Wylex adding MCB's

[wishy]

I have had extensive home improvement work carried out and I need to add some MCB's to my Wylex NHRs6604 unit.

I have installed a ring main into a conservatory (2.5mm2) a lighting supply and a single 2.5mm2 cable connected to aarmoured cable to supply the garden shed with power.

I know the type of MCB to use but where can I find information on what Amp rating the MCB's should be for these different jobs.

Am I Right in presuming that a NSB32 is necessary for ring mains. I have two spare NSB16 in the unit, could these be used for the lighting circuit and shed supply.

 

[ban-all-sheds]

I have had extensive home improvement work carried out and I need to add some MCB's to my Wylex NHRs6604 unit.

I have installed a ring main into a conservatory (2.5mm2)

30/32A MCB on the RCD side or 30/32A RCBO on the non-RCD side.

a lighting supply

Depends on size of cable and length, but 5/6A for 1mm and 10A for 1.5mm are reasonable guidelines for limits. Might as well use 5/6A in any case unless the lighting load is too high.

and a single 2.5mm2 cable connected to aarmoured cable to supply the garden shed with power.

15/16A MCB on the RCD side or RCBO on the non-RCD side. Or an MCB on the non-RCD side and RCD socket(s) in the shed. You could potentially use a 20A, but you'd have to be more careful about the length of the circuit, and how the cable is installed. And whichever you use you must also consider the rating, and length of the SWA.

I know the type of MCB to use but where can I find information on what Amp rating the MCB's should be for these different jobs.

here and the IEE On-Site Guide, amongst others.

Am I Right in presuming that a NSB32 is necessary for ring mains.

Yes.

I have two spare NSB16 in the unit, could these be used for the lighting circuit and shed supply.

No and yes respectively (subject to above caveats about the run of SWA).

 

[wishy]

As you have probably guesed I am no electrician, or anywhere close!
And I am very new to this Forum, but I must say how grateful I am for the help so far. Reading some posts has already enlightened me greatly.

I intend to install the circuits for both downstairs, upstairs, conservatory, and garden shed supply on individual NSB32 MCB on the RCD (80A/30mA)side. I also have a seperate kitchen ring main and Smeg A2 gas hob with electric double oven cooker connected on the RCD side both using the same rating of MCB's.
This means there would be six 32A MCB's on the RCD side filling all available connections.
IS THIS TOO MUCH!!?
Above the RCD are the lighting circuits, alarm, door bell and the new conservatory lighting supply cable, which I now appreciate needs to go through a NSB16 MCB

...how the cable is installed.

The SWA is 10m long 2.5 3 core connected via a 32 amp junction box indoors to 10 m 2.5 cable running to the CU. Already purchased a double weatherproof socket for shed, without RCD. Drat!

Hope this helps anyone who can help me!!.

Cheers ^w^

 

[ban-all-sheds]

There's a post gone missing since your last one, so I won't reply now in case it reappears and makes things confusing.

 

[ban-all-sheds]

No sign of the missing post surfacing, or any news on why it went west, so....

As you have probably guesed I am no electrician, or anywhere close!
And I am very new to this Forum, but I must say how grateful I am for the help so far. Reading some posts has already enlightened me greatly.

I intend to install the circuits for both downstairs, upstairs, conservatory, and garden shed supply on individual NSB32 MCB on the RCD (80A/30mA)side. I also have a seperate kitchen ring main and Smeg A2 gas hob with electric double oven cooker connected on the RCD side both using the same rating of MCB's.
This means there would be six 32A MCB's on the RCD side filling all available connections.
IS THIS TOO MUCH!!?

It's a bit steep. Even allowing for the fact that your shed cannot be on a 32A breaker, (as I said before, 20A would be the max), I make that, applying the generic diversity guidelines, just over 100A. Diversity is the concept that not all circuits will be loaded to their maximum simultaneously, and that you therefore make certain assumptions about how much load to allow for each circuit when assessing the total requirement. These "assumptions" are not a substitute for proper design, so if you were doing something out of the ordinary - e.g. you take in washing so in the utility room there are 2 washing machines, a tumble drier and an iron on continually all day, then the assumptions would not be valid, but in the absence of any particular knowledge, the wiring regs has a table of assumptions you can use. So if you had 6 x 32A socket circuits, you'd count 1 at 32A and the rest at 40%, making 96A. You can read more about this here .

I made yours more than that because one of your circuits supplies a cooking appliance, and there is a different calculation for that, but I had to assume that the appliance rating is the full 32A, or thereabouts (which it probably isn't), and that the cooker control unit has a socket (which it may not), making that circuit 21A.

So 4 socket circuits at 32A, plus 1 at 20A (shed), plus the cooker = 101.4A.

There are things you can do to get this figure down. You can run the shed from a 15/16A breaker. You can look at what your oven really draws (I doubt it's 7kW), and fit a lower rated breaker. You will struggle to get it down to below 80A though, so you may have to move a circuit to the non-RCD side.

Above the RCD are the lighting circuits, alarm, door bell and the new conservatory lighting supply cable, which I now appreciate needs to go through a NSB16 MCB

NO - this is too big for a lighting circuit - 10A max.

The SWA is 10m long 2.5 3 core connected via a 32 amp junction box indoors to 10 m 2.5 cable running to the CU.

Unless the 2.5mm cable indoors is buried in an insulated wall then you'd be OK for 20A.

Already purchased a double weatherproof socket for shed, without RCD. Drat!

Well - if you have to change it you have to change it, that's all there is.

I would strongly suggest you do a search here for words like shed, garage, outside, outbuilding, outhouse etc for fuller advice on best practices concerning the supply of electricity to the shed.

 

[Big_Spark]

Wishy, one thing to consider would be talking to your supplier and getting them to quote you for upgrading your supply from 80A to 100A.

It is likely that the head may be 100A rated and will simply need an uprated BS1361 HRC fuse. You can then have a bit more to play with and means the installation will be better designed.

One thing to remember, which I have not seen mentioned here, as far as the IEE and NICEIC go, part of good design and practise is to ensure that 20% of the ways available in the consumer unit/distribution board are left spare.

 

[ban-all-sheds]

There's me assuming again - the NHRS6604 is a split-load 100A-80A/30mA unit, so I never questioned that he'd already have a 100A supply...

 

[wishy]

May I start by saying thanks for the help so far guys. The fog of electrical wizardry is SLOWLY being lifted from my eyes by this discussion far more quickly than I could have hoped for.

Yes, I know it might seem like your talking to a brick wall but yes I did realise I'd made a mistake by still insisting on putting lighting circuits into a 16aMCB, but really I am paying attention. I know it should have been 6A, just testing!

Some very interesting comments can I begin by saying b-a-s seems to be spot on,

the NHRS6604 is a split-load 100A-80A/30mA unit, so I never questioned that he'd already have a 100A supply

It says 100A on the main switch in the unit so I'm presuming that means my supplier gives me that and has 6 connections above the RCD 6 below.

As regards the cooker, I have looked up the data for the Smeg A2-2 and it states

Nominal power: 6.3 kW, 30 Amp power supply required

Does this mean that it must go to a 32A MCB or could it be reduced? The hob is all gas.

The electricians who did the work in my house connected the cooker to a 13amp fused spur which then goes to a cooker control unit with a socket and then on to the CU. Which even I find strange for some, novice, reason.I also wonder what you guys think of the fact that the electricians have put both the upstairs and downstairs socket circuits into one 32A MCB?

I read the diversity info on the site you suggested b-a-s and found it extremely useful. For a household of two it is very unlikely that overloading should occur as you described. I then had a rethink of the CU layout, particularly after your statement of the 40% rule for the 32a MCB's supplying the socket circuits.

Knock me back down if you guys feel it necessary but how about this layout scenario. Be gentle though cos I'm really new at all this!

• Above the RCD;
  Upstairs lights 6A MCB
  Downstairs lights 6A MCB
  Conservatory lights 6A MCB
  Alarm/Door bell 6A MCB
  Shed 16A RCBO or (MCB + RCBO socket in the shed) which is best? Can't seem to find Wylex RCBO's for my unit.
  
  Below the 80 A RCD;
  Up ring 32A MCB
  Down ring 32A MCB
  Conservatory Ring 32A MCB
  Kitchen Ring 32A MCB
  Cooker 32A MCB

As regards the cable indoors supplying the SWA to the shed it runs through conduit below kitchen units.

Sorry FWL just less than 20% of the ways left spare this seems!

 

[Big_Spark]

I must be going blind in my old age, I missed the 100A comment earlier. Apologies if I confused you.

Ban, thanks for correcting me, appreciated

 

[ban-all-sheds]

It says 100A on the main switch in the unit so I'm presuming that means my supplier gives me

Not necessarily....

As regards the cooker, I have looked up the data for the Smeg A2-2 and it states

Nominal power: 6.3 kW, 30 Amp power supply required

Does this mean that it must go to a 32A MCB or could it be reduced? The hob is all gas.

6.3kW is 27.4A, so no, it can't be reduced, and I hope that the circuit is wired with at least 4mm cable.

The electricians who did the work in my house connected the cooker to a 13amp fused spur which then goes to a cooker control unit with a socket and then on to the CU. Which even I find strange for some, novice, reason.

That doesn't sound right - the cooker should come directly off the control unit (ignoring the socket, this is just a heavy duty switch). It may physically go via an outlet plate somewhere else for cosmetic reasons, but not via a 13A FCU. That won't be enough for a 6.3kW oven. Some ovens are 3kW or less, though, and can be fed by a 13A supply - in fact you can put a plug on them and plug them in, if you want. Do you know what the rating was of the one there before the new one? I'd get rid of the FCU if I were you, or put a 3A fuse in it and use it for the hob igniter or hood, if it's conveniently located. Whatever, you won't be able to use it for the oven - that will have to be wired to the cooker control switch using the same size (4mm/6mm?) cable that comes from the CU.

I also wonder what you guys think of the fact that the electricians have put both the upstairs and downstairs socket circuits into one 32A MCB?

I don't think this is against the regs, but IMOPO it's pretty shoddy work.

I read the diversity info on the site you suggested b-a-s and found it extremely useful. For a household of two it is very unlikely that overloading should occur as you described. I then had a rethink of the CU layout, particularly after your statement of the 40% rule for the 32a MCB's supplying the socket circuits.

Knock me back down if you guys feel it necessary but how about this layout scenario. Be gentle though cos I'm really new at all this!

• Above the RCD;
  Upstairs lights 6A MCB 0.3A per lampholder
  Downstairs lights 6A MCB 0.3A per lampholder
  Conservatory lights 6A MCB 0.3A per lampholder
  Alarm/Door bell 6A MCB whatever it's rated current is
  Shed 16A RCBO or (MCB + RCBO socket in the shed) which is best? Can't seem to find Wylex RCBO's for my unit. 6.4A. And you'll have to fit an RCD socket, or a small CU with RCD protection, out in the shed, as Wylex do not do Type B RCBOs for that board
  
  Below the 80 A RCD;
  Up ring 32A MCB 32A
  Down ring 32A MCB 12.8A
  Conservatory Ring 32A MCB 12.8A
  Kitchen Ring 32A MCB 12.8A
  Cooker 32A MCB 20.2A

As regards the cable indoors supplying the SWA to the shed it runs through conduit below kitchen units.

That's OK.

Sorry FWL just less than 20% of the ways left spare this seems!

Amazing how quickly they fill up - for anybody with a house and garden I'd not advise anything less than a 14-way these days, pref with variable split. Or two separate smaller units, one with RCD incomer, one without.

 

[wishy]

Thanks for all that info Ban & FWL. Believe me it is much appreciated.

Just been trying to digest it all and feel that the help youv'e given has put me in a positive mood as to how to design the supply.

However I am very worried about the cooker situation now. The cooker that was in the kitchen prior, when we moved in, was all gas. The new cooker has run through the 13amp spur as described for about a year now, with no apparent problem, even when using both ovens at full power.

So the answer seems to be to just take the spur out of the equation and wire direct to the cooker switch with socket. Then on to the CU.

And as regards the two ringmains into one MCB, if IYOPO this is shoddy work but there is no regs against it, what could be the consequences of this scenario?

Is it a fire hazard or something?

Cheers GUYS,

WORRIED NOVICE!

 

[Big_Spark]

Wishy, are you 100% certain the cooker is connected to a fused spur?

45A Cooler connection units are the same size as FCU's, but have no switch on them, 45A switches do, sometime a red switch, sometimes a white one.


This is a Switched Fused Connection Unit from MK.

And this is a single gang 45A cooker connection unit from MK

Here are a couple of examples of 45A Cooker Switches

Standard Size:

Single Gang Size:

Try to identify the switch your cooker is connected too. I would be very surprised if it is in a spur with a 13A fuse that has not blown when running both ovens.

 

[Big_Spark]

And as regards the two ringmains into one MCB, if IYOPO this is shoddy work but there is no regs against it, what could be the consequences of this scenario?

Is it a fire hazard or something?

Cheers GUYS,

WORRIED NOVICE!

Two circuits should not share one MCB. How many cores into each MCB? If there are two rings into it then you will have FOUR red cores in the MCB, if there are only two then you have a single ring main.

If two rings are together they should be split onto two circuits, however you need to confirm these are rings in the first place, it could be four radial circuits, or a ring with two radials taken straight from the MCB.

If it is the last one, it is not ideal and should be changed, radials should not be on 32A breakers unless fused down.

To be honest I am coming to the conclusion that perhaps you should call in a spark to get their opinion, they will be able to advise you in real time as they will see what you see, we cannot, with all the best will in the world.

 

[wishy]

Thanks for your prompt response, sorry it takes me a little longer to investigate your advice.

I quite agree it would be better to get an electrician to look at all these points but electricians, who had all the right recommendations and memberships and were registered with my local authority, were the people that did the wiring work in the first place. Recognised electrical contractors to put it their way! So I felt I had done all the necessary digging to get the right people in in the first place...it seems not

I am pretty certain it is a switched spur as it blew the first time we used the cooker and we were told it was because the fuse was too low, 5 amp, a 13amp was put in and it has run without a problem since!

I will delve deeper and get back to you FWL.

May even take some photies to send... how do you do that! see still alot to learn

 

[wishy]

The cooker plot thickens

On investigating further, following your last post FWL, I have discovered more reasons for concern.

The cooker panel I purchased for the electricians to connect my cooker to was here...still haven't worked out how to insert pictures, sorry.

It has never been switched off, never had any reason to, apart from now. Switched it off and surprise, surprise the cooker stays on. Switch of the fused spur in the cupboard and hey presto off goes the cooker supply. Definately a fused spur with a 13 amp fuse fitted like this one here, I know cos I've just had it apart!

So I guess its save to assume that the cooker goes from the 13 amp switched spur direct to the 32A MCB in the CU. Therefore I have a very pretty cooker panel switch & socket, which cost £17 and does nothing but look like a cooker panel switch & socket that cost three times the price of a normal socket But why has the cooker not blown the 13 amp fuse when cranked up to full power?

Still need to check out other points, watch this space.

I realise your restrictions guys in providing distance advice, but I feel totally enlightened by the advice so far and had I put my points to the forum earlier, and had your wisdom deseminated via your helpful postings, maybe I wouldn't have been in this position now.

Keep it up chaps I can see the light

Thank you for all your patience.