Armoured Cable Calculations

[SnapCracklePop]

Hmmm, Could anybody tell me which table I use for calculating armoured cable sizes in the on-site guide?
I'm self teaching and failing :-(

 

[buzzlightyear]

It is like normal cable. When you designing.
So if you say what cable to use.just to add protection of the cable.

 

[telectrix]

look up in bbb. table 4D4A and 4D4B. pages 407/408

 

[pc1966]

I don't recall seeing the SWA cable tables in the OSG, as Telectrix says, check the actual 18th edition BS standard (the "big blue book").

 

[SnapCracklePop]

Brilliant, Thanks for all your replies, I'm actually working my way through the 'student's guide to the IET wiring regulations' and this question was in the 'Test your knowledge section' I 'passed' the first question (standard thermoplastic sheathed) but struggled with the armoured cable question. Not sure I understand why armoured is the same as normal thermoplastic sheathed cable?

 

[pc1966]

It is not the same, but it is similar.

Same CSA so heat generated for a given current (per unit length) is the same. Same 70C insulation limit on how hot it is allowed to get.

But different outer diameter (so thermal resistance to ambient slight different) for general mounting cases, and typically the use-cases (i.e. "method") are different. E.g. you are unlikely to have SWA run through loft insulation, and similarly you should not be burying T&E directly in the ground.

 

[Mark Wright]

that's a good book, one thing I've not calculated yet is the armour as CPC there's some equation for this isn't there?

 

[SnapCracklePop]

It is not the same, but it is similar.

Same CSA so heat generated for a given current (per unit length) is the same. Same 70C insulation limit on how hot it is allowed to get.

But different outer diameter (so thermal resistance to ambient slight different) for general mounting cases, and typically the use-cases (i.e. "method") are different. E.g. you are unlikely to have SWA run through loft insulation, and similarly you should not be burying T&E directly in the ground.

Well when you put it like that It’s obvious ?, not sure if I’ve missed it, but it doesn’t appear to explain it like that in the book.
[automerge]1594763326[/automerge]

that's a good book, one thing I've not calculated yet is the armour as CPC there's some equation for this isn't there?

It’s a great book, really really helpful and informative but, not sure I’ve got to that bit yet. I’ll be reading this a few times over I think.

 

[David Prosser]

"but it doesn’t appear to explain it like that in the book".

Welcome to the wonderful world of electrickery !!

 

[pc1966]

that's a good book, one thing I've not calculated yet is the armour as CPC there's some equation for this isn't there?

The armour as CPC for SWA is not obvious. It usually has larger CSA than the conductors but as it is steel the resistance is around 8 times higher per mm^2. For smaller cables it is often adequate, better in fact than T&E for R2, but for large cables it can fail the adiabatic requirements so you may end up using 3-core for single phase, 5-core (expensive) for three-phase, or running a supplementary CPC of 0.5-1.0 phase size alongside.

The last page of this data sheet has the info you might be looking for:

https://www.whyprysmian.co.uk/BS5467.pdf

 

[Mark Wright]

thanks it's something ill come back to, I like to have notes on most things then ill be pestering you pc1966

 

[telectrix]

SWA armour is generally accepted as sufficient cpc for cables up to 95mm. however, on smaller cables it may not be adequate as a bonding conductor.

 

[SnapCracklePop]

The armour as CPC for SWA is not obvious. It usually has larger CSA than the conductors but as it is steel the resistance is around 8 times higher per mm^2. For smaller cables it is often adequate, better in fact than T&E for R2, but for large cables it can fail the adiabatic requirements so you may end up using 3-core for single phase, 5-core (expensive) for three-phase, or running a supplementary CPC of 0.5-1.0 phase size alongside.

The last page of this data sheet has the info you might be looking for:

https://www.whyprysmian.co.uk/BS5467.pdf

Brilliant, thanks for all the info. more blummnig reading . Although I'm finding it all very interesting, so much to learn.
[automerge]1594839603[/automerge]

SWA armour is generally accepted as sufficient cpc for cables up to 95mm. however, on smaller cables it may not be adequate as a bonding conductor.

Is this mentioned in the books anywhere? just trying to work out if I'm missing it.

 

[Mark Wright]

not sure it's in that book, very good book but does miss somethings, another book I like is Chris Kitcher guide to the Regs.

 

[davesparks]

SWA armour is generally accepted as sufficient cpc for cables up to 95mm. however, on smaller cables it may not be adequate as a bonding conductor.

It is also generally sufficient for cables over 95mm.
Plus it is almost never adequate as a bonding conductor when cables are sized purely on CCC. Its only once cable sizes are increased to cope with voltage drop that the armour becomes big enough to satisfy bonding requirements.