South African National Standard & BS7671:2008 - Defying Logic

[pweeks]

I posted another thread on here last night about differences in current carrying capacities between SWA cable in both sets of regs (SANS & BS7671:2008)...

Here are some typical examples as I have both sets of regs in front of me...

Contrast in calcs is beyone belief... How can this be, the ambient temps are 5 degrees different, correction factors come into play but wont change the calcs too much... See for yourselves and please somebody give me a clue

Lets take 120mm 4 Core SWA as an example here...

SANS = 120mm 4 core SWA...
Ambient temp = 25 degrees...
Conductor operating temp = 70 degrees...
Method Clipped Direct = 202A

BS7671 = 120mm 4 core SWA...
Ambient temp = 30 degrees...
Conductor operating temp = 70 degrees...
Method Clipped Direct = 267A

...................................................................................

SANS = 120mm 4 core SWA...
Soil temp = 25 degrees...
Conductor operating temp = 70 degrees...
Method Buried Direct in the ground = 295A

BS7671 = 120mm 4 core SWA...
Soil temp = 20 degrees...
Conductor operating temp = 70 degrees...
Method Buried Direct in the ground = 192A

......................................................................................

SANS = 120mm 4 core SWA...
Soil temp = 25 degrees...
Conductor operating temp = 70 degrees...
Method in pipes or ducts burried in the ground = 234A

BS7671 = 120mm 4 core SWA...
Soil temp = 20 degrees...
Conductor operating temp = 70 degrees...
Method in pipes or ducts burried in the ground = 192A

..........................................................................................

These are just examples of what i'm finding at the minute, defies logic in some areas and design must be carried out in 2 totally different ways by engineers from UK who I believe in and SA where I know nothing about in regards to standards of work !

Massive headache trying to work out what a cable actually does carry, excuse me for being stupid but i'm an electrician and not an engineer, on a project where either standard is accepted, we have SA electricians working to their regs and UK elecs working to ours, surely something somewhere will blow, more than likely the SA as ours are more stringent.

I ask this as i'm working in West Africa where the ambient temp is around 40 degrees every day, this will have a de rating factor further more to what both standards state..

Thanks for your advice in advance !

 

[Marvo]

As far as I know many of the figures in the SANS 0142 regs were taken directly from the ISO regs, I'm guessing the current capacities would be no exception.
There's also some fundamental differences with CPC regulations between the two sets of regs especially with SWA cabling.

 

[pweeks]

Ive noticed that too marvo...

No disrespect meant by this thread, its just me trying to get to the bottom of what the reality of cable current carrying capacity...

SANS Seem to guide you into using more amperes than BS in some areas, and BS seem to let you use more in other areas... Very grey area when you compare the 2 standards !

Thing is I am protecting a 120mm cable @ 200A which is a little over BS standard but within the SA standard by miles, Where as SA elecs are protecting @ 300A...

 

[Marvo]

No disrespect meant

None taken. It's a very pertinent question but one I've never had time to delve into. As far as I know the main limiting factors to a cable's current carrying capacity are the volt drop across it and its ability to lose heat which in turn determine its operating temperature. I agree this should be pretty much a constant throughout the various regulations regardless of geographical location.