Last Updated: 2026-10-19
Answer Capsule: Arc flash and electric shock protection boundaries are determined two ways. The limited and restricted approach boundaries protect against shock and come from NFPA 70E-2024 Table 130.4(E)(a) for AC or (b) for DC, based on voltage. The arc flash boundary protects against burns and is calculated in the study as the distance where incident energy equals 1.2 cal/cm2.
A label on a 480 V switchboard lists three distances: 4 ft 6 in, 3 ft 6 in and 1 ft 0 in. Your apprentice asks which one matters. The honest answer is all three. Each distance protects against a different hazard, and each comes from a different place in NFPA 70E. Get them mixed up and someone stands too close without the right PPE.
Print Pro AZ prints those three numbers on arc flash labels every week, and Brent Hanke gets the same question from foremen: how are arc flash and electric shock protection boundaries determined, and why is one calculated while the other two come from a table? This guide defines the three hazards, then the three boundaries, where each comes from, and which ones appear on the label.
What Is the Difference Between Arc Flash, Arc Blast and Shock?
Arc flash is the heat and light released when current jumps through air, arc blast is the pressure wave and flying material from that same event, and shock is current passing through the body from contact with an energized part. They injure in different ways, so NFPA 70E protects against them differently.
- Shock happens when you touch, or get close enough to arc to, an energized conductor. The defense is distance, insulation, and de-energizing the equipment.
- Arc flash happens when an arcing fault turns air into plasma. The injury is a burn from radiant heat. The defense is distance and arc-rated PPE matched to the incident energy.
- Arc blast is the pressure wave, sound and molten metal thrown by the same fault. NFPA 70E treats blast effects as part of the arc flash hazard and does not publish a separate arc blast boundary.
NFPA 70E-2024 handles shock and arc flash with separate risk assessments and separate boundaries. Section 130.5 covers the arc flash risk assessment, and Section 130.4 covers shock protection. The label brings both together.
How Are Arc Flash and Electric Shock Protection Boundaries Determined?
The shock protection boundaries are looked up from a table based on voltage, and the arc flash boundary is calculated in the arc flash study based on the available energy at that equipment. That is the most useful thing to know about the three distances on a label.
| Boundary | Hazard it protects against | Where it comes from | Depends on |
|---|---|---|---|
| Limited approach boundary | Shock | NFPA 70E-2024 Table 130.4(E)(a) for AC, Table 130.4(E)(b) for DC | Nominal system voltage |
| Restricted approach boundary | Shock | NFPA 70E-2024 Table 130.4(E)(a) for AC, Table 130.4(E)(b) for DC | Nominal system voltage |
| Arc flash boundary | Arc flash burn | The arc flash risk assessment, calculated per IEEE 1584-2018 | Fault current, clearing time, electrode configuration, enclosure |
The two shock boundaries are the same for every piece of equipment at the same voltage, because the table only looks at voltage. The arc flash boundary is different for every piece of equipment, because it depends on how much energy the fault can release and how fast the protective device clears it. Two 480 V panels side by side can have arc flash boundaries that differ by several feet.
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What Is the Limited Approach Boundary?
The limited approach boundary is the outer shock protection distance from an exposed energized conductor, and it comes from NFPA 70E-2024 Table 130.4(E)(a) for AC systems or Table 130.4(E)(b) for DC systems. The field label example for a 480 VAC switchboard shows a limited approach boundary of 3 ft 6 in.
Unqualified persons stay outside the limited approach boundary unless a qualified person escorts them and warns them of the hazards. The table gives different distances for a fixed circuit part and an exposed movable conductor. The label shows the fixed circuit part value, which applies at a panel.
What Is the Restricted Approach Boundary?
The restricted approach boundary is the closer shock protection distance, inside which only qualified persons with shock protection may work. It also comes from NFPA 70E-2024 Table 130.4(E)(a) for AC or Table 130.4(E)(b) for DC. The field label example for 480 VAC shows a restricted approach boundary of 1 ft 0 in.
Crossing the restricted approach boundary is energized electrical work under NFPA 70E. That brings in the energized electrical work permit rules, voltage-rated gloves and tools, and insulated barriers where they apply. The only way to remove the boundary is to establish an electrically safe work condition under Section 120.5. If your hands are going inside the restricted approach boundary, either the equipment is locked out or you are wearing shock PPE and working under a permit.
What Is the Arc Flash Boundary?
The arc flash boundary is the distance from the equipment at which incident energy equals 1.2 cal/cm2, the onset of a second-degree burn, and it is calculated in the arc flash risk assessment rather than looked up from a table. The field label example shows 4 ft 6 in for a 480 VAC switchboard with incident energy of 8.7 cal/cm2 at 18 in.
Inside the arc flash boundary, a worker needs arc-rated PPE matched to the incident energy at the working distance. Outside it, the expected energy is below 1.2 cal/cm2, so arc-rated PPE is not required for the arc flash hazard. Shock PPE still applies when the shock boundaries are crossed.
The calculation comes from IEEE 1584-2018. Its inputs are the available fault current, the clearing time of the protective device, the electrode configuration, the enclosure size and the working distance. The 2018 edition replaced the 2002 model. If your study still uses 1584-2002, the arc flash boundary on the label may be wrong, and the study should be re-run.
The arc flash boundary can be larger or smaller than the limited approach boundary. On high-energy switchgear it can extend well past the shock boundaries. Never assume one boundary covers the other.
Which Boundaries Appear on the Arc Flash Label?
The arc flash boundary is required on the label by both NEC 2026 Section 110.16 and NFPA 70E-2024 Section 130.5(H), and the limited and restricted approach boundaries are optional but appear on most labels. Arc blast has no boundary of its own and does not appear on the label. Here is the field label example for a 480 VAC main switchboard, equipment ID MSB-1:
| Label line | Value | Required? |
|---|---|---|
| Nominal system voltage | 480 VAC | Yes, both documents |
| Arc flash boundary | 4 ft 6 in | Yes, both documents |
| Incident energy at working distance | 8.7 cal/cm2 at 18 in | Yes, this or the PPE category, never both |
| Limited approach boundary | 3 ft 6 in | No, but common |
| Restricted approach boundary | 1 ft 0 in | No, but common |
| Assessment completed | 08/12/2026 | Yes, NEC 2026 110.16 |
Print Pro AZ includes the two shock boundaries on its standard layout, because a label with only the arc flash boundary leaves the worker to look up the shock distances. The Print Pro AZ post on how to read an arc flash label field by field walks through each line, and the Print Pro AZ guide to arc flash labels under NFPA 70E 2024 covers the full requirement.
How Do the Three Boundaries Work Together at the Panel?
At the panel, the three boundaries stack from the outside in: the arc flash boundary sets where arc-rated PPE starts, the limited approach boundary sets where unqualified persons stop, and the restricted approach boundary sets where energized work begins. Using the MSB-1 example:
- Beyond 4 ft 6 in: outside every boundary. No arc-rated PPE required for the arc flash hazard. Anyone can stand here.
- Inside 4 ft 6 in, outside 3 ft 6 in: inside the arc flash boundary. Arc-rated PPE rated for at least 8.7 cal/cm2 is required, so Category 2 gear at 8 cal/cm2 is not enough.
- Inside 3 ft 6 in, outside 1 ft 0 in: inside the limited approach boundary. Qualified persons only, or an escorted unqualified person, still in arc-rated PPE.
- Inside 1 ft 0 in: inside the restricted approach boundary. Qualified person, shock PPE, and energized work rules apply. Or the equipment is locked out under Section 120.5 and the shock boundaries no longer exist.
Here is a call Print Pro AZ gets every month. A crew reads 3 ft 6 in on the label, assumes it is the arc flash boundary, and stands at 4 ft in street clothes. The real arc flash boundary is 4 ft 6 in, so they are inside it. That is why Print Pro AZ prints the arc flash boundary and the two shock boundaries under separate headings.
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Frequently Asked Questions
What is the difference between the arc flash boundary and the limited approach boundary?
The arc flash boundary protects against burns and is calculated in the study as the distance where incident energy equals 1.2 cal/cm2. The limited approach boundary protects against shock and comes from NFPA 70E Table 130.4(E)(a) or (b) based on voltage. Either one can be larger.
Is there an arc blast boundary?
No. NFPA 70E treats the pressure, sound and flying material from an arcing fault as part of the arc flash hazard, and it does not publish a separate arc blast boundary.
Who can cross the limited approach boundary?
Qualified persons can cross it. Unqualified persons stay outside unless a qualified person escorts them and advises them of the hazards. Only qualified persons using shock protection may cross the restricted approach boundary.
Does the arc flash boundary change from panel to panel?
Yes. The arc flash boundary depends on fault current, clearing time, electrode configuration, enclosure size and working distance at that equipment, so two panels at the same voltage can have different arc flash boundaries. The shock boundaries are the same at the same voltage.
Conclusion
Three hazards, three boundaries, two sources. The limited and restricted approach boundaries protect against shock and come from NFPA 70E-2024 Table 130.4(E)(a) or (b), based on voltage. The arc flash boundary protects against burns and is calculated in the study as the distance where incident energy equals 1.2 cal/cm2. Arc blast is part of the arc flash hazard and has no boundary of its own.
A label that prints all three distances under clear headings keeps the crew from mixing them up.
Need labels that show every boundary clearly? Order the aluminum arc flash and shock hazard label from Print Pro AZ, or send your study data for a custom set. Questions? Call Brent: (602) 649-5305.
Sources
- NFPA 70E, Standard for Electrical Safety in the Workplace
- OSHA, Arc Flash Hazards
- IEEE 1584-2018, Guide for Performing Arc-Flash Hazard Calculations
Brent Hanke | Print Pro AZ | (602) 649-5305 | b.hanke@printproaz.com
Brent Hanke is the founder of Print Pro AZ, supplying NEC-compliant labels to contractors across the country.