Consumer Unit MCBs

MrGriff · Oct 9, 2021

Hi Everyone, I’m new to the forum and joined to gain more knowledge from you guys that are practicing electrical work. I studied 17th edition electrics at college a few years back and passed with great scores but never went on to use my electrical qualification in practice, I used my cabling skills and went into installing full office computer networks and config of the install bespoke to clients needs. My question is - I am currently building a summer house/office at home and have what you guys may think is a random question - I have a 4 bay consumer unit for the summer house 2x6a which will be 1 for internal and 1 for external lights, 1x 32a which will be for the ring main but there is a 16a MCB, having looked around on toolstation and Amazon most consumer units have a 16a MCB, I’m just wondering what I would use the 16a for? I am actually thinking of switching it out for another 32a MCB and creating a separate ring main circuit for a few outside double sockets to keep them on separate trips? Happy for any feedback that is constructive (I have seen plenty of forums where the response is - if you don’t know find an electrician) Thankyou in advance :c)

catdog1121 · Oct 10, 2021

Look up radial circuit.

Plus its a new circuit so needs to be certified with building control. I suggest you get an electrician in, to save money I sometimes have customers help me under my guidance..

SJD · Oct 10, 2021

What is the supply cable to the outbuilding (cable size, length) and what is the rating of the MCB/RCBO/fuse supplying it? So often I see a supply on a 16A or 20A device, then a socket ring final circuit in the outbuilding on a pointless 32A MCB.

Vortigern · Oct 10, 2021

I don't know when the existing CU was put in but you say they are MCB. If you are modifying socket circuits, especially for outside you will have to change them for RCBOs. As to changing an existing 16a MCB to 32a you would have to know if the cable that is coming off of the 16a would be protected by the upping of the MCB to 32a. Which I suspect would be in the negative. As above get a spark in for this one, could get messy!

MrGriff · Oct 10, 2021

Thanks for the advice, I wanted to pre wire everything first but leave all wires visible for an electrician to confirm everything is good before plasterboarding and then final inspection after the sockets and lights are in place. The feed cable is 16mm 3 core armoured cable, length is between 12m/15m and is rated for direct burial from Doncaster cables. Feed out from house is a 63a trip, My thought process was just a simple 32a ring for summerhouse sockets 32a separate ring for the shed end of the building, same for the lighting 2x separate 6a for 3 internal and 5 external lights.

Just never got why a 16a was supplied and if it was for a fused spur for a wall heater or an over sink water heater possibly?

Thankyou for not making me feel stupid asking, it will all be passed by an electrician near to me as safety of the build and the house is priority to me. This wasn’t about saving money just being upskilled and in the know. Most importantly peace of mind keeping the family safe with the electrics.

nicebutdim · Oct 10, 2021

MrGriff said:
Just never got why a 16a was supplied and if it was for a fused spur for a wall heater or an over sink water heater possibly?

Could be used for any number of reasons including, but not limited to, fixed appliances, water heater etc.

catdog1121 · Oct 10, 2021

A circuit should be rated for the current it requires.. There are some properties that have one 32A ring final circuit for the whole house so unless you are going to put a lot of heaters in this summer house 32A ring final I would say is way over kill... So is a ring circuit for outside sockets. I would think a 16A radial for both the inside and outdoor sockets would be ok but it depends what you plan to use them for.

16A or 20A can be used for socket circuits on a radial circuit, that is why most garage consumer units come with 16A and 6A, one for a socket radial circuit, the other for lighting. Probably really no need to have two lighting circuits unless you have a really big garden..

For an electrician to sign off this work he should be designing it, constructing it and testing it. I am all for a customer constructing it under my supervision but you should really be getting the electrician to design and test it. You may have gone way overkill and cost yourself a lot of money.

Have you correctly glanded the SWA cable? I could be wrong and you might have loads of outdoor lights and require a lot of load for this out house but I suspect that because you dont know what a 16A mcb might be used for you have put in a 32A ring because thats all you know rather than what is required. Get an electrician in now before you go to far..

MrGriff · Oct 10, 2021

catdog1121 said:
A circuit should be rated for the current it requires.. There are some properties that have one 32A ring final circuit for the whole house so unless you are going to put a lot of heaters in this summer house 32A ring final I would say is way over kill... So is a ring circuit for outside sockets. I would think a 16A radial for both the inside and outdoor sockets would be ok but it depends what you plan to use them for.

16A or 20A can be used for socket circuits on a radial circuit, that is why most garage consumer units come with 16A and 6A, one for a socket radial circuit, the other for lighting. Probably really no need to have two lighting circuits unless you have a really big garden..

For an electrician to sign off this work he should be designing it, constructing it and testing it. I am all for a customer constructing it under my supervision but you should really be getting the electrician to design and test it. You may have gone way overkill and cost yourself a lot of money.

Have you correctly glanded the SWA cable? I could be wrong and you might have loads of outdoor lights and require a lot of load for this out house but I suspect that because you dont know what a 16A mcb might be used for you have put in a 32A ring because thats all you know rather than what is required. Get an electrician in now before you go to far..

Thankyou for taking time to respond, you are 100% right for the electrician to come out and see what is in place, and then correct my thinking, you are also correct that I have tried to stay within what I know and probably better ways of doing this. My building is just a water tight shell and only the 2.5mm ring wires for plugs and 1.5mm for the lights, the wiring is all visible and can be amended as required without any hassle, I agree a ring is over kill at the moment but I was trying to future proof it’s use as I don’t know what it could be used for in years to come, at the moment it will be a couple of computers with dual monitors, a projector and possibly a small oil heater / panel heater depending on how cold it is over winter. I spent last winter working from a touring caravan in the same build spot but now for something more permanent.

Just for FYI on my story the armoured cable is only connected to the office/shed consumer unit, the other end is still on the roll by the house and will only be connected once an electrician is involved.

The attached pictures were taken as I was feeding wires not completed before it’s pointed out it’s radial in the pics :c)

DefyG · Oct 10, 2021

If you've bought a fully populated consumer unit, it will be fitted with a range of mcb's with no specific purpose in mind. Of course you can always swap one for a different rating depending upon your needs.

GLENNSPARK · Oct 11, 2021

There's a question of diversity here...

• the sockets in the summer house should be happy on a 20A radial in a 2.5/1.5 70° PVC twin

• lighting on a 6A radial in a 1.0/1.0 70° PVC twin

If you want to put your outside socket/s on the 16A then that'll be fine...

To give you an idea:
A typical 3 bed house will pull less than 10A during the day....
Peek demand (early evening)....can rise up to about 45A...for short durations until thermostats start kicking in on appliances and everyone's been in the shower etc...

When producing sheets for a customer I generally put a 3-4 bed property's MAX. Demand down as 45A
NOT "100A"....as I keep seeing on certs and paperwork...

richy3333 · Oct 11, 2021

Do you have a building warrant or have BC confirmed you don’t need one For the size and usage of the building? I’d not sign your work off and no decent spark would either unless you had someone in from the beginning. If you have your 17th Ed then you’ll know it’s not a ring main ? Agree with all other comments.

leebut · Oct 11, 2021

MrGriff said:
Hi Everyone, I’m new to the forum and joined to gain more knowledge from you guys that are practicing electrical work. I studied 17th edition electrics at college a few years back and passed with great scores but never went on to use my electrical qualification in practice, I used my cabling skills and went into installing full office computer networks and config of the install bespoke to clients needs. My question is - I am currently building a summer house/office at home and have what you guys may think is a random question - I have a 4 bay consumer unit for the summer house 2x6a which will be 1 for internal and 1 for external lights, 1x 32a which will be for the ring main but there is a 16a MCB, having looked around on toolstation and Amazon most consumer units have a 16a MCB, I’m just wondering what I would use the 16a for? I am actually thinking of switching it out for another 32a MCB and creating a separate ring main circuit for a few outside double sockets to keep them on separate trips? Happy for any feedback that is constructive (I have seen plenty of forums where the response is - if you don’t know find an electrician) Thankyou in advance :c)

Hi

If it was me installing the summerhouse I’d be taking the 32amp breakers and swapping them for two 20amp and I’d be running two radial circuits instead one internal and one for the exterior socket if your only installing one external socket use the 16amp

I would throughly recommend you rethink what your putting into the consumer unit to comply with RCD protection I’d be installing individual RCBO’s in a board with a main switch and not an RCD incomer

Lastly you will be supplying the summer house with an 6mm armoured cable don’t take the earth from the house into the summerhouse terminate it outside the building or use a compression gland on the consumer unit end and install an earth rod connected to the consumer unit earth bar

Electrics · Oct 11, 2021

Vortigern said:
If you are modifying socket circuits, especially for outside you will have to change them for RCBOs.

Do you HAVE to or was that just the wording you used?

GLENNSPARK · Oct 11, 2021

leebut said:
Hi

If it was me installing the summerhouse I’d be taking the 32amp breakers and swapping them for two 20amp and I’d be running two radial circuits instead one internal and one for the exterior socket if your only installing one external socket use the 16amp

I would throughly recommend you rethink what your putting into the consumer unit to comply with RCD protection I’d be installing individual RCBO’s in a board with a main switch and not an RCD incomer

Lastly you will be supplying the summer house with an 6mm armoured cable don’t take the earth from the house into the summerhouse terminate it outside the building or use a compression gland on the consumer unit end and install an earth rod connected to the consumer unit earth bar

Oh dear.....here we go again...

"Extend"...? Or "Export"...? ?