current carrying capacity of SWA cables

[pweeks]

Im working in africa right now, I have 2 sets of regs, SANS (South African national Standard) and BS7671;2008... anyway to cut a long story short, both regs state different current carrying capacity for eg 70mm 4 core buried direct, a huge difference, I know the BS7671;2008 regs as my own, and they actually state on there that the carrying capacity is based on conductor temperature @ 70 degrees rated, however there is no rating in the SANS... How can 2 recognised standards be yards apart?

By the way SANS states 210amp per phase for 70mm 4 core buried direct !

normally these are just a guide but I know most sparks take the regs as what the cables carry !

Whats the score chaps?

 

[Deleted member 26818]

It could be that the ambient temperature in SA is higher than in the UK.
Ground temperature in the UK is taken as being 20ºC.

 

[pweeks]

yes mate, good thinking, where I am working its around 34 degrees everyday of the year pretty much, not sure what the ambient is in SA, however its not stated in the SANS about the ambient and what the calcs are based on as in BS7671, the project we are on has 2 sets of regs which the client finds acceptable, little bit confusing if you ask me !

 

[IQ Electrical]

The BS7671:2008 rating is 143 A which is, as you say, a huge difference.

The nominal voltage and frequency is exactly as our harmonized voltage so that's that one gone!

The insulation operating temperature is the only real variable and I have heard of American designs based on 100 C

Other than that, I'm stumped!

 

[Deleted member 26818]

Weird, I would have expected the current carrying capacity to be higher where the ambient temperature is lower?

 

[pweeks]

we have quite a varied climate here in west africa, 6 months dry, 6 months wet, we bury cables direct around 500mm deep which is more than acceptable especially with no traffic... not sure how much the cables will be effected by the change in climate, thermal resistivity is a factor, still baffles me why the huge difference !

 

[IQ Electrical]

Weird, I would have expected the current carrying capacity to be higher where the ambient temperature is lower?

I know, it defies all logic!

Also, BS7671:2008 ratings are allowing an ambient air temperature of 30C and ground temperature of 20C.

 

[pweeks]

think SA has pretty much same climate as UK only a little warmer in their summer than ours, it gets quite cold there according to SA lads who work with us, strange bunch may I add

 

[Deleted member 26818]

I find all of our colonial brothers strange.
Saffos are the easiest to wind up though.

 

[dnk1983]

i have the same problem right now in afghanistan.

 

[SPARTYKUS]

I think non-UK cables have a higher level of copper coated resisitrons in them.


I just made that up.

 

[Darkwood]

The DNO use different regs over here and CCC is different as far as im aware, at a certain depth the ground over here remains constant 4C degrees regardless of temp above ground and this can be used as a factor to reducing the cable size but we are governed by 7671 which dosn't use this theory. How many times have you seen DNO cables a fraction of the size you expect the to be and you end up connecting a larger CSA to them to satisfy your own regs.

Im not stating this is the case but feel it may have some bearing, id check if the local version of the DNO follow the same SANS

 

[Darkwood]

I would also make sure you are not been confused with aluminium cables and copper cables as these also would explain the difference as these are also often used by the DNO but can't again comment on S.A. practices.

 

[Engineer54]

Perhaps Marvo might be able to shed some light on this??

 

[Richard Burns]

BS7671 has a "margin of safety" built in to the regs but it may be that SA does not, this could be the difference, e.g. if taking full BS7671 current the cable may only reach 60oC but SA current may actually reach 70oC (or more but not really melt the insulation!)