110.14(D) Installation.  Where a tightening torque is indicated as a numeric value on equipment or in installation instructions provided by the manufacturer, a calibrated torque tool shall be used to achieve the indicated torque value, unless the equipment manufacturer has provided installation instructions for an alternative method of achieving the required torque.

Comment:  Most electricians are familiar with the old adage “hand tight plus a quarter turn”,

but studies have shown that approximately 75% of terminations are not within +/- 20% of the manufacturer’s recommended torque value where a calibrated torque tool has not been used.  It is common practice to use a torque tool on larger switchgear, however the 2017 NEC now makes it clear that a torque tool must be used whenever a manufacturer provides torque values for the terminations on their equipment, so that the most reliable, low-resistance connection is made.