Confusion over table 7.1(i) OSG

It's late, so this might need a check with fresh eyes, but I think your issue is the way the OSG applies the “rule of thumb” to it's tables ie to compensate for cable temperature.

From BS 7671 Table 41.3 last column, we see the maximum ACTUAL Zs permitted for a B40 is 230x0.95/(5x40) = 1.0925Ohm or 1092.5mOhm (which matches the tabulated value in 41.3 of 1.09Ohm).

Taking off 800mOhm for Ze leaves a maximum R1+R2 of 1092.5-800=292.5mOhm.

Table I1 of OSG gives resistance/m of 6/2.5 as 10.49 mOhm/m at 20C. Table I3 then tells us that at full working temperature of 70C, we need to multiply by 1.2.

Which gives 10.49x1.2 = 12.588 mOhm/m

Therefore maximum length of cable = 292.5/12.588 = 23.24m.

Or if using the tabulated value of 1090mOhm -800= 290/12.588=23.04m.

Table B6 of OSG gives the maximum MEASURED impedance (ie at 20C). What they seem to have done is apply the compensation to the whole Zs including the Ze part, so  1.09 x 0.8 = 0.872 which nearly matches the 0.88 in the table, but not quite.

(The reason I was working with more digits was to see if 0.8 x 1.0925 gets it to 0.88. It gets it to 0.874 which is a bit closer, but still rounds down to 0.87 rather than 0.88).

So I can see why the answer is 23m and maybe the missing 0.01 will turn up in the morning ?

It's late, so this might need a check with fresh eyes, but I think your issue is the way the OSG applies the “rule of thumb” to it's tables ie to compensate for cable temperature.

From BS 7671 Table 41.3 last column, we see the maximum ACTUAL Zs permitted for a B40 is 230x0.95/(5x40) = 1.0925Ohm or 1092.5mOhm (which matches the tabulated value in 41.3 of 1.09Ohm).

Taking off 800mOhm for Ze leaves a maximum R1+R2 of 1092.5-800=292.5mOhm.

Table I1 of OSG gives resistance/m of 6/2.5 as 10.49 mOhm/m at 20C. Table I3 then tells us that at full working temperature of 70C, we need to multiply by 1.2.

Which gives 10.49x1.2 = 12.588 mOhm/m

Therefore maximum length of cable = 292.5/12.588 = 23.24m.

Or if using the tabulated value of 1090mOhm -800= 290/12.588=23.04m.

Table B6 of OSG gives the maximum MEASURED impedance (ie at 20C). What they seem to have done is apply the compensation to the whole Zs including the Ze part, so  1.09 x 0.8 = 0.872 which nearly matches the 0.88 in the table, but not quite.

(The reason I was working with more digits was to see if 0.8 x 1.0925 gets it to 0.88. It gets it to 0.874 which is a bit closer, but still rounds down to 0.87 rather than 0.88).

So I can see why the answer is 23m and maybe the missing 0.01 will turn up in the morning ?