Garage

[JKMRK1]

I have a 40amp typeB MCB (RCD protected) supply to an IP66 junction box in my garden which the runs in 4mm swa cable under my garden to the garage into a 13amp socket.

I want to fit a new consumer unit I'm my garage to run a lighting / ring main socket circuit. Question is can I just use the 32amp/6mp MCB supplied for this and do away with the 40amp 30mA RCD as it's RCD protected from the supply consumer unit in the house?

 

[telectrix]

does the cable to the IP66 box require RCD protection?if not, change the RCD to a MCB (40A is wrong, you need max. 32A) and use the RCD in the garage board.in the garage don'tinstall a ring, install a radial with a 20A or 16A MCB.

 

[westward10]

What is your earthing system.

 

[JKMRK1]

The existing supply to the IP66 junction box is a 40amp MCB supply, which has seperate RCD protection along with all other circuits on the main CU. Reason for leaving it as a 40amp supply and not changing it to 32amp is the RCD in the garage is 40amp? Surely having a garage 40amp RCD (Main) off a 32amp MCB supply is wrong? It will have the potential to draw more than the designated Supply MCB?

Also any reason why you recommend a radial over Ring?
[automerge]1580757962[/automerge]

What is your earthing system.

TT earth system..

 

[westward10]

Well you almost certainly cannot remove the existing rcd protection.

 

[JKMRK1]

Well you almost certainly cannot remove the existing rcd protection.

I never Intended to remove it (Main CU RCD). Just wanted to know could I do away with the RCD at the garage consumer unit if the supply circuit from the house is already RCD protected?

 

[telectrix]

The existing supply to the IP66 junction box is a 40amp MCB supply, which has seperate RCD protection along with all other circuits on the main CU. Reason for leaving it as a 40amp supply and not changing it to 32amp is the RCD in the garage is 40amp? Surely having a garage 40amp RCD (Main) off a 32amp MCB supply is wrong? It will have the potential to draw more than the designated Supply MCB?

Also any reason why you recommend a radial over Ring?
[automerge]1580757962[/automerge]

TT earth system..

the RCD in the garage is therefore redundant. you need to replace with a 2 pole main switch like the one in the house CU. it's current rating (A) does not matter as long as it's over 32A. it's not a protective device. a 100A one will do just as well. it's just the max. current it's rated for.

 

[JKMRK1]

So basically am external form of isolation for the garage?

 

[telectrix]

So basically am external form of isolation for the garage?

yes. it's just an isolator.

 

[westward10]

I never Intended to remove it (Main CU RCD). Just wanted to know could I do away with the RCD at the garage consumer unit if the supply circuit from the house is already RCD protected?

What is the rating of the existing rcd, 30ma?

 

[JKMRK1]

What is the rating of the existing rcd, 30ma?

Yes main RCD from house CU is 80amp 30mA.
[automerge]1580760156[/automerge]

yes. it's just an isolator.

Would you say this is Necessary? Surely supply MCB from the house is doing the same thing when isolated?

 

[westward10]

You need a two way consumer unit with a non-rcd main switch, two mcbs at 20A and 6A. The existing 40A needs reducing to 32A. You won't achieve selectivity between devices but that isn't the end of the world.
Oh yes this also needs notification to building control.

 

[radiohead]

A ring in a garage is generally over engineering things, a radial on a 16/20 is usually more than adequate for perhaps a freezer and a few power tools.
Personally I wouldnt bother with a CU, change the MCB at source to a 32a, extend the supply in the garage to however many sockets you want in 4.0mm, and use an FCU to feed the lights. You are then simply altering an existing radial, not installing a CU, no BC notification required.

 

[JKMRK1]

You need a two way consumer unit with a non-rcd main switch, two mcbs at 20A and 6A. The existing 40A needs reducing to 32A. You won't achieve selectivity between devices but that isn't the end of the world.
Oh yes this also needs notification to building control.

When you say selectivity between devices can you explain more please?

Yes of course.

 

[westward10]

When you say selectivity between devices can you explain more please?

Yes of course.

There would be no guarantee a fault on a garage circuit would not trip the device at source first. It is no big deal in your situation.

 

[radiohead]

When you say selectivity between devices can you explain more please?

Yes of course.

Best design practice is for only the device immediately upstream of a fault to trip, in the case of MCB's it's likely that a fault of negligible impedance at the load end will take out all upstream MCB's. Better practice is to have fuses at the supply end and MCB's at the load end