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Multiple circuits in a single MCB/RCBO

Former Community Member
Former Community Member
Having undertaken some inspections recently I'm finding a lot of older installations with multiple circuits within a single RCBO. Example being 2 light circuits (Upstairs/downstairs) being added to a single B6 RCBO.

The circuit design covers single pendants in each room and some downlights in the kitchen. The majority of bulbs are LED so from a risk point of view would this be unlikely to exceed 6amps. From the EICR however would this be considered C3 in peoples opinions due to risk level with a potential of splitting to separate RCBO's, or should this be C2 as there are technically 2 circuits fed from a single MCB?


Likewise the same installation has a 32amp MCB (RCD protected) supplying a ring final circuit downstairs. When work has been undertaken in the past, an additional radial circuit has been added to the same MCB in 2.5mm T&E. My view on this is that this instantly requires a C2 coding, as the radial circuit feeds multiple sockets (and there is no Fused spur within the radial line) it is not rated for the potential draw and the 32amp breaker will not trip with the overload of the radial line.


There are no spare ways in the board, so am I correct in claiming this would require a new replacement board to accommodate and separate the circuits? Would this be different if the radial only served a single double socket for ease of the wiring?


There appears to be a lot of confusion on when/if its acceptable to add radials to an existing breaker and I wondered if there was any clarity, given that these circuit amendments are historic and retrospective changing to meet current regulations is not essential providing it met previous regulations.


Many thanks

David
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  • 0
    As a point of definition the presence of more than one wire in an MCB does not make it 'two' or more final circuits. Just one final circuit that is not supplied at the end, rather centre-fed  or junction-fed or whatever you want to call it.


    There is no prohibition in BS7671 for circuits that are not simple daisy chains - a branching "Christmas tree" of a layout may be a pain in the wotsit to trace out, but it is not prohibited, nor are rings with spurs at the origin, or rings on the ends of radials (the "Lassoo" or "lollipop"  may not be in the annex or the OSG, but they can be made to work safely and properly and are permitted)


    Or indeed any other permutation,  so long as you can do the design sums to show that cable ratings are not exceeded, and VD and Zs are going to be OK.


    The exam question really  becomes 'is one (centre fed) lighting radial enough for a building with XX rooms?"

    That will depend on load and inrush, and maybe in some huge mansion, voltage drops. I'd argue in many cases it may not even be a C3, and not more unless the cable is clearly cooked or there is history of nuisance tripping.


    As regards the hybrid ring -radial - if it was to one double socket, no fault, as that is a normal spur just from the MCB.  If it is 4mm cable feeding any no of sockets, no fault. If it is 2.5mm cable feeding many sockets, then locally fusing down with a 13A fused spur, or creating a 2.5mm return leg to make a mini-ring or reducing the MCB to perhaps 25 or 20A, depending on cable routing,  may all be seen as fixes that do not involve a CU change.


    Mike.


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  • 0
    As a point of definition the presence of more than one wire in an MCB does not make it 'two' or more final circuits. Just one final circuit that is not supplied at the end, rather centre-fed  or junction-fed or whatever you want to call it.


    There is no prohibition in BS7671 for circuits that are not simple daisy chains - a branching "Christmas tree" of a layout may be a pain in the wotsit to trace out, but it is not prohibited, nor are rings with spurs at the origin, or rings on the ends of radials (the "Lassoo" or "lollipop"  may not be in the annex or the OSG, but they can be made to work safely and properly and are permitted)


    Or indeed any other permutation,  so long as you can do the design sums to show that cable ratings are not exceeded, and VD and Zs are going to be OK.


    The exam question really  becomes 'is one (centre fed) lighting radial enough for a building with XX rooms?"

    That will depend on load and inrush, and maybe in some huge mansion, voltage drops. I'd argue in many cases it may not even be a C3, and not more unless the cable is clearly cooked or there is history of nuisance tripping.


    As regards the hybrid ring -radial - if it was to one double socket, no fault, as that is a normal spur just from the MCB.  If it is 4mm cable feeding any no of sockets, no fault. If it is 2.5mm cable feeding many sockets, then locally fusing down with a 13A fused spur, or creating a 2.5mm return leg to make a mini-ring or reducing the MCB to perhaps 25 or 20A, depending on cable routing,  may all be seen as fixes that do not involve a CU change.


    Mike.


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