Arc Flash Boundary: Definition, Calculation, and a Worked Example

The arc-flash boundary is the distance from a potential arc source at which the incident energy falls to 1.2 cal/cm² (5.0 J/cm²). Inside that distance, an arc event could expose unprotected skin to a second-degree burn; outside it, the thermal energy has spread thin enough that it cannot. NFPA 70E requires the boundary on the equipment label (§130.5(H)) alongside the incident energy.

The 1.2 cal/cm² figure is not arbitrary: it is the incident energy for a 50% probability of a second-degree burn on bare skin, from the Stoll skin-burn research the electrical-safety standards are built on (documented in the IEEE IAS Electrical Safety Workshop proceedings). A second-degree burn is curable — that is the point of the threshold. The boundary is the line between "curable burn possible" and "PPE required," not a line between injury and no injury.

Last reviewed: July 18, 2026.

How the boundary is computed (IEEE 1584-2018)

The 2018 edition of IEEE 1584 computes the boundary analytically from the same incident-energy equation used for the working-distance result (Clause 4.7): instead of solving for energy at a fixed distance, you solve for the distance at which the energy equals 1.2 cal/cm². The inputs are the same ones that drive incident energy:

• Arcing current — derived from the bolted fault current, voltage, gap, and electrode configuration. The standard evaluates both the full and the reduced arcing current, because a lower current can trip the protective device more slowly and produce the worse result.
• Clearing time — how long the upstream device takes to open at that arcing current. Energy scales with time; so does the boundary.
• Electrode configuration — the five 1584-2018 configurations (VCB, VCBB, HCB, VOA, HOA) throw the plasma differently, so the energy falls off with distance differently.
• Enclosure size correction — box dimensions change the result for enclosed equipment.

This calculator's reviewed input scope stops at 2 seconds. It rejects longer clearing times instead of silently shortening them. A qualified engineer must decide whether a different duration, working practice, or engineering model is appropriate for the actual task.

Worked example, from a real calculation

Run through our calculator's engine (IEEE 1584-2018, both arcing-current branches evaluated):

Result: the arcing current computes to about 18.3 kA, incident energy at the 18-inch working distance is 9.5 cal/cm², and the arc-flash boundary is about 65 inches — roughly 5 ft 5 in.

Read that result the way an electrician would: anyone who may cross the five-and-a-half-foot boundary during the task belongs inside the arc-flash risk assessment. At the stated working distance, the 9.5 cal/cm² result requires an arc-rated system rated at least 9.5 cal/cm²; it is not converted into a PPE category. The boundary does not erase other hazards or replace site access controls.

Arc-flash boundary vs the shock boundaries

The arc-flash boundary is a thermal line. NFPA 70E separately defines shock protection boundaries in 130.4, and they are not the same thing:

On a high-energy panel the arc-flash boundary is usually the outermost line, which is why it governs where unqualified people stand. On a low-energy panel the shock boundaries can be the wider ones. The working rule: barricade and post at whichever boundary is largest, and never treat the arc-flash boundary as a shock clearance or vice versa.

What changes the boundary

• Clearing time dominates. Halving the trip time roughly halves the energy. Maintenance-mode trip settings exist for exactly this reason.
• Fault current, in the non-obvious direction. A lower available fault current can mean a larger boundary, because the device trips more slowly at the lower arcing current. This is the NEMA underestimate trap — see what is available fault current.
• System changes. A service upgrade or a protective-device swap moves the number; that is why the label carries an assessment date and why NFPA 70E caps the review cycle at five years (see how often an arc flash study is needed).

Compute the boundary for your own panel with the free arc flash calculator — every intermediate value is shown, so you can check the result against the standard. For the full label field list, see arc flash label requirements.