I guess you could consider if the cable is running at a temperature less than 70c, in which case the k value could be more than 115. If the formula is plotted it provides a straight line with k at about 163 at 0C and 115 at 70C. Run at a conservative 50C would give k at about 129.

The question of minimum csa of cpc to meet thermal constraints is a very common one on the 2396 Design course. There would be no expectation to adjust the k value, but I guess the various methods indicate some calculations in installation work are what Mapj referred to as measuring with a micrometer and cutting with an axe!

Using the 5900 figure that gives a csa of about 0.59mm2. As a matter of interest, you would be marked correct in the 2396 Design course using the simple method and the 1.38KA and a fixed time to disconnection of 0.1s, which would lead to a minimum csa of  around 3.8mm2. 

It is rare to see a 1mm2 circuit protected by a 6A breaker running at anything like 70C unless the ambient is unusually hot. Possible in parts of the middle east  - so if 1,1mm seems a squeeze remember the assumption about cable core temp before fault is probably most pessimistic.

Mike.

False precision indeed - I bet most 1mm2 cables are not made with copper of  exactly 17.241 milliohms per metre length(*) nor for that matter are they actually 1mm2 to within on part in 100,000 either. The world of mass manufacture is not that good, nor does it need to be.  Expecting to be within a few percent may be more credible, and new paper designs that squeak a pass only by that much should probably be re-thought, and any existing  design that fails by that much is no great risk for continued operation.

Mike.

(I tend to use a rule of 16 for  walking around looking at a cable type,  pacing and muttering about voltage drops - allows one to  identify the 'no problem' and the 'clearly a problem' cases very quickly, and the few that are a bit suck teeth and 'that looks borderline' need calculating or measuring to be sure anyway.)

False precision indeed - I bet most 1mm2 cables are not made with copper of  exactly 17.241 milliohms per metre length(*) nor for that matter are they actually 1mm2 to within on part in 100,000 either. The world of mass manufacture is not that good, nor does it need to be.  Expecting to be within a few percent may be more credible, and new paper designs that squeak a pass only by that much should probably be re-thought, and any existing  design that fails by that much is no great risk for continued operation.

Mike.

(I tend to use a rule of 16 for  walking around looking at a cable type,  pacing and muttering about voltage drops - allows one to  identify the 'no problem' and the 'clearly a problem' cases very quickly, and the few that are a bit suck teeth and 'that looks borderline' need calculating or measuring to be sure anyway.)