to find in a house.

[Stephen H]

OK, Why is it that in my old house, 2 x 500 Watt floodlights trips a Type B 6A MCB? I have checked the circuit, and there are just two 500 watt floodlights connected to that type B 6A MCB. The circuit breaker trips if both PIRs are triggered simultaneously. So if a 5A MCB will power 1380W, why does the 6A MCB nuisance trip?

For now, I have "solved" the problem by replacing one of the 500W tubes with a 300W tube. The nuisance tripping has stopped.

I know I could replace the Type B MCB with Type C 6A MCB, but it was cheaper to replace tube with a lower wattage than change the MCB......

:-)

Stephen

[John Rumm]

There are a number of standard circuits that are published in the On Site Guide. These have been extensively tested and proven in practice, so one an use them without further calculation in many cases. You can use a non standard circuit if you really need to, but would need to prove that it is acceptable from a design point of view. There is also the question of why bother, since there is no real benefit?

Note that use of conductors in parallel is actually a relatively complex design exercise.

[ARWadsworth]

The 5A was a typo. It should have said 6A:-).

Your MCB is probably tripping on the low side. I have seen 1000W of halogen switched on and not trip a 6A MCB.

[John Rumm]

May be overkill as others have said. Personally I find a 150W halogen over the front of the garage is more than adequate. 60W laterns on the top of wall pillars and wall mounted elsewhere (6 in total) give coverage for most other things. There is an additional 150W halogen on the shed.

Normally only need 16A unless fitting multiple heaters.

Chances are those could be combined into one 32A circuit. Alternatively the double oven is unlikely to need more than 20A

Good idea. Note that short term loads like kettles etc you can basically ignore from a design point of view.

Have a think about how you want to segregate outside circuits, and also think about what you might be doing in the garage... for example if working in it at night, would it be acceptable to lose lighting as a result of a RCD trip in its supply?

Have a read through this one for some of the special considerations for outside supplies:

13. 6A smoke detectors (need high integrity supply)

Battery backed detectors on a lighting circuit is also an alternative. Had the advantage of giving a ready indication of a circuit trip.

Usually wood burners with water connections need to be arranged such that they do not rely on power at all, and can get sufficient cooling via gravity circulation alone.

See the diversity article for more details. But basically you are into the "value judgement" area. Basically you have a house that currently is fine, and you are not in reality dramatically adding to the total overall load. All you are doing is segregating circuits for better control and discrimination. So the basic answer is yup you will be fine. You have not specified any particularly serious electrical loads, you have no space heating or underfloor heating, your water heating sounds like its for backup/summer use only etc. The cooker for all its theoretical power consumption will rarely suck more than 20A and even then not for long etc.

[Andrew Gabriel]

Type B won't handle the inrush current of large filament lamps. You would need a Type C.

Much cheaper in electricity, certainly.

[John Rumm]

Because the inrush current on a cold linear halogen will be significantly in excess of its normal operating current.

Yup. See the difference in response:

to run as well! (having said that, you can get MCBs for a couple of quid thee days)

[John Rumm]

That would not really be enough given the range of circuits proposed and their anticipated uses.

As a minimum he will probably need some non protected ways as well[1] so a two RCD plus main switch would be a minimum. three or four RCDs would be better.

Another option is more than one CU. For example having a secondary CU for all outside circuits.

I found a total of three split load CUs worked well for our needs. Since we are TT, the two in the house have 100mA trip type S RCDs in the incomer position.

One feeds the house circuits:

with a 30mA trip RCBO on the kitchen/utility room (lots of heater elements etc so potentially high leakage), three lighting circuits

Then 3 x lighting, 2 x normal power, one boiler, one immersion, one high integrity IT/Comms power circuit. Then the RCD side feeding 2x sockets for up and downstairs.

A separate "outdoor" CU has its own RCD incomer (that way one can't import any problems from outdoor circuits to the main house supply). It feeds a 30A fuse for the garage/workshop supply, and then one the 30mA RCD one circuit for outside lights, and another for external sockets.

There is a further split load unit in the garage. Min switch incomer (supply has 100mA Type S RCD at head end, and internal lighting circuits on incomer, with sockets, exterior lights, and submain to shed on the RCD side. (shed has a couple of sockets, and a FCU feeding internal and external lights).

(note all of the above was designed to 16th edition standards - hence the lack of 30mA RCD protection on lighting circuits with "vulnerable" cable runs)

[1] The OP has not mentioned what earthing system he has - the options change if he is limited to TT

See :

[Owain]

Better to split the kitchen and utility across two rings sharing with the rest of the ground floor, which won't take much.

Then you have potentially 60A available to the kitchen, and can plug in the kettle on one half whilst working on the other.

Or if you want to save a circuit, put half the kitchen on the ground floor and half the kitchen on the first floor.

Owain

[ARWadsworth]

I believe that it is an integral garage. The OP asked about moving his CU a week ago. He now seems Hell bent on rebuilding his house:-)

Well if he does, let's have photos.

[robgraham]

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"Usually wood burners with water connections need to be arranged such that they do not rely on power at all, and can get sufficient cooling via gravity circulation alone. "

I've always been lead to believe it was *essential* that wood burners did NOT have a pump in their circulation system for the very reason given. I did consider doing a pump when I installed a wood burner many years ago and realised that a stuck pump or by-pass valve would block the gravity flow and lead to serious problems. I do have a Dunsley Neutralise to allow me to run the wood burner in parallel with the main CH system oil burner.

Rob