MCB question

Petrolhead · 27 Sep 2018 at 13:51

I am putting a garage consumer unit into a shed thats a small workshop

I have taken the supply from a non used 32a MCB on the main house consumer unit via armored cable to the consumer unit in the shed. I am wiring a couple of sockets in the hut as a ring main.

What size MCB should I use in the hut?

Kol · 27 Sep 2018 at 15:27

@cms239 32A is what you need, unless of course you'd rather go with a radial (depends on what you are running in the garage, any high load equipment i.e. drier?), in which case a 20A for 2.5mm, unless you are using 4mm which you'd just use a 32A. I'd stick with a ring simply to ensure you've got the capacity.

As previously suggested, ensure the circuit on the main CU is protected by an RCD, it's unlikely (unless your install is very very new and even then most new builds are done cheaply) to be populated with RCBOs.

200sols · 27 Sep 2018 at 19:03

Remember though, work like this is notifiable

ZG002 · 27 Sep 2018 at 19:56

Really don’t see the need for a ring final circuit then.

Op

32a is fine if wiring a ring circuit in 2.5mm.

Could be easier running a radial though.

mattyy · 27 Sep 2018 at 20:45

OP. Leaving aside the incoming supply side of things and PME. You do not need a garage unit and as such the circuit will become (probably) a long final circuit.
You could even fuse down with a fuse spur of the ring or radial to run lighting for the work shop.

Installing a separate garage/consumer unit is the easiest way to make sure compliance and making sure you don’t get any nuisance tripping on the house CU.

Anyway, general rule of thumb, although all influences should be taking into consideration.
2.5mm radial - 20A mcb
2.5mm ring - 32A mcb
4mm radial - 32A/20A mcb
If you are putting a garage unit in then it’ll come with more than likely a 63a RCD with a 32a and 6a MCB.
The MCB back at the house board will determine what the maximum load will be.
If it was me I’d run a 6mm SWA to a garage unit. It would be impractical to run a 2.5mm ring main like people here have been suggesting without the garage board as you are suggesting using SWA so I’m assuming the run goes outside. Also keeping in mind the maximum floor coverage of rings and radials.
All circuits need to be RCD protected etc.