What size cable for a 32A radial ?

[SUNRAY]

Rental house with 3 socket circuits described as radials, one has 3 cables into the RCBO - 4mm² & additionally 2x2.5mm² believed [but not yet verified] to be supplying a DSSO each in insulated PB walls.
We have a difference of opinion on whether these should be 4mm² despite accepting 2.5mm² is acceptable for a ring spur.

Property is 9 years old and believed to be electrically untouched other than replacing damaged accessories.

 

[davelx]

Aren't those just unfused spurs off a radial per fig15B on Pg 506, but originating at the CU?

 

[SUNRAY]

Aren't those just unfused spurs off a radial per fig15B on Pg 506, but originating at the CU?

We don't have any books here, this job was sprung on us quickly!, thanks for the reference.

 

[EFLImpudence]

If there is only one (double) socket then the cable is protected by the plug fuses.

 

[JohnW2]

Rental house with 3 socket circuits described as radials, one has 3 cables into the RCBO - 4mm² & additionally 2x2.5mm² believed [but not yet verified] to be supplying a DSSO each in insulated PB walls. We have a difference of opinion on whether these should be 4mm² despite accepting 2.5mm² is acceptable for a ring spur.

Unless the installation method were 'extreme', 4mm² is obviously appropriate for the 32A radial.

There is never a problem having just one double (or single) socket supplied by a 2.5mm² cable, whether it arises from the MCB or (as a sort-of 'spur') from some point on a 4mm² (or 2.5mm²) radial (or a 2.5mm² ring final). Even Fig 15b of Appendix 15 of BS7671 illustrates such a situation.

Kind Regards, John

 

[SUNRAY]

If there is only one (double) socket then the cable is protected by the plug fuses.

Unless the installation method were 'extreme', 4mm² is obviously appropriate for the 32A radial.

There is never a problem having just one double (or single) socket supplied by a 2.5mm² cable, whether it arises from the MCB or (as a sort-of 'spur') from some point on a 4mm² (or 2.5mm²) radial (or a 2.5mm² ring final). Even Fig 15b of Appendix 15 of BS7671 illustrates such a situation.

Kind Regards, John

Personally I agree with these and as I mentioned there was no dispute about 2.5mm² spurs on RFC's.
However our discussion centered around a radial with one cable and one DSSO where the correct MCB size would be lower than 32A, we also need to double check the derating for insulated stud walls- google comes up with mixed views.

 

[EFLImpudence]

However our discussion centered around a radial with one cable and one DSSO where the correct MCB size would be lower than 32A,

Still protected by the plug fuses - but not sure why anyone would do it on purpose.

 

[SUNRAY]

Still protected by the plug fuses - but not sure why anyone would do it on purpose.

Again I agree with this but it still doesn't protect the cable per se.

 

[JohnW2]

Personally I agree with these and as I mentioned there was no dispute about 2.5mm² spurs on RFC's. However our discussion centered around a radial with one cable and one DSSO where the correct MCB size would be lower than 32A ...

Not really. To have one double socket supplied by 2.5mm² (Method C) cable protected by a 63A MCB would still be compliant with regs. The fact that there is also a 4mm² radial fed from the same MCB is irrelevant.

... we also need to double check the derating for insulated stud walls- google comes up with mixed views.

That's different. If the cables's CCC, taking the installation method into account, is not at least 26A (some might say 20A!), it is not suitable for feeding a double socket in any situation (i.e. regardlss of where it originates and what MCB is protecring it).

Kind Regards, John

 

[JohnW2]

Again I agree with this but it still doesn't protect the cable per se.

Downstream over-current protection of a cable (i.e. by the plug fuses, in this case) IS permitted by regs, provided only that there is adequate fault protection (which there will be).

Kind Regards, John

 

[EFLImpudence]

Again I agree with this but it still doesn't protect the cable per se.

You can't agree with it then say it doesn't.

Why not?

 

[SUNRAY]

Downstream over-current protection of a cable (i.e. by the plug fuses, in this case) IS permitted by regs, provided only that there is adequate fault protection (which there will be).

Kind Regards, John

Yes that's correct but I would not want to have a 2.5mm² cable meandering around a property for 25m [calcs not done so could easily be less than this permitted] protected by a 32A MCB - often seen but I don't like it.

 

[JohnW2]

Still protected by the plug fuses - but not sure why anyone would do it on purpose.

I don't see why one would never want "on purpose" to supply one socket with 2.5mm² cable from the origin of a 4mm² radial - e.g. as a matter of convenience if one wanted to add a socket close to the CU when there was already a 4mm² radial in place - after all, you presumably would not bat an eyelid if the socket were fed, using 2.5mm² cable, as a 'branch' from the first socket on the radial?

Kind Regards, John

 

[JohnW2]

Yes that's correct but I would not want to have a 2.5mm² cable meandering around a property for 25m [calcs not done so could easily be less than this permitted] protected by a 32A MCB - often seen but I don't like it.

In what sort of situation could you envisage an overload current (rather than a 'fault current') >32A arising anywhere along that 25m of cable. As I said, there is a requirement for adequate upstream fault protection, but that you will have.

Kind Regards, John

 

[EFLImpudence]

I don't see why one would never want "on purpose" to supply one socket with 2.5mm² cable from the origin of a 4mm² radial - e.g. as a matter of convenience if one wanted to add a socket close to the CU when there was already a 4mm² radial in place - after all, you presumably would not bat an eyelid if the socket were fed, using 2.5mm² cable, as a 'branch' from the first socket on the radial?

You are of course correct, but just like a spur you mention on a 63A MCB, I wouldn't do that on purpose in a customer's house.