Table 4B5 - multicore cables

[Rockingit]

In show world we use 19c 'socapex' cable as a staple, standard cable for connecting lighting dimmers to lights via 19 pin screwed connectors and we never think about it, ever.

I've just had cause on a design to work out some grouping factors and I'm momentarily stumped. A phone call with Eland technical has also got them stumped too....

The 'clarity' of BS7671 strikes again:
Table 4B5 (pg 395) applies here, ratings for cables having more than 4 loaded cores. Of those 19 cores we get 6 x L,N,E = 18 and one pilot (sometimes used to earth the actual connector housing).
So we're defining a 'loaded' core as both L & N, gives us 12 and a derate of 0.53 (roughly 15A @2.5mm) However..... Note 3 says ignore anything known below 30% as a loaded core. In order to work out that 30% we need to have first included it with an initial factor of 0.53, but then if we exclude it later than the overall factor drops (say for arguments sake we go from six loaded pairs to three, it goes up to 0.67)....and we go around again. Also, (and this is now being pedantic), dimmed lighting loads are a constantly changing thing anyway!

Also, note 3 says only allow the 30% once for grouping but isn't clear on whether that's before or after you've derated each cable as above, if you have multiple cables as we invariably do.

AND..... if our loaded pairs drop to three then we're in no-mans land as 4D2 only goes up to 4c cable!

Be interested to get people's thoughts on this.

 

[James]

I would decide if it is a loaded core when there are no derating factors involved.
I.e. above 30% of its maximum current capacity.
from there i would then use the tables to work out what the maximum current that can be drawn whilst the cable is in use in its intended environment.
a quick check to ensure there is no overload based on the maximum capacity after applying derating factors and its good to go.??

 

[Rockingit]

I would decide if it is a loaded core when there are no derating factors involved.
I.e. above 30% of its maximum current capacity.
from there i would then use the tables to work out what the maximum current that can be drawn whilst the cable is in use in its intended environment.
a quick check to ensure there is no overload based on the maximum capacity after applying derating factors and its good to go.??

We use a rule of thumb that says per pair 1.5mm = 1kw max and 2.5mm = 2kw max, but there's no actual 'science' behind that!

 

[James]

things like that tend to err on the side of caution, if you do the calcs for most i bet you will find that 90% of the 2.5mm circuits could be done in 1.5.

reminds me of the days when over engineering something was seen as a really good job that will last for a long time,
now the thought process would be that there is too much work and material put into something that is not intended to last that long.