Boundaries — who can stand where.
Three boundaries, two of them from a table and one of them from a study. The table has three columns, and reading the wrong one is the most common error in the field.
The limited approach boundary for every AC system from 50 V up to 72.5 kV is 10 ft 0 in (3.1 m). It does not change with voltage in that whole range.
The number that changes with voltage is the restricted approach boundary — and it has two columns, one for exposed movable conductors and one for exposed fixed circuit parts. In a data center you are almost always in the fixed circuit part column.
§ Shock approach boundary lookup
§ Reading the three columns
People see "3 ft 6 in" next to 480 V and record it as the limited approach boundary. It is not. At 480 V the limited approach boundary is 10 ft 0 in; 3 ft 6 in is the restricted approach boundary for an exposed movable conductor, and the number you actually want for switchgear is 1 ft 0 in — the fixed circuit part column.
| Column | What it means | Where you meet it |
|---|---|---|
| Limited approach | The shock-hazard boundary. Unqualified persons may not cross without a qualified escort and a hazard briefing. | 10 ft 0 in for essentially everything in a data center |
| Restricted — exposed movable conductor | A conductor whose position is not under the worker’s control. | Overhead lines. Rare indoors |
| Restricted — exposed fixed circuit part | Bus, terminals, lugs, breaker stabs — anything bolted in place. | Almost every task inside the building |
Energized work permits with "3 ft 6 in" written in the limited approach field for 480 V gear. It is the wrong column and the wrong boundary, and it makes the permit wrong on its face. The permit builder now prompts for all three.
These tables are cited as Table 130.4(E)(a) and 130.4(E)(b) — the 2027 numbering. Older material may cite the AC shock boundary table under a different subdivision letter; that is prior-edition numbering. Every citation and every value on this site is the 2027 edition.
§ Three boundaries, three different jobs
| Boundary | What it protects against | Who may cross | Where the number comes from |
|---|---|---|---|
| Limited approach | Shock | Qualified persons. Unqualified persons only with a qualified escort and after being advised of the hazards | Table 130.4(E)(a) or (b), by voltage |
| Restricted approach | Shock from arc-over combined with inadvertent movement | Qualified persons only, with shock PPE and a documented plan | Same tables — pick the movable or fixed column |
| Arc flash | Thermal burn from an arc flash event | Anyone inside must wear arc-rated PPE meeting or exceeding the incident energy at the working distance | Your arc flash study, or Table 130.7(C)(15)(a)/(b) |
The arc flash boundary is frequently outside the restricted approach boundary and can exceed the limited approach boundary — Table 130.7(C)(15)(a) puts the arc flash boundary at 20 ft for 600 V class switchgear and 40 ft for metal-clad 1–15 kV gear, both beyond the 10 ft limited approach. Being outside one boundary tells you nothing about the other.
§ The arc flash boundary
The distance from an arc source at which the incident energy equals 1.2 cal/cm² — the threshold for a second-degree burn on bare skin. Inside it, arc-rated PPE is required.
Unlike the shock boundaries, this one is not a lookup by voltage alone. It comes either from the incident energy analysis for that specific equipment — available fault current, clearing time, gap, enclosure geometry and working distance — or from the arc flash boundary column of Table 130.7(C)(15)(a)/(b) where those tables' parameters are met. Work the category tables →
| Equipment (table method) | Category | Arc flash boundary |
|---|---|---|
| Panelboards or other equipment ≤ 240 V | 1 | 19 in (485 mm) |
| Panelboards or other equipment > 240 V and ≤ 600 V | 2 | 3 ft (900 mm) |
| 600 V class MCC — fault ≤ 65 kA, clearing ≤ 0.03 s | 2 | 5 ft (1.5 m) |
| 600 V class MCC — fault ≤ 42 kA, clearing ≤ 0.33 s | 4 | 14 ft (4.3 m) |
| 600 V class switchgear and switchboards | 4 | 20 ft (6 m) |
| Other 600 V class equipment | 2 | 5 ft (1.5 m) |
| NEMA E2 motor starters, 2.3–7.2 kV | 4 | 40 ft (12 m) |
| Metal-clad switchgear, 1–15 kV | 4 | 40 ft (12 m) |
| Arc-resistant equipment, doors closed and secured, within rating | N/A | N/A |
Teams that treat the arc flash boundary as a fixed distance across a room. It is per-equipment, and a 20 ft boundary on the main switchboard means the boundary extends past the wall into the corridor.
§ Elevation
The restricted approach distances in both tables are based on an elevation not exceeding 900 m (3,000 ft) and must be adjusted for higher elevations — air is a less effective insulator at lower pressure, so arc-over distance increases. Relevant for campuses in Denver, Salt Lake and comparable elevations. The tables reference 29 CFR 1910.269 Table R-5, ANSI/IEEE C2, and IEEE 516 for the correction.
Get the crew on the right column
Weekly NFPA 70E qualified person training on the 2027 edition, including the DC tables and the elevation correction.
See the schedule Bring it onsite