UK some help with grouping of cables

[Speeka]

According to note 8 of table 4c1 of bs 7671, cables of differing operating temperatures when grouped together, the current rating is to be based on the lowest temperature of the group.

could somebody elaborate on this in regards to a 90c thermosetting being grouped with a 70c cable.
do the C factors such as ambient air temperature based on the 70c column etc...

cheers.

 

[pc1966]

The point is you cannot allow a set of cables rated for 70C operation to be heated by those happy to run at 90C, so if you are looking at a section for that group, you should treat them all as if they were rated at the lowest temperature (e.g. 70C) when calculating and if you get an acceptable outcome then no problem.

The other aspect to consider is if accessories are rated for, say, 90C operation. Again that very much depends on the use-case of the cable, and if you took an example of, say, 6mm and found the 70C case such as Table 4D2A works for all segments of the route (with possibly differing "method" and grouping) you have nothing to worry about.

So if you had a 6mm cable going through a hot area, or an area of high thermal insulation (like method 103, etc) you might select 90C type of insulation, then you could do checks to see:

• Is the CCC of 90C insulation in a hot area with de-rating acceptable?
• Is the CCC of 90C traversing in, say method 103, acceptable?
• If I use the same CSA in 70C insulation, am I meeting the CCC in method B for common accessories?
• When grouped with other cables with a 70C rating, if I used 70C parameters for my cable would I meet CCC grouped?

If none of those has a "no" answer then your choice of cable is safe, subject to meeting VD and Zs for ADS.

 

[Speeka]

The point is you cannot allow a set of cables rated for 70C operation to be heated by those happy to run at 90C, so if you are looking at a section for that group, you should treat them all as if they were rated at the lowest temperature (e.g. 70C) when calculating and if you get an acceptable outcome then no problem.

The other aspect to consider is if accessories are rated for, say, 90C operation. Again that very much depends on the use-case of the cable, and if you took an example of, say, 6mm and found the 70C case such as Table 4D2A works for all segments of the route (with possibly differing "method" and grouping) you have nothing to worry about.

So if you had a 6mm cable going through a hot area, or an area of high thermal insulation (like method 103, etc) you might select 90C type of insulation, then you could do checks to see:

• Is the CCC of 90C insulation in a hot area with de-rating acceptable?
• Is the CCC of 90C traversing in, say method 103, acceptable?
• If I use the same CSA in 70C insulation, am I meeting the CCC in method B for common accessories?
• When grouped with other cables with a 70C rating, if I used 70C parameters for my cable would I meet CCC grouped?

If none of those has a "no" answer then your choice of cable is safe, subject to meeting VD and Zs for ADS.

thanks for the help, i think i understand, but...

so the table i would use would be for the lowest rated cable in the bunch,( in this case 70c) to select the current carrying capacity of my cable.

lets say that my scenario is an 18 metre run on perorated tray with the other cables (2 of) for most of its length & 25c ambient air temperature. for the ambient temperature do i choose my actual cable column or the 70c column values, obviously for the Zs i would choose the 90c or my actual value would be ----... no? along with my thermal calculations which fault currents would be based on incorrect fault loop impedance.

sorry if that sounds dumb, but im finding this part a little confusing.

ta.
S.

 

[Rockingit]

The exact combination of numbers doesn’t really matter - what matters is that they match the lowest temperature rated cable. So for example if you have a 70deg, 90deg and 120deg all bunched together, it’s irrelevant that the 120deg cable can carry 1000A at full monkey because the 70deg cable can only carry 100A @ 70deg.

 

[pc1966]

As above, for each segment of a complicated arrangement you use the lowest cable temperature rating of that group in any CCC computation.

Zs it trickier because you might have X meters at 70C due to a grouping with a lower rating, and in another area Y meters at 90C as your new cable can meet that and needs to for a specific use-case (high thermal insulation 'method', or running in hot area like next to heating pipes, etc), etc.

However, if you meet Zs on the assumption of max temperature everywhere it is a safe bet. If you don't then your situation is more difficult, but you can split in to segments and do the computation for working temperature's R1+R2 for each in turn to add up.

Note also that your working temperature won't necessarily be the limit in each case, again a bit tedious to compute and only worth doing if it is a marginal case in terms of meeting Zs.