95mm cable entry into switch fuse

[Monchichak]

Just noticed your post with image IPF, thanks for sharing, due to existing layout I will need to do a similar s bend with the trunking.

Ok,, thanks everyone - I'll get some tri rated tails, >150/150 trunking with swept edges & make or (modify an existing) flared flange to provide some wriggle room.

Onwards and upwards ?

 

[ipf]

Get some paxolin or, if not, some good quality 6mm ply wood for mounting between trunking and unit, smooth slotting as required for safe cable entry. I find using grommet with such cable and lack of space is a no no.

 

[Monchichak]

Interesting, rather than cut a slot, I could cut individual holes for each cable to help align cables & provide support.
Will look into this thanks

 

[Spoon]

Interesting, rather than cut a slot, I could cut individual holes for each cable to help align cables & provide support.
Will look into this thanks

Just to clarify, you are thinking of putting individual holes for each cable in the paxolin/ply wood and not the trunking?

 

[Monchichak]

Yes, potentially individual holes in paxolin, not metallic material ?

 

[UKMeterman]

Normally I would go for tri-rated wire because it is so much easier to wrangle, and mostly I have worked on systems more akin to "control panels" than power distribution where '6491X' is the norm.

But I don't have the tools or ferrules for anything over 25mm so would be looking at having to add something like an extra £150 or so to the cost to get them. Some places sell them individually instead of packs of 100 or so, such as these folks (who also sell the tri-rated cable and have been helpful to me in the past):

Red Pre-Insulated Bootlace Ferrule 95mm2 | CET95-RED

Buy Red Pre-Insulated Bootlace Ferrule 95mm2 | CET95-RED | Order By 1pm For Same Day Shipping |

www.cse-distributors.co.uk

Example of crimp tool:

SWA RPB50-95 Ratchet Tool 50-95mm

<p>Specialised Wiring Accessories (SWA) is one of the UK's market leaders when it comes to manufacturing essential cable terminations, fixings, tools and fastenings. With over 25 years' experience, SWA is a name that is associated with quality and reliability within the electrical industry. </p>...

www.tradesparky.com

Of course it might also be the case that your fused-switch has bolt terminals (such as Schneider SQB3153K which would be my choice having used the smaller ones of the same series) and in that case you would need crimp lugs for 95mm cable & matching tool irrespective of it being fine-stranded "tri-rated" or the usual coarse-stranded 6491X.

Beware some DNOs will not allow tri rated, discuss with them before using tri-rated.

 

[pc1966]

Beware some DNOs will not allow tri rated, discuss with them before using tri-rated.

Good point. In all the cases I have used it, it was never directly to the DNO's equipment so that issue never came up.

 

[Monchichak]

Thanks, checked with SSEN who say they do not have any specific requirements on meter tails and trust the accredited electrician carrying out the work is competent to select the correct part.

Unfortunately, they were unable to confirm if their meter terminals are rated for 90 Deg C, does anyone know the answer to this? I will try to find the meter datasheet once I receive model details.

What is the BS number of the tri rated cables you would recommend?

 

[pc1966]

You would need to see the meter, though I imagined that meters much above 100A would probably have separate current transformers so the cable might well go to the cut-out or whatever.

Looking at the tables for 70C (Table 4D1A in the regs) is appears that 95mm is rated for around your 315A requirement when spaced by one diameter for air circulation (columns 11 & 12) so if your cable is arranged like that it ought not to be above 70C at the ends.

But if it is in trunking (column 5) then it is only 207A for 70C, or 269A (Table 4E1A) for 90C, so neither are meeting the 315A specification for the fused-switch.

 

[pc1966]

Tri-rated is OK to 105C but really at that point you have to worry about what is in contact with it, etc.

https://clyndercables.co.uk/wp-content/uploads/sites/36/2020/03/Tri-rated-Cable-Spec.pdf

 

[pc1966]

Turns out they also do double-sheathed tri-rated wire so it could be used with open cable tray for cooling and not have the primary insulation exposed:

Double Insulated Tri-rated Cable | Clynder Cables

clyndercables.co.uk

 

[Monchichak]

Thanks for your input, I noticed a post on IET which I plan to look into that deals with this exact issue: https://---------------/forums/foru...OHoZTIDC0Ccclv1jPnacR5Zxiu80eSmAaAkszEALw_wcB
Any thoughts and experience would be welcomed.

Cable tray was my initial through, but I agree that trunking would be better, I'm attending site tomorrow so I'll be able to make a better judgement. I would definitely swing towards cable tray before switching to 240mm cables but I hope to find that 95mm cables installed within a 1m piece of trunking are acceptable, even if I have to provide vents.

This is a future proofing upgrade to incoming switch gear on a growing site, maximum demand will not be required for a long time, but at the same time I don't want to have to do a job twice.

 

[pc1966]

That link seems not to work, so maybe easier to copy & paste the text?

If you need to enclose the cables for whatever reason it might be worth looking at using some parallel runs, it might be that, say, 2*70mm is going to meet the CCC even allowing for the grouping factor, or if you have the 95 already, then doubling it up. Parallel cables have more opportunity for the heat to escape than just increasing the CCC in a single cable.

For bolted lug terminals you can often put them back-back and they still sit nicely on the assembly to be bolted down. However, you would have to look at the meter end to see exactly how the cables will terminate there and if it is practical to have two ferruled ends in to a clamp, etc. (maybe going for uninsulated ferrules, or cutting the plastic sheath off an insulated one).

Some of the other folk on here have far more experience than me so might have better advice on running a high current cable setup like this.

 

[Monchichak]

Will try the link again: https://---------------/forums/foru...OHoZTIDC0Ccclv1jPnacR5Zxiu80eSmAaAkszEALw_wcB

Parallel cables seems a little extreme for this task but I'll keep it in mind, interesting to learn the trick of bolting back to back.

 

[Monchichak]

Link failed.

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Text copied from link:

I'm working on a current project where the meter tails need to be upgraded. The the maximum capacity allowed at the meter is 240mm2 but this is not an adequate size for the current carrying capacity required if you take into account that the existing containment is metal trunking.

If the cables were clipped direct or installed on a tray then it complies with the tables in BS7671.

There is only a few metres between the meter enclosure and the main switch. I am told that it is OK to size the cable without taking into account the fact it is in trunking. However I have not found anything to justify this statememt.

What do you think?

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Fairly common practice in my experience.

The switchroom is rarely at high ambient temperature, the full load of the installation is usually not connected, are you concerned with overload protection anyway and short lengths of cable in trunking isn't really Ref method B as the heat losses from a single circuit (and the trunking) usually conspire to give you the equivalent of a clipped direct value.

Use a few reasonable corrections for the above and you'll soon see that most tails are grossly oversized anyway. If you want some concrete basis for design, do a temperature analysis on the cable based on the probable ambient and the probable load.

Regards

OMS

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The answer is in 533.2.1 and Guidance Note 6 section 2.2.3 which explains that the thermally equivalent current can be calculated for varying loads. However the guidance note does not detail how to do the calculation.

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I recall it does - usually a seven step process that requires you to know (or predict) how the load varies with time.

Personally I would first adjust the current carrying capacity for a lower ambient temperature (say 20C) and for the design current (as it's extremely unlikley to be equal to the tabulated CCC for a 240mm.

From there you can determine an actual conductor temperature (assumed steady state) and a suitable rating factor.

If this won't show that the tails in trunking comply then use the GN 6 method (assuming you have a varying load).

Finally, as I suggested, do you actually need to protect the tails from overlaod if the building connected load is less than the CCC of the 240mm - ie you use 433.3.1 (ii)

Regards

OMS

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Back in the room:
Thanks for advice, making a judgement on whether 95mm tails grouped within trunking will be safe is beyond my experience and understanding so there really is only 1 way forward and that's clipped direct (double insulated cables) with adequate spacing.