Duet 2 Wifi: Max input current and power supply requirements
I am upgrading a custom printer with a Duet 2 Wifi board and am having a hard time finding the max allowed input current and appropriate wire gauge. I am aware of the documentation Duet 3D provides discussing how to calculate the max current draw of a given machine however there is no mention of the physical current limit of the board or input terminal block. My plan is to use the two 12V 6-pin PCIE connectors on a 700W FSP700-50ERN power supply. According to the internet this is only rated to provide 150W (75W each) of power. Given I want to use a 200W heated bed, 30W heater cartage, 4 motors, and some fans this puts me around 250W (20.8A at 12V) for the printer. I have some concerns with the PCIE connects as I am over the wattage spec. Id like to leave the power supply unmodified if possible. I'd be interested what the max input current is, typical input wire gauges, and what others have done with adapting unmodified ATX power supplies to a Duet.
There is a 7.5A fuse (stepprs, misc), a 1A fuse (fans) and a 15A fuse (bed), so if you were to be pedandic you'd need wires sized for 23.5A to supply the Duet.
If you were to be pragmatic you'd review https://duet3d.dozuki.com/Wiki/Choosing_the_power_supply
Yes that is a good resource and I did calculate my loads to the best of my knowledge and it came out to roughly 20.8A . It looks like another thread provided information on the terminal which looks to be rated up to 32A. 12AWG only gets me to 20A so I may go with 10 AWG wire or, now thinking of it, buy 2 PCIE cables stripping them and crimp them together in a single wire ferrule. That would be pretty clean. Id still be interested in other PCIE adapter ideas though.
Tbh I think you'll be fine at 12AWG, the heaters will only be on full power for a limited time, and full power also decrease rapidly as the temperature increase as does the resistance, lowering the power.
You can also delay energizing the motors until the printer has been brought to temperature, and even stagger the hotend and bed heating to keep the maximum current down considerably. I'm sure you'll find ample headroom to 20A if you do some real world measurements.
But do take into account the ampacity ratings are usually given for room temperature, and might have to be derated for elevated temperatures if that is an issue.