Reading Table K-1
29 CFR 1926.403(i)(1) sets working space in front of electrical equipment at 600 volts nominal or less. Table K-1 is read by nominal voltage to ground, not the line-to-line rating. A 480Y/277-volt panelboard is 277 volts to ground, so it uses the 151-600 V row. A 208Y/120-volt panelboard is 120 volts to ground, so it uses the 0-150 V row.
The three conditions describe what faces the worker. Condition (c) is exposed live parts on both sides of the workspace, not guarded, with the operator between them.
The clearance figures
- 0-150 V to ground: 3 ft under conditions (a), (b) and (c) alike
- 151-600 V to ground: 3 ft under (a), 3½ ft under (b), 4 ft under (c)
So the 480Y/277-volt panel with condition (c) needs 4 ft, and the 208Y/120-volt panel with condition (c) needs only 3 ft. The trap: using 4 ft at 0-150 V is a listed common error. The 4 ft figure exists only in the 151-600 V row.
Width
Under 1926.403(i)(1)(i), the working space in front of the equipment must be at least 30 in (762 mm) wide, even where the equipment itself is narrower. A 24 in switchboard in a recess with a 24 in space falls short by 30 − 24 = 6 in. Forgetting the 30 in minimum is a listed common error. The 36 in and 42 in figures are the Table K-1 depth clearances of 3 ft and 3½ ft, not width rules.
Headroom
Under 1926.403(i)(1)(v), headroom of working spaces about service equipment, switchboards, panelboards or motor control centers must be at least 6 ft 3 in (1.91 m). The trap is the NEC figure of 6½ ft (6 ft 6 in): the OSHA construction standard is 6 ft 3 in, and substituting the NEC value is a listed common error. Neither 7 ft nor 6 ft appears in the rule, and 6 ft is short of the minimum.