Thickness will differ for different type "A", "B" , and "C" GI pipe.
1. Light- Class A: The pipe is marked in yellow color for identification
2. Medium- Class B: The pipe is marked in blue color for identification.
3. Heavy - Class C . The pipe is marked in red color for identification.
Therefore, Class A GI pipe is thinest GI pipe than other.
GI Pipe
In the Compound Lighting Layout Plan drawing, It will note that all road crossing cable shall be laid in 150MM ( diameter) class "C" GI Duct. Since GI pipe can support the high stress, underground cable that cross road will laid inside GI pipe for protection of high press from heavy transport that pass through everyday.
http://jagvirgoyal.com/content/view/121/8/