What derating table do I use for multicore armoured thermosetting cable in insulation.

Other thought, is rather than use 16mm² 3-core, use 25mm² 2-core (it's about the same diameter) and if you don't trust the armour as a PE for downstream, add a 16mm² G/Y along side (all permitted these days). The 90 degree version with a method C rating of 146A should be good for 63A under almost any conditions.

   - Andy.

Other thought, is rather than use 16mm² 3-core, use 25mm² 2-core (it's about the same diameter) and if you don't trust the armour as a PE for downstream, add a 16mm² G/Y along side (all permitted these days). The 90 degree version with a method C rating of 146A should be good for 63A under almost any conditions.

   - Andy.

I had considered 25mm two core. How does the oval shape work with standard swa glands, do I need a special gland. The widest diameter of the cable is the same as 3 core, do I gain a lot in flexibility. It's  a TT earth, so as long as I have some sort of earth path it will be enough for the 100mA time delayed RCD I will have up front anyway.

Alan B said:

I had considered 25mm two core. How does the oval shape work with standard swa glands,

A 2-core SWA is round, not oval.

Alan B said:

It's  a TT earth, so as long as I have some sort of earth path it will be enough for the 100mA time delayed RCD I will have up front anyway.

RCDs can't provide protection against overcurrent .. for protection against overcurrent, you'd need to take into account the maximum prospective fault current, which is not the same as the minimum fault current which is used for ADS.

Just worth a check there are no parallel paths that might increase prospective fault current.

AJJewsbury said:

Other thought, is rather than use 16mm² 3-core, use 25mm² 2-core (it's about the same diameter) and if you don't trust the armour as a PE for downstream,

Why would you not trust the armour of a 2-c 25 mm2 ?

I can see that there may be circumstances where the armour is not up to it (although)

I am also aware that not all courses provided the appropriate "tools" to work out the loop impedances, but if it's safe, you can always test that to verify at an appropriate socket-outlet.

"Corrosion" or "can't get a good connection" is NOT an answer ... if it is, then one of the following is awry, and is, of course, a non-conformity to BS 7671:

• poor workmanship
• equipment not suitable for external influences

AJJewsbury said:

add a 16mm² G/Y along side (all permitted these days).

Doesn't mean it's always valid do to that and everything is covered for all fault conditions. In particular, if the cable is direct-buried, BS 7671 requires the armour to be suitable as the protective conductor (Reg 522.8.10) ... but a designer might, in other circumstances, be advised to take into account that there are other conditions in which the cable could be completely severed, with or without the parallel conductor being severed, rendering the parallel protective conductor useless.

gkenyon said:

Why would you not trust the armour of a 2-c 25 mm2 ?

Just a common preference not to (similar to drawing in a g/y into steel conduit) - the OP had spec'd 3C 16mm² after all. I was just forestalling a possible objection.

   - Andy.

AJJewsbury said:

Just a common preference

Yes, this has, for some reason become a 'preference', but the only issue I can find that drives this decision, is that glands are not made off properly, or the wrong type of gland is selected for a particular application.

The wider problem comes when:

• the parallel cpc becomes damaged (it is not protected in the same way as the armour or armoured cable), or
• the terminations for the parallel cpc are not adequately protected against corrosion, or
• there is a situation as I alluded to in my previous post, where the parallel cpc becomes ineffective in the specific conditions.

An internal conductor used as a cpc is not quite as "frail", but perhaps the issue of severing a buried cable brings the same issue.

A straight "parallel conductor", where either the armour, or parallel cpc, are each separately adequate to act as the cpc is the safest approach.

gkenyon said:

Yes, this has, for some reason become a 'preference', but the only issue I can find that drives this decision, is that glands are not made off properly, or the wrong type of gland is selected for a particular application.

I do wonder where it had its origins but it was very common on specifications from consultants back in the 1980s, at least those presenting designs for the Department of Education. Back then, my team did school after school, all with the SWA cables and separate cpc. Cables were often clipped along corridors. 
Similarly, the steel conduits all had separate earths for each circuit. Interestingly, the plethora of mims used the copper sheath without question. 

Not sure if the influence was Table 54.7 (or what preceded) being interpreted incorrectly and as the only option.