What gauge wires are used throughout the system?
18 gauge for the motors (recommend 18/4 shielded security wire).
22 gauge for the homing switches (recommend 22/2 shielded security wire).
If you are in the US you can get this at Home Depot.
You can get away without using shielded wire. I believe the assembly instructions don’t mention the shielding or how to properly use it, and most forum members seem to avoid using it as intended as well.
There were a few cases where not using the shielding on the cables caused problems:
When the spindle was on its way out it would send out so much EM interference that it would cause the stepper motors to stutter and lose steps.
When trying to use the limit switches as true limit switches rather than homing switches. Turning on the “hard limits” boolean in the grbl config cause the interference the motor was generating to create a false positive in the switch wiring, which shut the machine down, thinking it had hit an axis limit.
For most people, especially now that many of us have switched away from that first spindle motor, the shielding isn’t necessary. Heck, I rarely use the homing feature on my machine at all. I just home it manually job-by-job.