replacing garage cu

[kung]

Hi all

I am about to replace my garage cu for Niceic assessment as old one has dodgy main switch (very stiff) i have 2.5 swa feeding the garage thats under concrete and is run the 10ft from the house to the garage-in good order ! i have the new garage cu its a europa 63a rcd protected and 32a mcb and 6a mcb fed cu thats supplied from the house cu on rcd side i was going to take it off the house rcd side and put on non rcd side to remove fault discrim on 2 rcd's but was told by a spark in local wholesalers to leave it on rcd side as one should opp in the event of a fault ! whats your views ? also as radial on power was going to replace 32a mcb with 20a or convert radial to ring as this will be better as will be able to show understanding etc of r1/r2/rn as well as R1/R2 etc the garage has 2 double sockets at mo and 2 lights so was going to add another double socket or 2 and conv to ring. also the wiring is on wooden boards (secured to metal frame) and not protected apart from pvc as t & e would it be better to trunk it all and as metal frame block concrete garage i will be 10mm bonding ext con paths.

Regards

Kung.

P.S. main house supply is TN-C, BS1040 100A 80A max demand.

Any pointers would be very much appreciated.

 

[ianmacd]

What is the breaker in the house end of the line rated at?

Ian.

 

[ianmacd]

Welcome to the forum btw.

Ian.

 

[kung]

What is the breaker in the house end of the line rated at?Ian.

Hi Ian its a 40a type b mcb 6ka b/c on a 63a 30ma rcd side at the moment.

Thanks for the fast reply.

Regards

Kung.

 

[ianmacd]

Whats the maximum current carrying capacity of the cable (taking into account voltage drop too)?

Ian.

 

[kung]

Hi Ian will be back online later and will look in onsite or 17th for this info sorry cant think offhand.

Regards

Kung.

 

[ianmacd]

What I am trying to ask is if the 40A might be a bit large to protect the cable.

Ian.

 

[ianmacd]

cu on rcd side i was going to take it off the house rcd side and put on non rcd side to remove fault discrim on 2 rcd's but was told by a spark in local wholesalers to leave it on rcd side as one should opp in the event of a fault ! whats your views ?

If you are putting a CU in the garage then this is a distribution circuit (not a final) so shouldn't need an RCD so you can satisfy the RCD discrimination, as you said, by moving the MCB for the garage off the RCD.

also as radial on power was going to replace 32a mcb with 20a or convert radial to ring as this will be better as will be able to show understanding etc of r1/r2/rn as well as R1/R2 etc the garage has 2 double sockets at mo and 2 lights so was going to add another double socket or 2 and conv to ring.

A ring is useful as they like to get you to show testing r1,r2,rn along with R1+R2. Think they get their kicks from it (only joking). Depending on the MCB at the house end, you may want to rethink that 32A (even on a ring) to allow for Discrimination of overcurrent devices too.

.

P.S. main house supply is TN-C, BS1040 100A 80A max demand.

Is that TN-C-S or TN-S as you may have issues with exporting the earth with PME as discussed in detail here before (I'm sure Apache with his super search abilities will provide links later ).

Ian.

 

[Apache]

Hello and Welcome Kung

You want expoted earth links - I can do exported earth links

http://www.talk.electricianforum.co.uk/showthread.php?t=483

http://www.talk.electricianforum.co.uk/showthread.php?t=1559

read them both carefully - there will be a test at the end

 

[M107]

The ocpd at the house end is over rated for the 2.5mm submain for starters.. Needs down rating to a 20amp.

Leave the submain on the rcd side of house cu.

Just have a main switch on garage cu...discrimination betwen rcd's wont happen if you use one at house & one at garage.

also as radial on power was going to replace 32a mcb with 20a or convert radial to ring

Wont be able to do that discrimination again with submain ocpd (that will be 20amp), you'll need a 16amp as a radial or ring.

t & e would it be better to trunk

For the cost difference I'd re-wire in singles in pvc conduit.. looks neater & added mechanical protection.

I wouldnt export the pme either, go for TT at the garage end & bond garage frame if metal.

 

[M107]

If you are putting a CU in the garage then this is a distribution circuit (not a final) so shouldn't need an RCD so you can satisfy the RCD discrimination, as you said, by moving the MCB for the garage off the RCD.

I dont have the regs to hand but doesnt the distribution circuit need to be greater than 32amp on a TN supply to do away with the rcd?

 

[ianmacd]

I dont have the regs to hand but doesnt the distribution circuit need to be greater than 32amp on a TN supply to do away with the rcd?

Nor do I but I thought that was only for final circuits not distribution circuits.

 

[M107]

Nor do I but I thought that was only for final circuits not distribution circuits.

You could well be right, I'll nip out to the van when I get a few mins & grab the book.

 

[ianmacd]

You could well be right, I'll nip out to the van when I get a few mins & grab the book.

Stop wasting time on here and get your shoes on then otherwise this will bug me all afternoon .

Ian.

 

[M107]

Right then the way I see it is no mention of an rcd for a distribution circuit.

The >32amp is for discon time of 5sec on a TN system 411.3.2.3

"in a TN system, a disconnection time not exceeding 5 s is permitted for a distribution circuit & for a circuit not covered by regulation 411.3.2.2"

411.3.2.2

"The maximum disconnection time stated in table 41.1 shall be applied for all circuits not exceeding 32A".

Having said that if the submain is buried in a wall prior to going external to the garage it would fall foul of 522.6.7 & require rcd protection..........unless the swa armour is terminated at the house cu & connected to earth.

Another thought is.....if the garage cu is metal clad & the garage end is TT'd the submain would need rcd protection, unless the tails enter the garage cu through one of them insulated supply grommetts.

Now back to my cup of tea

 

[ianmacd]

SO we are saying (assuming swa run with armour carrying earth and PME system), remove from rcd at house end (and downgrade to 20A), swap 32A in garage cu for 16A, TT garage and bond any metalwork in contact with earth, wire as ring or radial.

Have I missed anything?

Ian.

 

[kung]

Thanks for your reply guys will be on here later after work and quick check in regs & onsite etc its a pitty i cant do a mock board and wire that into the garage CU with a ring/lighting like the one i made for practicing testing etc or attend a site and do mock install inc CU like when you get the domestic installer qual save the bucks for building control !

 

[kung]

SO we are saying (assuming swa run with armour carrying earth and PME system), remove from rcd at house end (and downgrade to 20A), swap 32A in garage cu for 16A, TT garage and bond any metalwork in contact with earth, wire as ring or radial. Have I missed anything?

Ian.

Hi again Ian

Funny enough i did buy an earth rod and clamp set from screwfix few weeks ago ! should be setup for electricfix at screwfix soon save money !

 

[ianmacd]

That does also assume an insulated CU in the garage so that the earth on the swa is not in contact with any other earth from the new TT (or nylon terminators as M107 said).

Ian.

 

[M107]

SO we are saying (assuming swa run with armour carrying earth and PME system), remove from rcd at house end (and downgrade to 20A), swap 32A in garage cu for 16A, TT garage and bond any metalwork in contact with earth, wire as ring or radial. Have I missed anything?

Ian.

No I think we've covered it :^O