I upgraded the 5 resistor plate to a actively cooled 12 resistor plate to limit the current draw and therefore the power and heat created in the components (resistors, cables, printhead, transformer).

With the resistor plate and the continuous current operation I could measure the current draw for the first time - my amperemeter couldn't measure the 250ms pulses before.

When idle, the system draws 1,240A and if the transformer secondary is shorted (toolhead on buildplate, copper on copper) it draws around 3A.

I also measured the temperature while testing with a infrared thermometer.

When idle the resistors and the transformer stay below 50°C. The system can stay idle the whole time without overheating.

I tested measuring the temperature when shorted, too but unfortunately the area of the cable above the toolhead got too hot, so that I must abort testing. The toolhead will definitely need some cooling. The nozzle got very hot, too and changed its color to a more brown color.

I tried to measure the current while melting wire and it looks like it even drops under the measurement form idle mode, but it's hard to say whether this was a correct measurement because I could not hold the hight and feedrate steady per hand. Will continue working on X, Y, Z and E axis for testing it again.

A thought about cooling:

I think I could use a powerful water cooler for the hotend if air cooling would not be enough and if the cables heat up too much I could use 35mm² or 50mm² cables. For the resistors it seems like air cooling would be good enough, but will see that in longer test runs. Maybe I could add heat sinks underneath the fans for better cooling if needed.