The issue with thin wire tends to be that as it passes current, it heats up and as it heats up, the resistance goes up slowly, not by much, but after a while it becomes a limiting effect.
FWIW: I would personally dump the PTC fuses and use real fuses, which might gain you a small amount. If you want to get even more out of the board, I'd lay a bit of solder or even a piece of wire on top of any existing tracks on the RAMPS PCB related to D8. This will reduce the volt drop between 2a and 2b.
You also definitely want to heatsink and to actively cool the mosfet. Longer operation with high current could lead to the mosfet heating up a bit over time. Lots of on/off switching will also lead to it heating. The hotter the mosfet is, the higher it's internal resistance will be, leading to higher voltage drops across it. This will reduce the volt drop at 2c.
Notes:
- You probably want to repeat these readings as the temp rise levels out a bit, as that seems to be when the system is in stress.
- Are you running BANG-BANG or PID in firmware. For a heated bed, you really don't need PID control for the bed, and it usually causes issues. You might find that the firmware defaults to PID and this is why the temp increase slows down so much.
- PTC fuses don't have well defined trip points, and they take AGES to trip (20secs is fairly fast for a PTC). The closer they are to their trip current or max current (these are different values), the higher the resistance across them, leading to a larger voltage drop. IMO they're ill-suited as protection devices and in fact cause a LOT more problems than they solve. With a high current PSU and a high current short, it's quite possible to make a PTC fuse catch fire, as it gets so hot internally that they explode before the fuse itself trips.
FWIW: I would personally dump the PTC fuses and use real fuses, which might gain you a small amount. If you want to get even more out of the board, I'd lay a bit of solder or even a piece of wire on top of any existing tracks on the RAMPS PCB related to D8. This will reduce the volt drop between 2a and 2b.
You also definitely want to heatsink and to actively cool the mosfet. Longer operation with high current could lead to the mosfet heating up a bit over time. Lots of on/off switching will also lead to it heating. The hotter the mosfet is, the higher it's internal resistance will be, leading to higher voltage drops across it. This will reduce the volt drop at 2c.
Notes:
- You probably want to repeat these readings as the temp rise levels out a bit, as that seems to be when the system is in stress.
- Are you running BANG-BANG or PID in firmware. For a heated bed, you really don't need PID control for the bed, and it usually causes issues. You might find that the firmware defaults to PID and this is why the temp increase slows down so much.
- PTC fuses don't have well defined trip points, and they take AGES to trip (20secs is fairly fast for a PTC). The closer they are to their trip current or max current (these are different values), the higher the resistance across them, leading to a larger voltage drop. IMO they're ill-suited as protection devices and in fact cause a LOT more problems than they solve. With a high current PSU and a high current short, it's quite possible to make a PTC fuse catch fire, as it gets so hot internally that they explode before the fuse itself trips.