MCB sizing

[Jako13!]

Is there any issues in having 10amp breaker if the circuit is pulling just under 6 amps? I know the breakers there to protect the cable and the cable is 2.5 3 core armour. Just curious.

 

[Jako13!]

a) Where the circuit load is not limited to below the cable CCC by design, i.e. the user could plug in or dial up more load on the equipment until the cable is overloaded.

b) Where equipment manufacturers specify it. Or rather, they will sometimes specify a maximum MCB or fuse size, not necessarily as overload protection but as short-circuit protection for the wiring and components inside the equipment. E.g. you might choose a 32A MCB to protect a 6mm² cooker circuit, where the oven is connected by 2.5mm² flex, and so long as the adiabatic is satisfied the flex will be protected against short-circuit. The flex won't handle 32A but the oven can't draw 32A for long enough to damage it, even in the event of an element failure. But the oven instructions might well specify a maximum 20A MCB because the oven internal wiring splits into multiple 1.0mm² cables, and for those the 32A MCB does not offer adequate short-circuit protection.

IMO the extractor fan 3A fuse business is just daft, though!



What governs the level of fault current available to trip the MCB is not so much the normal load rating of the 'stuff' but the resistance of the wiring supplying it. It takes a higher fault current to trip the larger MCB, which puts a lower ceiling on the loop impedance, and might require heavier cables. A long lighting circuit might have to be wired in 2.5mm² simply to enable a 16A MCB to trip fast enough, which is pointless if the total load is less than 6A.

So my understandings is it’s more about fault current to trip the breaker under fault conditions the higher rated breakers will need a lower impedance to satisfy the required disconnection time. The breakers primary function is to protect the cable from over heating so as long as the cable is rated higher then the breaker. As for (a) what are examples of overload situations?

 

[pc1966]

As for (a) what are examples of overload situations?

Simple case is someone putting on too many heater (or kitchen appliances of comparable draw) on their 13A sockets.

A typical RFC design is for 32A max, which is about 7.4kW, or you might have a 20A radial for a couple of sockets, which is around 4.6kW, so if you put on 3 * 3kW you are over the design limit (but usually they are not all on simultaneously for any significant time).

 

[Lucien Nunes]

Example: AV equipment rack with a PDU (power distribution unit) i.e. a box with lots of sockets mounted inside the rack to feed Blu-ray player, amplifier, matrix switcher, audio processor etc. If the total equipment load is 8A you might be tempted to run a 2.5mm² circuit for the rack, but protect it with a B32 MCB to avoid nuisance tripping from inrush. However there is nothing to stop an event caterer who has run out of nearby wall sockets plugging their hotplates and tea urns into the rack PDU sockets instead and overloading the cable. In this case there was a design current (8A) within the rating of the circuit, but the nature of the equipment does not absolutely guarantee that the user won't exceed it.

 

[Jako13!]

I can see from that example 2.5 on a 32 amp breaker can be overloaded but say if it’s a 16 amp radial on 4mm isn’t it hard to overload a circuit like that as the cables well over rated for a 16 amp breaker. Even with the RFC the MCB should break the circuit before it can be overloaded am I correct? Thanks for your help guys

 

[Lucien Nunes]

I was giving an example in answer to your question of a load that could exceed its design rating, therefore not a fixed load and requiring overload protection by the MCB as well as fault protection for the cable. Had the PDU been internally fused at 16A total, it would not have been able to overload the circuit.

4mm^2 on a 16A MCB will be protected under almost all conditions, but the point was whether it needed to be proetcted against overload or just faults.