need to fully understand ... and im just not getting it

[andysparkfree]

Hi guys

i need an idiots guide to the following and i know you guys know your stuff

swa armouring as a cpc only

now i think i know the following , but how to put it altogether and where 8.8 comes from is just confusing me , so if i start again and you good people

so
4 core 35mm swa has csa of 78mm
and swa for a cpc needs to be at least 46 to be adequate

as per attachments

how do i actually find out the actual copper equivalent of the swa so i can say 35mm swa = 9.2mm earth

really need to understand this as seems to be cropping up on a weekly basis
and the more i read the more confused im getting lol

thanks guys

 

[polo1]

Deleted.

 

[Ian1981]

Guidance note 8 suggests a figure of 8.5 copper equivalent ratio to steel and 1.68 for aluminium to copper ratio.
So if you had 85mm steel then 85/8.5= 10mm copper equivalent.
35mm steel divide by 8.5 would be 4.11mm copper

 

[andysparkfree]

Ian thank you !

 

[Ian1981]

Ian thank you !

No problem

 

[Pete999]

Is it doing the calculation or reading/interpretation of the table you are having a problem with?
Using your example, a 35mm, 4 core SWA has swa copper equivalent of 37.2mm2 or 25mm2 if xple - taken from the table.

Agree Polo, not sure but it may be the OP is trying to find out the csa of the cpc for the cable he is specifying i, e, for example is the tabulated CU size is suitable for the 35mm2 he is speaking of, mind you that's how I look at it I may be wrong, but not disputing your answer.

 

[andysparkfree]

so 35mm 4 core swa with steel at 78mm / 8.5 = 9.17mm copper conductor

 

[andysparkfree]

Pete , is different job , as was pulled up last inspection for leaving cpc blank when steel armouring being used as earth , just trying to get things right

 

[Ian1981]

so 35mm 4 core swa with steel at 78mm / 8.5 = 9.17mm copper conductor

Yeah really if your considering using the steel armour as a protective bonding conductor at domestic level then to comply with the minimum CSA of 10mm copper conductors needed then 85mm is the minimum steel CSA you can use of copper equivalent

 

[davesparks]

That table is a at best a rough guide, and at worst just plain wrong.

There isn’t a direct ‘copper equivalent’ for use as a CPC as there is more to the calculation than just comparing conductivity.
The simple answer is to use the equations given in BS7671 to calculate it as and when required, either the adiabatic equation or the simplified method. Actual steel CSA of the armour should be in the manufacturers data for the cable you are using.

If you are considering the steel armour for use as a bonding conductor then it can be simplified to equivalent conductivity. Steel has roughly 9 times less conductivity than copper, so you can multiply the required copper csa by 9 to get the required steel csa. Using this method for a cpc would result in a ridiculously oversized steel csa!

 

[andysparkfree]

yes , this one is office with no bonding conductors