What Mr. Krug said for sure.
The problem with cutting copper isn't cutting copper, It is when the beam isn't penetrating the material and just bouncing off the surface.
With a 1500 watt, I wouldn't go any thicker. And if you don't care about edge quality so much, use full power for pierce, full frequency full duty, and Oxygen at about 0.2 MPa or 2 Kg
If the edge is super important, use Oxygen for the pierce and then switch to Nitrogen, but cut super slow. 10 or 20 Ipm to start off with. Again, full power full duty.
Using Oxygen you maybe able to get it up to 30 ipm but start slow.
Good luck.