Help in choosing mcb/rcd for power to garage. Ask questions and find answers to many subjects relating to electrics and electrical work

[remelad]

Hi am at college and have been studying part p. Have been offered my 1st job, to run power to a garage. S.W.A having been supplied by customer is 18 mts long 3 core 2.5. Im intending to run a further 10 mts in 2.5 t&e inside the main house to the consumer unit connecting to a 20 amp m.c.b. Inside the garage i intend to use a consumer unit with a 30 amp 30m/a rcd with a 16amp/6amp m.c.b,s is there any problems with this as the rcd inthe garage is 10amp higher than the 20amp mcb in the main consumer unit. The power to be used in the garage is minimal 150w for door opener 500w for security light and the use of a lawn mower. Many thanx in advance for any help

[sparx]

Hi remelad, you obviously know you can't do this work legally! untill you have registered with conforming body, NAPIT/ECA/NICEIC.
the rating of the RCD is not an issue as long as it's capable of carrying/switching more than the upstream protective device,in your case 20A, regards SPARX

[remelad]

Hi Sparx thanks for your reply i thought that was the case with the RCD, it was just the lecturer said that the mcb at the main c/u would trip 1st. As the 2 mcb,s are lower rated in the second c/u i couldn't see where he was coming from. Yes i know about registration. i'm planning to go wit NIC EIC and am going to use these works for one of my assessments,also i will be informing the building control. Once again many thanks

[sparx]

Hi Remelad your on the ball, very best of luck with course & assessement later on, so many people who ought to know better think that an RCD will also act as an MCB which you obviously didn't fall for!
Of course your 6/16A garage mcb's will go before the supply end 20A one!!!
Confusion arises with 2 RCD's in line as supply end (up stream ) one will usually trip before load end (down stream ) device because both will 'see' leakage in final circuits the same but the supply one will also 'see' any additional leakage in the sub main supply between them!
Keep up the good work, & question everything!!!!
Regards SPARX