You don´t need 500W ! - all you need is WM (MeanWell) HRP-300-15 an the Rep-Rap Power-Expander !!!
My Config for ABS print: on both Extruder Heater 12V 40W = 2x66W on 15V and the Custom MK3 headbed (320W)
I don´t know why but it works exellent !
i get 110 degrees for the heatbed after 4:30 minutes heating and with 4mm glas - after the heating the HRP-300-15 is on 45 degrees and the Power-Expander is on 55 degrees.
i also heating the extruder heater with the heatbed together. no problem for the HRP-300-15
the power expander go after heating on print to 45 degrees and the HRP-300 to 40 degrees
it realy works exellent - maybe the HRP-300-15 have much more reserves
i have print jobs more then 20 hours with 110 degrees heatbed and both extruders and never problems !!
and i have more than one K8400
since 2 weeks i install the power-expander in airstream from the buttom fan - before he go up over 100 degrees but he works good
here is one more picture from my other K8400 - here you see the power-expander in airstream - on the other pictures he is in the black box
i have try much more power supplies - stronger and i also try two power supplies … but nothing works so good like the HRP-300-15 (Reichelt 80 euro)