2002 NEC 670.5 Clearance.
Where the conditions of maintenance and supervision ensure that only qualified persons service the installation, the dimensions of the working space in the direction of access to live parts operating at not over 150 volts line-to-line or line-to-ground that are likely to require examination, adjustment, servicing, or maintenance while energized shall be a minimum of 750 mm (2? ft). Where controls are enclosed in cabinets, the door(s) shall open at least 90 degrees or be removable.