What are the requirements for dedicated equipment space around panels?
Two different rules get mixed up on this question. Working space under 110.26(A) is the room you stand in to work on the gear. Dedicated equipment space under 110.26(E) is the volume directly above and below the panel that other trades are not allowed to use. You have to satisfy both.
The dedicated space itself
Section 110.26(E)(1)(a) defines the indoor dedicated space as the footprint of the equipment, meaning its width and depth, extending from the floor to a height of 6 feet above the equipment or to the structural ceiling, whichever is lower. Nothing foreign to the electrical installation may be in that zone. That means no plumbing, no gas line, no duct, no sprinkler pipe passing through, no shelving. This is the rule that catches the plumber who ran a copper line across the top of a panel.
Section 110.26(E)(1)(b) allows foreign systems above the dedicated space, so a duct at 8 feet over a panel whose top is at 6 feet 6 inches is acceptable if there is protection against damage from condensation, leaks, or breaks in the system above. A drip pan is the common fix.
Section 110.26(E)(1)(c) makes an allowance for sprinklers. A suspended ceiling with removable panels is not considered a structural ceiling, so the dedicated space still runs to the actual deck above in most commercial buildings.
Working space, the other half
Depth in front comes from Table 110.26(A)(1). For equipment operating at 150 volts or less to ground the depth is 3 feet under all three conditions. Above 150 volts to ground it is 3 feet for Condition 1, 3 feet 6 inches for Condition 2, and 4 feet for Condition 3.
Width under 110.26(A)(2) is 30 inches or the width of the equipment, whichever is greater, and the space has to allow the enclosure doors to open at least 90 degrees. Height under 110.26(A)(3) is 6 feet 6 inches or the height of the equipment, whichever is greater. Section 110.26(A)(3) also states that other equipment associated with the electrical installation located above or below may extend not more than 6 inches beyond the front of the electrical equipment.
Outdoor gear
Section 110.26(E)(2) covers outdoor installations. Outdoor electrical equipment must be in suitable enclosures and protected from vehicular traffic and from accidental contact by unauthorized personnel or by objects. The working space requirements of 110.26(A) still apply outdoors.
Field takeaway
Keep a clear box the size of the panel from the floor to 6 feet above it, keep 3 feet in front and 30 inches wide with the door swinging free, and get a drip pan under anything wet that crosses higher up.