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sheepdog43
What size wires go to the bed? 16Ga wires are not enough. I used two 16ga and while it worked, it wasn't great. I switched to 14 and while it heated fast,
It changed because you wanted it to change. Switching from 16 to 14ga should have very minimal effect at 12v/10amps.
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it eventually killed the PSU because it drew more than the PSU could handle.
Then you had a very cheap power supply. A power supply is only going to produce the current that it can produce. It's not like the heated bed can "suck" more power out of the power supply than it can provide. If the bed can consume more than the power supply can produce, then the power supply's overload protection circuitry should kick in.
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Which is the next issue, 20 amps may not be enough, I killed a few 20amp psu's this way.
If you killed a 20 amp power supply drawing < 20 amps, then it wasn't a 20 amp power supply.
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4 motors = Nearly 4 amps
Hot end = More than 3 amps
Heated bed = 10-11 amps, some as high as 12 depending on resistance.
Fan = 1/3rd amp ea.
You still have to account for anything else needing power and the fact that China uses their own math for rating power supplies, which is usually about 20 (sometimes 50%) lower than rated, and that you shouldn't run a power supply at peak all the time.
Granted, not everything runs full power all the time, so 20 will get you going, it's not enough.
It's amazing how 7.3+ amps is able to continuously flow through a RAMPS 5 amp PTC without blowing it. Either the PTC is way out of spec on all the boards, or you're not drawing anything over 5 amps combined. And your fans draw 1/3 amp, EACH?!?!? What are you using, a furnace blower to cool your hot end and/or prints?