PCB source

[Jonathan]

Oh come on Jonathan, stop being such a tease. What is your 2 minute calculation and how exactly does it change the measured result shown in the video?

It's just resistors in parallel - it doesn't change the result, just shows it without having to watch the video which takes longer.

While you're at it, what is a 'demanding application', what is 'a lot',

It depends...One where you are trying to conduct a higher current through the track than reccomended, and one where that current changes from a high to low value frequently but at a low frequency.

what is the difference in the heating effect of a fluctuating current and a steady current,

The peak temperature will be the same, but if the current fluctuates at a low frequency (say the device is turned on for 5 mins then switched of, then turned on after 10 mins) the temperature of the solder and copper wire will rise and fall, and since they expand at different rates this results in metal fatigue, so the material can fail over time.

I know linking to another forum is not a great reference, but here it is anyway:
DipTrace Forum • View topic - How do i make certain tracks soldered rather than painted

There's some good information on the topic in this thread:
current capacity of a solder loaded track - Page 2

Post #23 in this thread says that adding solder to the track does not comply with CE regulations, although I've seen it in plenty of ATX PSUs I've dismantled.

This discussion is a bit over the top though for a PSU powering stepper motors. Although the transformer is presumably capable of delivering 15A constant current, it never will in this application since at 69V that is well over the power rating of 4 Nema 24 stepper motors. From m.marino's build log we can see that he's using the standard Nema 24 3Nm motors, so from the datasheet the motor is rated for 4.2A and 5.46V, so the power rating is 4.2*5.46=23W per phase, two phases so 46W. Four motors so 184W. If the average power drawn from the PSU is any more than that then the motors are being run outside their ratings and should overheat. Therefore the rms current drawn from the PSU is 184/69=2.7A which is far less than the 15A we were previously designing for. It cannot be higher for a significant amount of time, otherwise you are exceeding the motor's rating. Clearly there will be current spikes well above 2.7A, but the duration of these is small so they are not important for thermal considerations