Tip run Pos from PSU direct to hotbed only run GND to the in and out on the Relay board it will save you some cable and help the hotbed heat up a bit faster.
I'm using a good 30AMP psu and it has a hard time with the hot bed my amp meter says full load printing with hexagon hotbed and all axis on draws 29.4amps

i replaced the hotbed cables with 12awg silicone ultra flex cable used in R/C cars it's like a wet noodle compared to the junk on the bed.
you will need a good 60w with a 5mm tip or a 120w gun type to solder to the bed as it acts like a heatsink and suck the heat away
(i used my 60w hakko and a 4.2mm 30deg CH tip it did fine 3 or 4 sec per wire at 460c)