Garage DB problem

r10ley · Feb 7, 2011

Hi, just quickly im a new, young but enthusiastic spark : D so some of my questions might sound obvious but bare with me : )

Im looking to install a small DB in a garage which is 4 way, a DP 40A incomer with a 16A radial mcb and a 6A lighting mcb (you know the type) anyway.....

The incomer is 40A therefore im lead to believe due to discrimination the mcb at the main Db should be 40A or bigger. My problem is this, without any correction factors, to carry atleast the 40As my breaker will be rated at I need to use 6mm cable.

However the load of my small DB is only 16A plus (using diversity) 66% of 6A being roughly, only 20A. Now if my breaker at the main board were a 20A or 32A I would beable to use a 4mm or even 2.5mm cable and it would still be protected.

As I say to most experienced sparks the answer is probably straight forward but any help is appreciated.

Thanks

r10ley

telectrix · Feb 7, 2011

first. are you coming from a RCD protected way in the house. when you say 40A incomer, do you mean the rating of your main switch? i can't see any problem with supplying the garage with 4mm via a 32A MCB.

r10ley · Feb 7, 2011

Yes the new mcb will be RCD protected at the main mcb. Yeah I meant the mian switch, just seems weird to have a main switch that wouldnt in relaity ever pop : ? The main switch is also RCD protected would this be a problem having two RCDs on the same circuit?

Thanks for your help

r10ley

Johnny-G · Feb 7, 2011

The 40a incomer is just what it's rated at, it's not 'fused' at 40a.

r10ley · Feb 7, 2011

Oh see thats what you pros are here for to steer us young bucks in the right direction : D will the two RCDs on one circuit matter?

Thanks!

telectrix · Feb 7, 2011

you don't want 2 rcds. leave the garage one out for the simplest way. just means that if there,s a fault on the garage, you'll have to go and resert at the house. the 40Aswitch is a switch for isolation purposes, it does not trip, 40A is the max. rated load for the switch.