What size cable for a 32A radial ?

[EFLImpudence]

I'm sorry, I don't know where I went wrong.

7671 says 60A wire is 1,33mm diameter which of course is 1.39 mm².


However, what happened is as I stated.

 

[SUNRAY]

I lied, I have 16th edition:

1.53mm dia is well over 1.5mm², more like 1.75 or 1.8mm² so under overload conditions I'd expect the cpc to fail before the 60A fusewire.

 

[EFLImpudence]

Oh dear, I should go to Specsavers.

You are correct.

The rest of my posts are right, though.

 

[JohnW2]

1.53mm dia is well over 1.5mm², more like 1.75 or 1.8mm² so under overload conditions I'd expect the cpc to fail before the 60A fusewire.

It's a good few decades since my "adiabatic processes" education, but I'm not at all sure that the adiabatic behaviour of many metres of a conductor of a given CSA (in a cable) would necessarily be the same as the behaviour of a centimetre or three of a conductor of the same CSA and material in a fuse.

Kind Regards, John

 

[sparkwright]

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.

So, 3 cables at the RCBO:-
1) a 4mm2 as a radial doing majority of sockets (fine if vast majority of this is all 4mm2)
2) a 2.5mm2 as a spur serving one double socket (fine, it's a spur)
3) another 2.5 mm as a spur serving another double socket (open to debate - a 32amp RING allows only one 2.5mm2 spur from any point if you read the text books, yet most here would state you can spur off a point twice... so based on all that stuff, your radial sounds ok)

Obviously you have a 32amp radial circuit, yet I believe many principles of a 32 amp ring circuit can be applied where questions arise.

 

[JohnW2]

... (open to debate - a 32amp RING allows only one 2.5mm2 spur from any point if you read the text books, yet most here would state you can spur off a point twice...

I don't know what text books you're talking about, but neither BS7671 nor 'electrical common sense' says that as a generalisation.

The only time it matters about more than one spur arising from a single point on a ring is if that point is close to one end of the ring - and essentially the same problem arises if umpteen spurs arise, each from a different socket, all those sockets being very close together and very close to one end of the ring - and, indeed, the same issue arises if one simply has lots of sockets close to one end of the ring, with no spurs.

BS7671 says that the designer should ensure that the cable of no part of the ring (i.e. the shorter leg) will be 'likely to' carry a current over its CCC, and the designer must keep in that mind when deciding whether it's OK to have several spurs originating from the same, or very close, points near the end of the ring AND when deciding whether it's OK to have several sockets very close to the end of the ring (the latter being something which some electricians seem to forget).

Of course, in the context of this thread, this is all totally irrelevant. If one could fit them into the MCB/RCBO terminal, there would be absolutely nothing wrong in having 20 spurs originating from that MCB/RCBO, since none of the current from any of those sours would flow through any part of the ring cable!

Kind Regards, John

 

[SUNRAY]

It's a good few decades since my "adiabatic processes" education, but I'm not at all sure that the adiabatic behaviour of many metres of a conductor of a given CSA (in a cable) would necessarily be the same as the behaviour of a centimetre or three of a conductor of the same CSA and material in a fuse.

Kind Regards, John

This was a response to a fusewire of >1.75mm² and a cpc of 1mm², and which will rupture first.

 

[JohnW2]

This was a response to a fusewire of >1.75mm² and a cpc of 1mm², and which will rupture first.

Exactly - which is why I said that I wasn't sure (couldn't remember) whether, under adiabatic conditions, the 'rupturing behaviour' of a long length of a conductor of a particular CSA (e.g. a cable's CPC) would necessarily be the same as that of a much shorter length of the same conductor (i.e. a fuse) - e.g. whether it would necessarily be the case that a long length of 1mm² would be expected to rupture (somewhere) before a very short length of >1.75mm² would. I can't really think why the two situations should be appreciably different but, as I said, I'm not sure.

Kind Regards, John