House to Garage Supply question(s)

[ianbrookes]

I have installed a new CCU in my garage. The circuit is as follows, and my question is are there any issues with the 25amp RCD being used in this way. Thanks.

Distribution board in house: 63amp RCD, 16amp MCB supplies through 3 meters of 2.5 T&E to a box inside which connects to 10mm SWA (all terminated correctly etc.). SWA runs 10 meters, into the garage to another box which again connects to 2.5mm T&E which goes to the new CCU.

The new CCU has a 25amp RCD and two MCB's:
1 - 32amp for radial power (couple sockets, security light) - circuit uses 4mm T&E
1 - 6amp for lighting - two lots of 2 100w lamps in series.

so recap: house: 63amp RCD - 16amp MCB to supply garage, 25amp RCD (main switch) in garage with - two MCB's, 1*32amp, 1*6amp.

should the garage supply have a larger rated RCD, e.g. 40amps?

thanks again.

 

[Sintra]

RCD's are rated in mA so both your RCD's are more than likely 30mA. The figure you have quoted is the max current that can be drawn through it.

If the circuit is protected by an RCD at the house then there is no need to have it protected by another RCD in the garage.

 

[pennywise]

Personally I would be putting the supply from the house CU on a 20 amp breaker, then at the garage CU have power on a 16amp breaker and the lights on a 6.

 

[Sintra]

Have to agree with that also.

 

[ianbrookes]

Thanks for the advice. My plan was to allow for future expansion, hence the 32amp and 4mm on radial for power (BS7671 corrigendum - July 2008). I see your points, and am thankful that this all looks ok. went overboard with RCD's, but the unit came as a "Builders Supply" - at a good price from TLC, with all the bits. cheers.

 

[pennywise]

If your wanting to allow for the future, I would be upgrading the 2.5 from the CU to the SWA and the Garage run of 2.5, as this will be undersized for what your planing in the future

 

[ianbrookes]

yep, thanks, spot on. I realise now! der.. that I can only push 27amps through the 2.5, before voltage drop and other effects. Ta.

 

[davelerave]

the 2.5 is too small for the sub-main basically

and the discrimination issue with the 16amp and 32amp

 

[ianbrookes]

Thanks to all. I just had the installation assessed. I was told that the RCD's in series are not allowed as per 531.2.9 of BS7671, e.g. as the first RCD is in the house and the second in the garage the first RCD could trip which could cause equipment to be isolated (life saving stuff, compueters, etc. etc..). where the first RCD may be unreachable - e.g. doors locked etc. The example was given where an installation was completed just before and then tested whilst someone was away for a few days, the first RCD happened to have a freezer on the circuit (for whatever reason), on installation and testing the first RCD was tripped during RCD testing and no access was available to reset the RCD. I am not sure how reasonable this example is because I would always expect access to the whole (necessary parts) of the entire circuit. But I guess it makes sense.

So, as people have pointed out, if the circuit is RCD protected then it IS protected.

another lesson learned.

thanks.