Selecting the proper stepper motors, question on current drawl  

Selecting the proper stepper motors, question on current drawl

Hello everyone!

After getting my feet wet with the I3 MK2 and learning a few things, I am ready to try building my own based on the Prusa design.

I am basing this build on the same Mini Rambo 1.3a and had a question regarding the integrated stepper drivers.

The motors I was looking at buying for the project is a set of 5 Nema 17s 1.8 deg, 2A motors with a holding torque of 45Ncm.
https://www.amazon.com/dp/B00QEXSCE8/_encoding=UTF8?coliid=I5PUOGFSLP82E&colid=39C3SEO8S08CA

What I hadn't thought about until now was that the rated current of each motor is 2 A (1amp per phase)
Checking a few forums I saw between 1 and 1.5 amps thrown around (high performance setting) for what the stock Prusa i3 MK2 motors rated current.

This also had me puzzled as the mini Rambo board's supply of power to the motor drivers is fused at 5 amps, but how could this be possible if all five motors are pulling 1-1.5 amps a piece? I assume that during the printing process all motors are pulling current to stay on their current step, and do not de-energize when not currently moving.

Is the rated current listed the maximum current the motors should be run at? I had assumed this was the current the motors were designed for and providing less then the rated current would impact their performance.

My ultimate question is if the mini Rambo is only able to supply 5 amps between all 5 steppers, it would seem that using 2 amp rated current motors would suffer performance issues if I am mistaking the term "rated current" as designed current, rather then maximum current.

If this is the case, I assume I should look for lower rated current motors, but even then it would seem the stock prusa steppers are being under supplied.

What rated current motors then should I be looking for if I intend to drive them with the mini Rambo board?

Secondly, I was considering using a .9 deg stepper for the extruder. I found a smaller Nema 17 stepper
https://www.amazon.com/Stepper-Bipolar-15-6oz-42x42x20mm-4-wires/dp/B00W96BBF6/ref=sr_1_1?ie=UTF8&qid=1504160372&sr=8-1&keywords=nema+17+stepper+motor+.9+deg
which would be lighter (150g vs 280g) on the x carriage, but it's torque is listed as 11Ncm. Is this too little torque to use for the extruder?

Thank you!