Garage Supply - 2 way metal consumer unit in a seperate garage

[Dunc88]

Hi,
I'm installing a 2 way metal consumer unit in a seperate garage, the earthing arrangement in the property is TNCS. I am planning to install a new 4mm2 ciruit from the main consumer unit in the house, in 4mm2 swa to a garage consumer unit (5M run of cable).
There are no extraneous-conductive parts in the garage, so can I gland this swa directly into the garage consumer unit and have an RCD main switch feeding 1x 16a & 1x6a mcb? If so, how do I prevent both RCD's tripping (garage & main consumer unit)?
Would RCBO's still trip the RCD back in the main consumer unit?
Could I turn the garage into its own TT system? I could gland the swa into an adaptable box next to the garage consumer unit, & join the earth from the 4mm core to the swa inside that box, carrying the Live & Neutral through into the garage consumer unit, from there I would take a seperate earth down to an earth spike.
Thoughts/advice please!
Cheers

 

[telectrix]

as long as the feed cable from the house CU does not require RCD protection ( i.e. SWA and/or internal feed in house not subject to 522.6.101 then MCB the feed at the house CU. I'd up the cable to 3 core 6mm on a 40A MCB ... 1. to get a bit of discrimination. 2. to allow for increase in load in the future. then tere's only 1 RCD to worry about. and exten the TNC-S to the garage.

 

[Tink]

No need for RCD in series.
Use a 40A RCBO on the non-RCD side at the house CU.

 

[westward10]

Assuming the board has non rcd ways.

 

[Tink]

Ya absolutely right – I’ll hoping to shift alone the protective devices make room

Assuming the board has non rcd ways.

Ya absolutely right – I would be hoping to shift alone the protective devices make room to insert the RCBO

 

[westward10]

Why make room for an rcbo.

 

[Midwest]

No need for RCD in series.
Use a 40A RCBO on the non-RCD side at the house CU.

That's confused my small brain

PS OP there's a handy download on the home page halfway down on the right.

 

[DPG]

No need for RCD in series.
Use a 40A RCBO on the non-RCD side at the house CU.

Hopefully this is a typo.

 

[Dunc88]

Assuming the board has non rcd ways.

Thanks for the feedback all! Unfortunately not, no space for an rcbo! Its fed via an mcb on a split load board. It also starts as 4mm twin & earth before entering a weatherproof box (where i'm glanding the swa in), so from there could I not just enter the garage consumer unit and gland the cable straight in? Then just have a main switch as the circuit from the main consumer unit is protected by the rcd?

 

[Dunc88]

as long as the feed cable from the house CU does not require RCD protection ( i.e. SWA and/or internal feed in house not subject to 522.6.101 then MCB the feed at the house CU. I'd up the cable to 3 core 6mm on a 40A MCB ... 1. to get a bit of discrimination. 2. to allow for increase in load in the future. then tere's only 1 RCD to worry about. and exten the TNC-S to the garage.

I like this idea, would mean altering the work i've already done slightly, id have to remove the small peice of 4mm twin & earth, & raise the height of the adaptable box on the external wall so i can just run the cable straight into the back of the consumer unit once its glanded in... would mean the main consumer units rcd would trip but at least theres no RCD's in series?

 

[DPG]

Why do you need an RCBO for the swa?
What do you mean by 'have a main switch'?

 

[DPG]

I like this idea, would mean altering the work i've already done slightly, id have to remove the small peice of 4mm twin & earth, & raise the height of the adaptable box on the external wall so i can just run the cable straight into the back of the consumer unit once its glanded in... would mean the main consumer units rcd would trip but at least theres no RCD's in series?

Why would the main rcd in the house trip? The rcd protection would be in the garage. I think I'm missing something here.

 

[Des 56]

Yes gland it directly into the garage unit

 

[Des 56]

He has a Rcd protected supply to the garage
He can have a isolator as the main switch in the garage
Pointless TT ing and Rdc ing the garage end if its fed from a household Rcd,it would be a ---- up which pops first

Is ---- as in coin a swear word

 

[DPG]

But as per telectrix why doesn't he feed from an mcb in the house - he's using armoured so no rcd protection needed.

Yes fully agree with not tt'ing.

 

[Des 56]

Well Dunc,It seems to be going round in circles in this thread
The 64K dollar question,can you feed it from a non Rcd protected mcb or no

Add
How has the twin and earth feed to the adaptable box been wired

 

[Dunc88]

But as per telectrix why doesn't he feed from an mcb in the house - he's using armoured so no rcd protection needed.

Yes fully agree with not tt'ing.

I've got 14 ways at the main consumer unit, all of which are protected by the 2x rcds (7 & 7), so the 4mm comes from one of those mcbs, I was just hoping to avoid it tripping at the house (& knocking off other circuits) but because there is no space for an rcbo/none rcd protected mcb it looks like i'm going to have to just use a main switch & mcb's in the garage and deal with it if a fault does ever occur. To be honest i'm only having a few twin sockets & a couple of lights, just being over cautious!

 

[Dunc88]

Well Dunc,It seems to be going round in circles in this thread
The 64K dollar question,can you feed it from a non Rcd protected mcb or no

Add
How has the twin and earth feed to the adaptable box been wired

Tricky one isnt it!
I took a 4mm t&e from the mcb and buried it in the wall before drilling it and feeding it directly into the back of the adaptable box on the external wall...

 

[Des 56]

Seems to me,like you stated, just fit main switch cons unit in the garage off a Rcd protected Mcb and be done with it

 

[DPG]

Either that or some Henleys.

 

[Dunc88]

Seems to me,like you stated, just fit main switch cons unit in the garage off a Rcd protected Mcb and be done with it

Yeah, that will do the trick! Thanks all

 

[Nimmasaurus]

time delayed rcd in the house protecting the garage circuit, that way the rcd in the garage consumer unit will trip first.

 

[Midwest]

Somewhat complicated solution, for an event unlikely to happen?

 

[Octopus]

time delayed rcd in the house protecting the garage circuit, that way the rcd in the garage consumer unit will trip first.

The op has already said the cu is a dual RCD unit with no spare ways - how is adding a time delay going to improve the situation? And why do people think by TTing a shed or garage would prevent the house cu RCD tripping?

 

[Nimmasaurus]

Replace one rcd with the delay one? OP wants to avoid the garage potentially tripping an rcd in the house, so if there's a short in the garage the garage cons unit should trip before the house one if it's a timer delay one. On a 5m run it's not that big a deal to have to flick a switch in the house c.u. We recently did a farm shed that was about 120m from the main supply and that's what we did - time delayed rcd in house c.u. and normal rcd in the shed. 16mm cable to run the power to the shed because of the length of run with voltage drop.

 

[Anthony_wayne]

Shame it's not a high integrity dual rcd board, I just installed a similar set-up, but used an mcb to protect the 4mm swa for the garage which had a 63A RCD main switch with 2 mcbs in the unit.
Never been a fan of large consumer units with no none RCD ways.

 

[Toneyz]

Replace one rcd with the delay one? OP wants to avoid the garage potentially tripping an rcd in the house, so if there's a short in the garage the garage cons unit should trip before the house one if it's a timer delay one. On a 5m run it's not that big a deal to have to flick a switch in the house c.u. We recently did a farm shed that was about 120m from the main supply and that's what we did - time delayed rcd in house c.u. and normal rcd in the shed. 16mm cable to run the power to the shed because of the length of run with voltage drop.

Because it would make the other existing circuits non compliant

 

[Dave OCD]

I ordered a time delay RCD last week but it was delivered the same day !

 

[Had8Lives]

The only thing that's not been mentioned and I'm sure someone will correct me, I was told by an ELECSA assesor that you're not allowed to export a TNCS earth in less than 10mm cable. So, as you can't get an mcb unprotected in the house consumer unit, have a main switch in your garage consumer unit and put an earth rod in to earth your garage consumer unit.

 

[Octopus]

I'd rather export a decent earth than rely on a twig ....

Tin hat on..

 

[Had8Lives]

I'd rather export a decent earth than rely on a twig ....

Tin hat on..

Ah, but, if you happened to be touching a metal clad socket and lightning struck a power line and it burned your arm you'd wish you'd put a twig in lol

 

[Midwest]

The only thing that's not been mentioned and I'm sure someone will correct me, I was told by an ELECSA assesor that you're not allowed to export a TNCS earth in less than 10mm cable. So, as you can't get an mcb unprotected in the house consumer unit, have a main switch in your garage consumer unit and put an earth rod in to earth your garage consumer unit.

Here goes then; in a typical domestic property, the 10mm bit only relates to the minimum size cable if you have extraneous conductive parts to bond. Or run a separate 10mm protective bonding conductor to the extraneous conductive part in the garage, and run an appropriate sized swa for load/vd only. Or as you suggested turn supply into TT, bonding locally as necessary.

Don't think there is an extraneous conductive part in the situation described by the OP.