20a MCB supplying a 20A radial (DB2)

[ianphilip007]

hi guys

have a problem. customer currently has a 2.5mm(SWA) RCD protected supply to an outbuilding from TNS supply. They are requesting a 2 way cu be fitted for 2 lights and 3 double sockets ( one external). The supply cable is protected with a 20A type B MCB ( which is correct due to installation method )

now the lighting circuit for outbuilding will be protected by a 6A type B MCB as usual and i would usually protect the radial sockets with a 20A type B MCB as per OSG standard circuits.

my problem lies in the fact that there is no discrimination between the 20A supply MCB and the 20A MCB protecting the radial sockets. Is this acceptable within regs or should the supply cable be increased to 4mm 0r 6mm (30/32AMCB)



many thanks any help appreciated

 

[mogga]

reduce the mcb in shed to 16A 30ma rcbo ?? or are they going to be drawing a heavy load?

 

[malcolmsanford]

There is nothing to stoping you doing a A3 radial and fiting a 16amp breaker or even a 10amp breaker if you wish, you will need to identify what the client intends to use the sockets for.

If it is just for a few power tools, perhaps a kettle or a small fan heater for the winter, then 16amps will be fine. If he intends to plug in welders, piller drills, lathes and such then he will need to redesign the installation to suit.

 

[ianphilip007]

thanks for your reply no the heaviest load will be a lawn mower no heavy loading at all. i thought about reducing the mcb for the sockets to 16A which would be fine for one socket (13A) but there are 3 double sockets and the OSG states 20A minimum. unless im mistaken.

 

[ianphilip007]

thanks malcom, so 3 double sockets which have very small loading can be protected by a 16AMCB if thats the case then theres no problem i must have misunderstood the OSG lol not the first time

 

[malcolmsanford]

Remember Ian the OSG is a GUIDE and what both Mogga and I suggest is a safer circuit than the guides A3 20amp 2.5mm as you are fitting a smaller protection device to a 2.5mm cable, so your erring more on the side of caution.

 

[mogga]

isnt it 20A max not Min??

 

[ianphilip007]

yes malcom i agree, a 16A is safer option than a 20A on a cable (2.5mm) rated at just over 20 A . i have taken the 20A as a min from the OSG rather than as a max as stated by mogga. manythanks guys least the quotes alot cheaper now,