I am trying to make a square pipe of a square curve.
Every time I make a square pipe I get a cross-section of a pipe as a rectangle.
Switching the curve shape to 2D got me the desired result but as soon as I applied the transformation operations I got the error “Rotation/Location can’t apply to a 2D curve”.
All I want is to get a square cross-section of a pipe using curves.
Are you sure scale is applied in object mode?
This is not going to work. 
add
If it’s the same way as the image below, it seems possible without deformation. 
tsXX.blend (955.8 KB)
The answer is it already is square. The little square still has a square shape, but its placed a 45 degree at the corners of larger final shape. ( turn on wireframe to have a look)
So the walls look smaller because,if you look through a square that stands in front of you at a 45 degree angle it no longer looks square.
So you want the extruded wall to look square you have have to scale your small square along the x axis to be sqrt(2) = 1.4142. But thats just true for 45 degree angled extrusions.
Not sure what you mean with this, but if you work with 2D curves instead you can use simply use menu → object → convert → to mesh
Check if profile curve is applied too, could be that
Can also use this:
Good luck!
The problem is not with the scales of curves.
They all are set to 1,1,1.
The problem is with the default behavior of 3D Curves.
I am getting the desired output with 2D Curves but the only problem with that it is limited to 2D plane only.
you are right, it creates a rectangle because (as Debuk says) it is using the corner as the profile on the flat, but if you change the angle of the Rail of your sweep it pops into a square. Excuse the messy shape, it just to show what I’m saying.
This is working, just need to change the mode to Complex on the Solidify modifier.
