Cable Selection for Ring Main

[rjb]

Can somebody please explain. In the 16th Edition reg a 2.5mm cable current rating when clipped is 27A for 2 core cable (Table 4D2A).

Why is it that the ring main circuit is protected by a 32A Type B MCB.

Does this mean you could have the situation where the cable is running with an overcurrent of say 30A of load and the breaker wouldn't trip this overcurrent.

Clearly I am confused! Where am i going wrong with this calculation?

 

[Spark123]

A ring final circuit has 2 legs hence current can flow around both ways. The table you need to look at for flat twin and earth is table 4D5A

 

[rjb]

So in reality am I right in saying the cable is bigger than 2.5mm as it is feeding in both directions.

Is therefore the table I mention earlier for radial circuits only?

 

[Spark123]

The table you mention is not specifically for flat twin and earth.
Take a look at regulation 433-02-04. The Iz of the cable must not be less than 20A.

 

[rjb]

Spark123

I thought I was just getting it but now I am not so sure! Table 4D5A still states that 2.5mm clipped is 27A. I understand about the ring being fed from both direction, but still how does 32A protect a wire that has a 27A maximum current capacity?

 

[Spark123]

The current can travel both ways around a ring hence for overcurrent protection the ring cable only needs to be rated for 20A for each leg (subject to reg 433-02-04). For spurs etc the cable is protected from overcurrent by the plugtop fuse downstream.
For fault current protection the whole of the ring (including the CPC) should meet the requirement of the adiabatic equation S=sqrt I²t/K.