Connection of outbuilding

[phoyland]

In my new workshop (converted shed) I have a small consumer unit with a 40A RCD, a 16A MCB (connected to my sockets) and a 6A MCB (Connected to my lights).

click me

In my garage I have a consumer unit. At one end I have the main breaker followed by other MCBs for the cooker circuit, lights etc. At the other end is an RCD with two MCB for the sockets.

Do I connect the cable from the workshop to a new MCB or straight to the main breaker. If I connect it to another MCB do I put that in the same lot as the other MCBs, the RCD protected MCBs or on its own connection from the main breaker?

cheers

Paul

 

[plugwash]

you need a rcd in the system and for reasons of nusence tripping it is generally inadviseable to use your main house rcd

i notice you already have a rcd in the workshop CU so this is fine

also when terminateing armoured cable (with proper glands of course) its best to terminate to a metal box and most domestic CUs are plastic

i would put a 40A breaker in non-rcd side of the main CU and wire from it in 6mm twin and earth
from here i would go to a 45A metalclad isolator (http://www.mkelectric.co.uk/products/item.asp?itemid=6182&rangeid=1032)
from here i would run 6mm steel wire armoured out to the workshop cu

if the workshop CU is metalclad you can terminate the armoured cable directly to it otherwise i would use a metal adaptable and a short length of 6mm twin and earth

 

[breezer]

what cable did you use to connect to the "shed"

 

[ban-all-sheds]

You must not under any circumstances connect it to the main breaker.


What size cable feeds the shed CU? - This will determine the size of MCB you use.


As for RCD/Non-RCD, the shed CU provides RCD protection for the sockets etc in there, but that leaves the supply cable unprotected. There are differing opinions on whether you need to bother with that.

I think you should, but there is a risk that a fault with something powered via the shed will trip the main house RCD, not the one in the shed, which could be annoying. You could get round that by using an RCBO on the non-RCD side of the main CU, but that is a more expensive option.

 

[plugwash]

the whole point of armoured cable is it can be buried safely

once you hit the armour your spade will be earthed
if you go through the armour and hit live you will have a dead short which will take out the mcb almost instantly

service cables have much higher current ratings and are never rcd protected!

 

[phoyland]

I used amoured cable with glands into a connection box and then into the CU.

 

[breezer]

I used amoured cable with glands into a connection box and then into the CU.

but what SIZE cable?

 

[phoyland]

...sorry, its here

paul

 

[ZenStalinist]

That cable's only good for 24 amps. It'd be better if you run it from a circuit on your main CU, with a 20 amp MCB on RCD side, or 20 amp RCBO.

Then take it to the outbuilding, and terminate it at a metal double pole 20 amp switch (isolation is required locally in outbuilding). Run a 2.5mm cable from there. Run your sockets off it radially and any lights off an FCU with a three amp fuse.

You might want to use a separate RCD configuration, but I don't see much reason for the extra effort and expense. (let's face it, how often does it trip anyway?)
Never caused any problems to us, and you then have your armoured cable protected by 30ma rather than 100ma RCD.

 

[plugwash]

since he has already got the CU by the sounds of things he may as well use it for isolation and breakering in the shed

and since there is already a rcd in that CU he is imo better putting it on the non-rcd side of his main CU

 

[ZenStalinist]

No... That wouldn't protect the cable... The cable should also be RCD-protected.

 

[plugwash]

that isn't a problem provided the r1+r2 figures are ok

the whole point of earthed armour is you hit it first giveing your tool a good earth and taking out the mcb quickly

 

[phoyland]

that isn't a problem provided the r1+r2 figures are ok

the whole point of earthed armour is you hit it first giveing your tool a good earth and taking out the mcb quickly

What does that mean?

 

[ban-all-sheds]

What does what mean?

 

[BR]

that isn't a problem provided the r1+r2 figures are ok

the whole point of earthed armour is you hit it first giving your tool a good earth and taking out the mcb quickly

What does that mean?

ignore it, nonsense.

It is not essential that the cable is RCD protected and to be honest this is not a major issue in a domestic installation. The ideal solution to your problems however would be the following.

A 32A 100Ma RCBO (MCB / RCD combined) fitted to the non - RCD protected side of your CU. (this is probably where the lights are connected). This means you have an RCD protected cable and the tripping current of 100Ma should prevent both RCD's tripping should a fault occur in your garage.