Grey area in regs ??

[glorysparks]

Hi All,

What's your thoughts on this. If you have a 2.5mm radial bunched in with a ring main on a 32a breaker then that's a C2 but you can spur off a ring main out in the field no problem...

What's the difference ?

 

[littlespark]

A radial circuit usually means more than one point.... whereas a spur is just one point.

The spur can come off a ring at any point, including the OCPD, in your case the 32A.

A radial, in 2.5, would have to come off a 16 or 20A.... serving more than one point.

A spur from the origin has never been a C2, but a radial circuit would be, when serving more than one point.

 

[glorysparks]

A radial circuit usually means more than one point.... whereas a spur is just one point.

The spur can come off a ring at any point, including the OCPD, in your case the 32A.

A radial, in 2.5, would have to come off a 16 or 20A.... serving more than one point.

A spur from the origin has never been a C2, but a radial circuit would be, when serving more than one point.

Yea i see the thinking, but 1 x local socket bunched into the 32a MCB can't go un-noticed ? Protective device is deemed over-rated surely ?

 

[Pretty Mouth]

Current carrying capacity of ring final conductors must be at least 20A.

A spur will feed no more than 1 double socket, so is not expected to draw more than 20A (BS1363 double sockets can withstand a steady 20A). It won't overload the cable.

A radial can feed infinite sockets, so it is possible to exceed the current carrying capacity of the 2.5mm cable (max. 27A method C). The cable will be protected if the breaker is rated less than the cable, but a 32A breaker isn't, so it won't.

 

[Lucien Nunes]

Exactly. The 32A MCB protects 2.5mm against short-circuit, the single point served protects it against overload. Job done.

Ah, you say, the official loading of a double socket might only be 20A but it could be loaded to 26A, even to 40A for some time before the two 13A fuses or the 32A MCB respond. Well, just like the RFC, it works fine in practice because 2.5 clipped to surface can handle 50 amps or more without failure (although it does go a bit soft!)

RFC - triumph of empirical success over theoretical doubt.