Dear Raam,
during the construction, first the metallic line is installed, and after the insulation is fixed on it.
During the installation the temperature of the pipe is the ambient temperature, so the dilations will occur starting from this non insulated situation.
Often in calculation the installation temperature taken in account is the minimum ambient temperature, to be more conservative.
After the ending of construction, and after the start up, the pipe heats up and reaches its operative temperature.
So, no matter if the line is insulated or not, delta T is:
(Operative or Design temperature) - (Minimum ambient T, or ambient T).
Sometimes, if the operative temperature is low, you have to take in account the effect of solar radiation that can heat the pipe during installation up to 50°C or more, and calculate Delta T as:
(Temperature caused by solar radiation) - (Minimum fluid temperature).