National Electrical Code: General Requirements and Definitions
Scope and definitions
Article 100 definitions drive every later calculation: branch circuit, feeder, service, GFCI, and arc flash hazard. Article 110 is the enforcement backbone: listed and labeled equipment (110.3), terminations (110.14), working space (110.26), available fault current marking (110.24), and lockable service disconnects (110.25).
Working space rules
- 110.26(A) depth is measured from live parts, not from the door or enclosure front.
- Width is 30 in. minimum or the width of the equipment, whichever is greater.
- Height is 6.5 ft.
- Table 110.26(A)(1) conditions: Condition 1 has no live parts on either side; Condition 2 has live parts on one side and grounded parts on the other; Condition 3 has live parts on both sides.
Worked example
A 480 V motor control center has exposed live parts on one side and a grounded metal wall opposite. Condition 2 applies, so Table 110.26(A)(1) requires 3.5 ft of depth for 301–600 V. If the opposite wall instead held another 480 V panel with exposed live parts, Condition 3 would apply and the depth becomes 4 ft. The measurement starts at the live parts, not at the enclosure door.
Traps
- Measuring working space from the door, which understates the depth.
- Using 3 ft for all voltages; 480 V Condition 2 needs 3.5 ft.
- Ignoring 110.26(E) dedicated equipment space above and below the panel.
- Forgetting that 110.14(C) temperature ratings limit conductor ampacity at terminations.