thermal derating factors

[rasell]

When calculating a thermal insulating rating factor from Table 52.2 [523.7] if the figure is between two of the given numbers do you go up to the next figure or down? For example if the insulation was 150mm would you count it as 200mm (derating factor 0.63) or 100mm (derating factor 0.78).
Usually I round up for safety, but the regs say that if the insulation if equal or bigger than 500mm to use a rating factor of 0.5, so I was not sure what to do when a figure is in between two values in the table.

 

[johnelectrical]

Table 52.2
if the cable is surrounded by thermal insulation for less than 50mm = no derating factor
50 to 100mm = 0.88
100 to 200mm = 0.78
200 to 400mm = 0.63
400 to 500mm = 0.51

and from the text above the table over 500mm = 0.5

 

[trebor]

When calculating a thermal insulating rating factor from Table 52.2 [523.7] if the figure is between two of the given numbers do you go up to the next figure or down? For example if the insulation was 150mm would you count it as 200mm (derating factor 0.63) or 100mm (derating factor 0.78).
Usually I round up for safety, but the regs say that if the insulation if equal or bigger than 500mm to use a rating factor of 0.5, so I was not sure what to do when a figure is in between two values in the table.

I asked the NICEIC about this and the simle answer was split the factor, so for your example 150mm the factor would be 0.70 in round figures