Multiple consumer units

[ElectricEddie]

Hi guys,

im currently in the middle of doing a rewire on a large domestic property which has a single phase supply, in total I will have 4 consumer units and a distribution board of some kind - also we do lighting control so all the light circuits are separated and going back to a Cbus lighting board which along with the cus will be in each divided zone.

I have run my feeds for my cus back to the distribution point in 16mm L/N and also run in a 16mm Earth to go back to the MET.

So my question is what sort of board is going to be best to have these 16mm feeds going to it im sure I'm right in thinking that they will have to be rcd protected as well as having the rcds in each consumer unit but correct me if I am wrong.

thanks

ed

 

[Andy78]

This rewire should have had these decisions made at the planning stage. RCDs in series, or RCDs covering distribution circuits within the installation with the set up you describe is unbelievable to be honest.

 

[ElectricEddie]

Could you advise what would be best to do please

 

[ElectricEddie]

There will be RCDs covering the distribution circuits that is the whole plan the only question I am asking is what type of board is going to be best to use

 

[Andy78]

Could you advise what would be best to do please

Start again with the distribution circuits run so they don't require RCD protection to offer better division of circuits for the final circuit arrangements, and have a think about what sort of OCPD would provide the best discrimination for the distribution circuits bearing in mind the devices fitted to the final circuits in the sub boards.

 

[Darkwood]

The point been made here by Andy was its poor design to have submains RCD protected, you design to supply your submains in say a SWA which would negate the need for rcd protection then have a duel RCD split board or RCBO boards for the final circuits, your also in the middle of the rewire and your asking advice on design that should have been worked out long before you got to this stage..

Regardless how far on the job is we can try to help but first a few questions -
-When you say you have 16mm L/N + seperate earth ..what kind of cables and install method are the L and N.?
-You say the 16mm earth goes back to the MET - is this the earth for the submain in which case it would be better to put it in the earth terminal for that particular distribution way to limit confusion.

 

[ElectricEddie]

Ok that is very helpful thank you for your input, I will be able to calculate the sort of over current protective device easily enough via the Way I have arranged the circuits off of each sub board .it was only the distribution element that I was unsure of.

please know also that this job is still very much in the first fix stage so anything is changeable and no consumer units or DB's have yet been chosen.

in your opinion what would be the best way to run the distribution so that it needn't require RCD protection.

Thanks again

 

[ElectricEddie]

It's 16mm t/e with a separate 16mm2 Earth reference method 103 so through stud walls and joists etc my plan was to use the 16mm Earth in order to ensure my resistance was low enough to meet the ads times for each submain all submains having an Earth back to the main Earth at point of distribution

 

[Darkwood]

It's 16mm t/e with a separate 16mm2 Earth reference method 103 so through stud walls and joists etc my plan was to use the 16mm Earth in order to ensure my resistance was low enough to meet the ads times for each submain all submains having an Earth back to the main Earth at point of distribution

This method of choice would require rcd protection unless your somehow deeper than 50mm for the full run, like I mentioned before, if you can design your submain run or change the cable to be able to omit the rcd protection then its a better method.

 

[ElectricEddie]

This method of choice would require rcd protection unless your somehow deeper than 50mm for the full run, like I mentioned before, if you can design your submain run or change the cable to be able to omit the rcd protection then its a better method.

This is exactly what I thought. I have made sure that throughout the run I am more than 50mm deep due to the fact that all the walls have bee studded off so I have run behind all the stud work I'll measure out at all the shallowest points to double check tomorrow.

with this method of installation would I then just be able to run All the circuits from a single isolation point

either that or swap to swa

thanks very much for your help

 

[Leesparkykent]

Is the stud work metal? Are you planning on putting the distribution circuits on MCB's then?

 

[davesparks]

Surely running one cable to each point is easier/cheaper than running two?

 

[ElectricEddie]

Is the stud work metal?

no it's all timbers but I am behind it not in it

 

[ElectricEddie]

Surely running one cable to each point is easier/cheaper than running two?

Would the cpc not have sufficient surface area though with a16mm t/e with it only being 6mm

 

[Leesparkykent]

no it's all timbers but I am behind it not in it

But technically still in a partition?

 

[davesparks]

Would the cpc not have sufficient surface area though with a16mm t/e with it only being 6mm

What does the adiabatic say?

 

[ElectricEddie]

Theoretically but it will be deeper than 50 mm

 

[Leesparkykent]

Theoretically but it will be deeper than 50 mm

What size OCPD are you planning on using for the 4 x distribution circuits?

 

[ElectricEddie]

What size OCPD are you planning on using for the 4 x distribution circuits?

Do do you mean on each of the submain boards ?

 

[Leesparkykent]

Do do you mean on each of the submain boards ?

The over current protective device that is protecting each 16mm T&E.

 

[ElectricEddie]

The over current protective device that is protecting each 16mm T&E.

63amp each I was planning to use

 

[davesparks]

63A what?

 

[ElectricEddie]

63amp each I was planning to use

That is of course providing that I keep the set up in the same way distribution wise and don't use swa but my 16mm the is below 50 mm all the way through the run

 

[ElectricEddie]

63a 30ma rcd

 

[ElectricEddie]

I think I need to think about this a bit more and do some more calculations that you for everyone's input I will update in a couple of days

 

[ElectricEddie]

* thank you

 

[Leesparkykent]

The thing is Eddie if you use an mcb for the distribution circuits then you have next to no chance of discrimination with the final circuits.

edit: An RCD doesn't provide over current protection.

 

[davesparks]

63a 30ma rcd

He asked for the OCPD, not the additional protection!

 

[Leesparkykent]

Best thing I think you can do is sit down and revaluate the whole design.

 

[ElectricEddie]

Yer I am going to cheers guys the issue regarding distribution has been the bit I have been stuck I want to get it right hence seeking advice on here I will get someone with more experience to come in and have a look but looks like I'm going to run it in swa i will properly calculate the csa's and get back to this forum afterwards, I appreciate this should have been done beforehand

 

[ElectricEddie]

The thing is Eddie if you use an mcb for the distribution circuits then you have next to no chance of discrimination with the final circuits.

edit: An RCD doesn't provide over current protection.

if I ran swa out of a fused isolator switch for each submain would that provide me with the protection that I need dependant on demand etc

 

[davesparks]

if I ran swa out of a fused isolator switch for each submain would that provide me with the protection that I need dependant on demand etc

Maybe, it depends on what protection you need for a start.
Follow the design process to select cable type, size, OCPD etc etc

And I think the term you are looking for is 'switchfuse'

 

[Simonslimline]

if I ran swa out of a fused isolator switch for each submain would that provide me with the protection that I need dependant on demand etc

Do you own a copy of BS7671?

 

[ElectricEddie]

Do you own a copy of BS7671?

Of course

 

[Simonslimline]

Of course

It may be a good place to look for the answers to these questions perhaps!

 

[DuaneMHunt1976]

Hope you get it sorted Eddie, I'm trying to keep up with this post and learn a bit more
Humans hay, keep learning and changing or fade away

 

[Leesparkykent]

Hope you get it sorted Eddie, I'm trying to keep up with this post and learn a bit more
Humans hay, keep learning and changing or fade away

Sounds like it could be a decent job if it's designed properly.

 

[Marvo]

Could you advise what would be best to do please

What size OCPD are you planning on using for the 4 x distribution circuits?

63a 30ma rcd

Yer I am going to cheers guys the issue regarding distribution has been the bit I have been stuck I want to get it right hence seeking advice on here I will get someone with more experience to come in and have a look but looks like I'm going to run it in swa i will properly calculate the csa's and get back to this forum afterwards, I appreciate this should have been done beforehand

I think by your own admission you're not very experienced in this type of design and this thread is turning into blow by blow instructions on an installation that only you have seen so it's guesswork at best by those attempting to help you out and this is why I'm closing the thread. I'd say your suggestion about getting assistance, or at least some guidance on site from someone with more experience in designing this type of installation might save you a lot of problems.

If you've got a more specific electrical question rather than blanket design assistance please feel free to open a new thread.