External electics . .

[tsyrett]

I'm installing a consumer unit in an outbuilding which amongst other things will, for a short period in the Summer, supply power to a marquee. I have been advised to have two separate armoured cables routed to the site of the marquee which will supply lighting and power to various appliances. Each of these cables connected to 16A MCBs on the CU.
As some of these appliances might be freezers, etc I have since been advised that one cable should be RCD but the other, for the chillers, should not so as to avoid nuisance tripping. The makers of the CU I have used do not sell a 16A RCD, although they do have a 32A RCD. Would the latter be appropriate?
While I'm at it, is it normal to run the armoured cable direct into the CU, which can be tricky to manipulate?
rgds

 

[BR]

Well it depends on what your total load is likely to be. Personally, I would run a single length of 6mmsq 3 core Armoured cable to the external consumer unit. This should be protected by a 40A non-RCD protected MCB in the house/main building CU. This cable can be run directly into the external consumer unit and the various circuits RCD protected there. (Freezer sockets/FCU's and lights should be left out).

The only problem I see with this setup is that the cable itself is not RCD protected, but provided it is correctly installed and protected this should not present a real problem.

The rating of the RCD itself must be above the amount of current it could have handle or else it will nuisance trip. I would recommend using a 40A minimum in your case.

 

[tsyrett]

Thanks for that . . . You describe the set up I had in mind for the feed from the main building to the outbuilding (single cable 40A MCB) which will itself have lighting and sockets . . .but I was mainly asking about the protection for the cables that will run from the outbuilding to the marquee.
The reason I had in mind for the two cables from the outbuilding to the marquee was so that one could be non-rcd protected. Are you saying then that the RCD protected cable can be on a 32A/40A RCD?
thanks again
rgds

 

[BR]

There are two ways you could do this:
1. Run a second length of 6mmsq non-RCD 3 core armoured cable to the marquee and have a second CU/RCD in the marquee itself.

2. Run a single length of 2.5sq 5 core armoured cable from the outbuilding CU to the marquee.

The first option is probably the best but would be more expensive (second CU, RCD etc.) and if it’s only for a short period...........

However the second option would also be effective, You could set it up like this.

Core 1. RCD protected Neutral
Core 2. Non-RCD protected Neutral
Core 3. 10A Non-RCD protected Live (10A MCB) (Lights)
Core 4. 20A RCD protected live (Sockets)
Core 5. 20A non-RCD protected Live (freezers)

Earthing?
You have several options:
Run a single core earth wire back to outbuilding CU (Probably Best)
Use cable armour as earth. (Not recommended)
Install a separate ground rod close to the marquee. (Easily done)

Note that 2.5 sq cable is rated for 27A and therefore the max breaker you can normally use is 25A.