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# copper equivalent of steel

Discuss copper equivalent of steel in the Electrical Forum area at ElectriciansForums.net

#### wirepuller

Forum Mentor
Does anyone have the copper equivalent of a two core 16mm SWA to hand? Or the minimum SWA 2 core CSA in which the steel would be equivalent to a 10mm copper conductor? Don't have the info to hand.
Thanks.....

#### Leesparkykent

##### Super Moderator
Staff member
Super Moderator
Does anyone have the copper equivalent of a two core 16mm SWA to hand? Or the minimum SWA 2 core CSA in which the steel would be equivalent to a 10mm copper conductor? Don't have the info to hand.
Thanks.....
In conductivity or as in CSA?

#### westward10

##### In echoed steps I walk across an empty dream.
Staff member
Moderator
For conductivity off the top of my head as no books to hand 95.0 or above.

#### Ian1981

Forum Mentor
The ratio in gn8 for copper to steel is 8.5.
So a 85 mm steel conductor -equivalent conductive copper conductor is 10 mm
85/8.5=10

#### happyhippydad

##### Member
Electrician's Arms
This really isn't my area as it's rarely used in domestics.

However, I've found from one table on the net (https://www.dungannonelectrical.co.uk/dun1-shop/pdf/cable/SWA.pdf) that 2 core 16mm will have a 42mm CSA of steel.

I am then not sure if you divide by 8.8 or 2.75.

AJJewsbury explains it below: (IET Forums - SWA Size of steel armour for CPC - https://www.theiet.org/forums/forum/messageview.cfm?catid=205&threadid=47544)

"If the armour is only being used as a c.p.c. (and not as PME bonding conductor) then you're not really interested in its actual resistance (either absolute or copper equivalence) - just its ability to survive an earth fault current for the required time - i.e. it's "k" value (tables 54.2 and following). E.g. if the cable runs at max 70 degrees in normal service with a maximum ambient of 30 degrees, then k for copper is 143 and steel 52 - making copper just 2.75 times "better" than steel for the same c.s.a.

So if the cable had 76mm2 of steel that would be the equivalent of 27.6mm2 of copper - easily satisfying table 54.7. "

#### spinlondon

Forum Mentor
I believe the minimum CSA for the armour to have the equivalent conductivity of a 10mm2 copper conductor is 95mm2.
I think it would be cheaper to run in a 3core.

#### wirepuller

Forum Mentor
Many thanks for the replies.
Just to clarify an EICR on a recent build (new <5yrs ago,) steel framed outbuilding has a 2core 16.0mm SWA feeding from a TNCS supply in the main building. No main bonding has been installed to the steel frame, the supply head is a TPN 100a so a minimum 10mm copper equivalent is required to serve as main bonding and cpc. Just to complicate matters a supply back from the outbuilding serves a pump installation in the main building so converting to TT would result in two separate earthing systems in the main building.
Why the original installer put in a 2 core is beyond me, but it is what it is now.

Last edited:

#### Ian1981

Forum Mentor
This really isn't my area as it's rarely used in domestics.

However, I've found from one table on the net (https://www.dungannonelectrical.co.uk/dun1-shop/pdf/cable/SWA.pdf) that 2 core 16mm will have a 42mm CSA of steel.

I am then not sure if you divide by 8.8 or 2.75.

AJJewsbury explains it below: (IET Forums - SWA Size of steel armour for CPC - https://www.theiet.org/forums/forum/messageview.cfm?catid=205&threadid=47544)

"If the armour is only being used as a c.p.c. (and not as PME bonding conductor) then you're not really interested in its actual resistance (either absolute or copper equivalence) - just its ability to survive an earth fault current for the required time - i.e. it's "k" value (tables 54.2 and following). E.g. if the cable runs at max 70 degrees in normal service with a maximum ambient of 30 degrees, then k for copper is 143 and steel 52 - making copper just 2.75 times "better" than steel for the same c.s.a.

So if the cable had 76mm2 of steel that would be the equivalent of 27.6mm2 of copper - easily satisfying table 54.7. "
A 76mm steel conductor would equate to around 8.9mm copper conductive equivalent

#### Ian1981

Forum Mentor
Many thanks for the replies.
Just to clarify an EICR on a recent build (new <5yrs ago,) steel framed outbuilding has a 2core 16.0mm SWA feeding from a TNCS supply in the main building. No main bonding has been installed to the steel frame, the supply head is a TPN 100a so a minimum 10mm copper equivalent is required to serve as main bonding and cpc. Just to complicate matters a supply back from the outbuilding serves a pump installation in the main building so converting to TT would result in two separate earthing systems in the main building.
Why the original installer put in a 2 core is beyond me, but it is what it is now.
Yep a 16mm 2 core armour isn’t going to cut it

#### buzzlightyear

##### please let me back in to the prison cell.
Electrician's Arms
where's Pete with the information he mostly on the ball .stop taking his job.

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