I can't tell you anything about the gauge of wire, but I can tell you that pushing wire through a 1 1/2" conduit is MUCH easier than 1 1/4". I had the same idea. I purchased about 500' of 1 1/4" but ended up not using it because it was too difficult to push those 3 wires through there. The 1 1/2" PVC conduit is just a little more money and it maked the job much easier.
