opinions, please?

[RMS]

Hi all,

Came across this setup today,

20amp MCB supplying a 10mm twin and earth cable feeding a double socket and switchfuse spur for an unswitched single socket. The 10mm cable entered the double socket backbox and was joined to 30amp connectors as the conductors were too short to terminate in the socket. Out of the connectors was 6mm that terminated into the socket with a 2.5mm cable feeding the spur.

any thoughts and opinions on this in regards to BS7671 greatly appreciated.

Thanks

 

[industryspark]

20amp MCB supplying a 10mm twin and earth cable feeding a double socket and switchfuse spur for an unswitched single socket. The 10mm cable entered the double socket backbox and was joined to 30amp connectors as the conductors were too short to terminate in the socket. Out of the connectors was 6mm that terminated into the socket with a 2.5mm cable feeding the spur.

thats what you call DIY!

suppose the mcb is protecting all cables although its very very poor worksmanship.

bet the 10mm wa a b*st*d to terminate!

if it wa me id rip it out and do it properly.

 

[Spark123]

The cables are a bit on the big size, ex cooker socket? As its on a 20A MCB then I would say A3 radial, as the cable is 2.5mm or larger it is fine subject to the normal criterion.

 

[plugwash]

it certainly sounds like an ex cooker circuit adapted to a radial (my guess is the double socket replaced the CCU and the FCU was installed alongside) and as long as the terminations are sound i can't see how it would fail to comply (unless cables are excessively long or something)

i'd probablly change the link to the socket from the terminal block to 2.5mm to avoid mixing cable sizes in one terminal (which can sometimes make them a bit loose) and to make the backbox less cramped.

 

[RMS]

Yes i think it was an existing cooker point,

i know this is not a very well designed circuit but the mcb does protect the cable. Am i right in thinking that when designing a circuit you can increase the size of the cable for part of the length normally for parts of the run that are affected by thermal effects, grouping etc, providing the protective device protects the smallest cable and voltdrop and disconnection times are adequate?

obviously this circuit has not been designed that way and was most likely for convenience purposes only.

 

[Spark123]

Yes, you've hit the nail on the head. You sometimes need to increase a cable size for correction factors. Too big is OK, its when a cable is too small thats the problem.

 

[RMS]

Thanks for the clarification,

its just i was told that a switchfuse is needed whenever a cable is reduced in a circuit, although this is rare.

Probably only relevant when really big cables are used for long runs and the price of the cable becomes an issue.

 

[Spark123]

Not entirely, it is correct that a fuse needs be provided to protect the cable downstream from short-circuit current, in your case the MCB at the board is adequate to protect both the 10mm cable and the smaller cable.

Example:
Theoretically the 10mm cable may be fused up to 63A. A 63A fuse is almost certainly too much to protect a 2.5mm cable with 1.5mm cpc from short circuit current hence a smaller fuse is required where the cable size drops to 2.5mm.

 

[flashbanger13]

So, are you guys says it would be acceptible practice to...for example.

If running a 2.5mm radial, 16A MCBsupplying socket outlets. If the cable at some point went through some thermal insulation, you could increase it to 4mm just through the affected part of the run, returning to 2.5mm when in the clear?

Obviously the cable(s) would be adequately protected by the MCB, but i was told at college not to mix cable sizes on final circuits? Is this wrong?

 

[Spark123]

I don't see much of an issue with what you suggest, as long as the cable is adequately protected along its length. I'm thinking it might not be a good idea on a ring final circuit to mix cable sizes as it might cause it to imballance, but I'll wait for a 2nd opinion on this

 

[plugwash]

mixing cable sizes on a ring will certainly affect the imbalance whether it will increase it or decrease it depends where the loading is placed.

when looking at balance on a mixed ring instead of length think length/CSA

if a ring has two approximately equal length runs back to the CU at the ends then theese should ideally be the same size. If one end run is longer it will improve balance if the longer one is also thicker (provided it isn't so much thicker it tips the balance the other way).

if you use a little bit of thicker cable to run past thermal insulation it isn't going to affect the balance a great deal.

 

[Taylortwocities]

Going back to the OP. Suggest you label the CU to indicate that the contact breaker must not be changed for one of a higher rating.
That's in case someone decides to up it to 45A beacuse the cabel 'appears' to me 10mm².

Regards