Conductor Operating Temperature Correction Factor Ct for Volt Drop Calculation

I have a query relating to the Ct formula in Appendix 4, Section 6.1 of BS7671 for calculating conductor operating temperature correction factor is :

Ct = 230 + Tp - (Ca2 x Cg2 - Ib2/It2) x (Tp - 30)) / (230 + Tp)

where :
Ct = Temperature Correction Factor to account for the actual temperature reached with load current flowing taking into account the heating effect of grouping and ambient temperature.
Tp = Maximum Operating Temperature of the conductor (for PVC = 70 deg C and for XLPE = 90 deg C).
Ca = Ambient Temperature Correction Factor.
Cg = Grouping Correction Factor.
Ib = Design Load Current (in Amps).
It = Tabulated Continuous Current Carrying Capacity or Current Rating of Conductor at 30 deg C (in Amps).
30 = The Ambient Temperature (in deg C) the tabulated cable current ratings are designed to.

For cables installed in underground ducts (installtion method D), the curent carrying capacities are calculated using an ambient ground temperature of 20 deg C.

So for calculating the Ct for cables in below ground ducts, should the Ct factor have (Tp - 20) instead of (Tp - 30), i.e. so the formula becomes :

Ct = (230 + Tp - (Ca2 x Cg2 - Ib2/It2) x (Tp - 20)) / (230 + Tp)

For example, say I'm installing a Copper/PVC cable in underground duct, so Table 4D4A & B are applicable: In Table 4D4A & B the current ratings and volt drop values are for an ambient ground temperature of 20 deg C (i.e. if the above ground ambient air temperature is 30 deg C, the ambient temperature in duct below ground would be lower at 20 deg C).

So I'm figuring that to apply the Ct correction factor to the mV/A/M values derived from Table 4D4B for cable in underground duct then the appearance of 30 deg C in this Ct formula would be meangingless as we don't have 30 deg there to start with, we have 20 deg C and the mv/A/M values are for a conductor temperature of 70 deg C in an ambient temp of 20 deg C, so if the ambient temp in duct changes to a different temperature then Tp - 20 would seem more appropriate than Tp - 30 ??

So I think the general formula should have (Tp - Tr) instead of (Tp - 30), i.e. :

Ct = 230 + Tp - ((Ca2 x Cg2 - Ib2/It2) x (Tp - Tr)) / (230 + Tp)

Tr being the temperature the current carrying capacity values are calculated at, which for above ground installations is 30 deg C and in ducts below ground is 20 deg C.

(Tp - Tr) is the temperature rise above ambient, so for calculations of voltage drop for cables in below ground ducts I would consider the temperature rise above ambient to be (Tp - 20) rather than (Tp - 30).