Secondary consumer unit

TL;DR is at the bottom of the page

Hi thanks for reading

I want to install a secondary consumer unit out to my garden shed.  I want a couple of double plug sockets, a shed light and some garden lights.

I have a spare way in my current consumer unit.

I plan to install an a2 radial circuit in the shed on a 32amp RCCB, then two seperate 6amp RCCBs for the two lighting circuits.  I think I'm going to install an AFDD too since its a wooden shed.  I also plan to run all the cables in metal conduit and for the socket outlets to be metal clad. Is there any other suggestions you could make to improve on the design of the installation? 

My real question though is how big does the MCB device have to be on the original consumer unit?  I'm going to run the supply to the secondary consumer unit in 6mm SWA cable reference method D, which according to the table I'm looking at allows me 53amps.  So obviously the MCB I take the supply off from needs to be smaller than this.  How much smaller does it have to be though?  If there was such a thing as a 52.9amp MCB would that be ok? How much leeway do I get?

Then how much smaller do all the loads on the secondary unit have to be?  Is there a regulation for all of this? 

BASICALLYYYYYYY  can I feed my secondary consumer unit that has a 32amp, 6amp and 6amp MCB off from a 40amp MCB on the original board?  I'm guessing this is all down to diversity at the end of the day, but I can't find regulations on diversity for secondary consumer units.  Another question out of interest is how big would an installation have to be before you were not happy with working on it until it is upgraded to a three phase supply?

And a different question entirely, can I spur the light switch off of the radial circuit? I don't want to but I am just wondering if I am allowed to that?  Would it have to go to a fused connection unit first?

thank you for your time, sorry for the abudance of questions

can I feed my secondary consumer unit that has a 32amp, 6amp and 6amp MCB off from a 40amp MCB on the original board? What's the rule/regulation/whereabouts of relevant information?

Could I feed a consumer unit with 10, 30amp MCBs with 1 32amp MCB?  What regulation would this break?

jumpjamesjump said:

What regulation would this break?

Click to expand...

Part P for a start by the sounds of it unless you are able to notify!  

Too much information that isn't relevant.

Not sure why you would need an afdd?

What type of earthing, what distance from the db, what is the intended use of the shed? 

-all factors which determine the design of the shed supply.

Oh yeah and that part pee malarkey....

Blue Duck said:

Too much information that isn't relevant.

Not sure why you would need an afdd?

What type of earthing, what distance from the db, what is the intended use of the shed? 

-all factors which determine the design of the shed supply.

Oh yeah and that part pee malarkey....

Click to expand...

thank you for the reply, sorry for irrelevant information.

421.1.7 reccommends AFDD for additional protection in CA2 locations.

It's a wooden shed.

TNCS, and probably about 8metres away.  Some power tools, work shed, storage and stuff, general use. 

thanks

Onoff said:

Part P for a start by the sounds of it unless you are able to notify!  

Click to expand...

thank you for your reply.  

I am planning to get someone qualified to sign it off.  

I'm just wondering how you would go about sizing the MCB on the original consumer unit that feeds the secondary.   

Would you prefer 40amps or 50amp?  I think 63amp is too big due the cable being only 6mm SWA

thanks for your time

jumpjamesjump said:

thank you for your reply.  

I am planning to get someone qualified to sign it off.  

I'm just wondering how you would go about sizing the MCB on the original consumer unit that feeds the secondary.   

Would you prefer 40amps or 50amp?  I think 63amp is too big due the cable being only 6mm SWA

thanks for your time

Click to expand...

Im getting a real sense of deja vu again....]

So, you saying you are not a qualified sparks yourself then? 

jumpjamesjump said:

I am planning to get someone qualified to sign it off.  

Click to expand...

To sign which bit???

Design?

Construction?

Verification?

Part-P building regs notification?

Some or all four???

I wouldn't trust anyone who claims to be qualified who just signs the end bit with no involvement with the design.... 

There is no point messing around with AFDD you mention quoting reg 421.1.7.. IF you are skipping past and ignoring the correct guidance for design & construction.. :shakehead

have a gander at section 644.1 -> 644.5...

Then it will be best speak to your qualified person first who you are hoping to sign the paperwork for you,

ask them what design they are happy to put their name and associated legal responsibilities to.

  :coffee

Would you like me to move this to the DIY section?

I think my main fuse is 80A and when I add up all the MCBs in the consumer unit it comes to a much higher number........... [not an issue, fuse is to stop wires of the circuit melting in the event of a fault]

Why 32A radial? Why metal conduit? Having outside and inside lights on seperate circuits I can kind of see, but with modern LED lighting you can put a lot of lighting on a 6A breaker.

Not qualified in anything electrical except PAT testing, but a 20A radial and a 6A lighting circuit with plastic conduit would be ample IMO. I'd stick it all on something like a 32A MCB in the main CU.

Is this trade or DIY  ?   

sprocketflup said:

Im getting a real sense of deja vu again....]

So, you saying you are not a qualified sparks yourself then? 

Click to expand...

SPECIAL LOCATION said:

To sign which bit???

Design?

Construction?

Verification?

Part-P building regs notification?

Some or all four???

I wouldn't trust anyone who claims to be qualified who just signs the end bit with no involvement with the design.... 

There is no point messing around with AFDD you mention quoting reg 421.1.7.. IF you are skipping past and ignoring the correct guidance for design & construction.. :shakehead

have a gander at section 644.1 -> 644.5...

Then it will be best speak to your qualified person first who you are hoping to sign the paperwork for you,

ask them what design they are happy to put their name and associated legal responsibilities to.

  :coffee

Click to expand...

I am not qualified.  My bad if I posted this in the wrong place. 

All 4 of them.  The plan was to message a whole bunch of electricians since obviously most of them are going to say no, then pay them to come before the installation and after the installation to discuss anything they want done.  I just quite fancied designing it myself, carrying out the work and then testing it,  hence the questions.  I guess I'm still in the design phase?  So is there a particular reason I would consider not having an AFDD?  

Apache said:

Would you like me to move this to the DIY section?

I think my main fuse is 80A and when I add up all the MCBs in the consumer unit it comes to a much higher number........... [not an issue, fuse is to stop wires of the circuit melting in the event of a fault]

Why 32A radial? Why metal conduit? Having outside and inside lights on seperate circuits I can kind of see, but with modern LED lighting you can put a lot of lighting on a 6A breaker.

Not qualified in anything electrical except PAT testing, but a 20A radial and a 6A lighting circuit with plastic conduit would be ample IMO. I'd stick it all on something like a 32A MCB in the main CU.

Click to expand...

sure.

Yes ofcourse I understand this.

Metal conduit because its stronger than plastic and there's going to be powertools.  Yes I guess a 20amp radial would suffice, not like 32amps is going to break the bank,  Seperate breakers for the lights just in'case one gets tripped and it's dark.

thanks everyone

moved to DIY.

My shed wired in metal clad with T&E clipped to the walls. The powertools havn't left the bench and drilled through anything as yet. Plastic conduit would be fine but do as you please

jumpjamesjump said:

The plan was to message a whole bunch of electricians since obviously most of them are going to say no, then pay them to come before the installation and after the installation to discuss anything they want done.  I just quite fancied designing it myself, carrying out the work and then testing it,  hence the questions.  I guess I'm still in the design phase?  So is there a particular reason I would consider not having an AFDD?  

Metal conduit because its stronger than plastic and there's going to be powertools.  Yes I guess a 20amp radial would suffice, not like 32amps is going to break the bank,  Seperate breakers for the lights just in'case one gets tripped and it's dark.

Click to expand...

OK right procedure for getting an electrican involved - no one likes being called in after a job has been done.

AFDD - total wate of space really mean't for the American market that some arse is trying to foister on the UK.

Conduit - are you planning to chop through the conduit with said power tools ?  :^O  Plastic does for most things and is much easier to install

20 A is roughly 5 KW, that's a 'shed load' of leccy for most applications ( bad pun intended coz I have a terrible sense of humour  :^O ). 

I think you are slowly realising why getting a qualified sparky in is easier than trying to design circuitry yourself - you are clearly getting bogged down in the nonsense of BS7671 interpretation, of what is a simplish job. But please feel free to keep trying, it's good for the brain matter apparently  .

Just to kkep the brain matter working , do please research exporting earthing on a  TN-C-S system ( hint, this is not a good thing!) 

Can't knock the OP for going and spending £80 on the copy of the regs though.