Arc Flash Incident Energy Calculations Explained

Arc flash incident energy is the thermal energy a worker would absorb during an electrical arcing fault at a specific working distance, expressed in calories per square centimeter (cal/cm²). Engineers calculate this value to determine arc-rated PPE requirements, set arc flash boundaries, and identify equipment where energized work is unacceptably dangerous. The calculation method recognized by OSHA and NFPA 70E is IEEE 1584, which uses empirical equations developed from thousands of laboratory arc tests across 208 V to 15 kV systems.

This guide walks through what drives an arc flash incident energy calculation, how the IEEE 1584 model actually works, and how the resulting numbers translate into PPE decisions on the floor.

At the time of writing this article, the IEEE 1584-2018 edition is being used. The standard is presently undergoing revision, with a revision due within the next year.

What Is Arc Flash Incident Energy?

Incident energy is a measurement of heat flux delivered to a target surface during an arcing fault. The standard reference distance is 18 inches from the arc source, which represents the typical position of a worker’s chest and face during equipment interaction. For a broader primer on the physics and workplace consequences of these events, see our overview of arc flash basics and why arc flash hazards matter.

The number matters because human skin tolerates very little thermal exposure before serious injury. Exposure above roughly 1.2 cal/cm² is sufficient to cause the onset of a second-degree burn on unprotected skin, which is why that value defines the arc flash boundary.

Two related outputs come from the same calculation:

• Incident energy at working distance drives PPE selection.
• Arc flash boundary defines the distance form the arc at which incident energy falls below 1.2 cal/cm².

Both depend on the same set of system inputs.

Why IEEE 1584 Is the Calculation Standard

IEEE Std 1584 is the industry-adopted method for predicting arcing current and incident energy. The 2018 revision replaced the 2002 model and introduced significant changes in how electrode geometry and enclosure size are handled.

The 2018 model was developed from more than 1,800 new arc tests and produces materially different results than its predecessor for certain configurations. In some cases, incident energy predictions roughly doubled when equipment was reclassified from a vertical-conductor box (VCB) to a horizontal-conductor box (HCB). For facilities that completed studies under the 2002 model, this is the main reason to recalculate.

The standard is recognized by federal agencies, including OSHA, and is referenced by NFPA 70E as an acceptable method for performing an incident energy analysis. NFPA 70E gives employers two methods for selecting PPE: the incident energy analysis method or the PPE category table method, but not both on the same equipment.

The Five Inputs That Drive Every Calculation

An arc flash incident energy calculation is only as accurate as the data fed into it. Five variables dominate the result.

1. System Voltage and Bolted Fault Current

The model is valid for AC voltages between 208 V and 15 kV. Outside this range, IEEE 1584 does not apply. The three-phase bolted prospective fault current at the equipment bus is the starting point. Lower fault current does not always mean lower incident energy because protective devices may take longer to clear a lower arcing fault. 

The 2018 standard expanded the lower limit of analysis to systems less than or equal to 240 V AC with a bolted fault current of 2000A. The standard states that arcs are unlikely to sustain below these limits.

2. Arcing Time

Arcing time is the duration of the fault, set by the upstream (or final) protective device’s clearing time at the calculated arcing current. This may have a large influence on incident energy. A 0.2-second clearing time at 25 kA produces dramatically less energy than a 1.0-second clearing time at the same current.

3. Working Distance

Incident energy decreases with the square of the distance from the arc source. The default working distances differ by equipment type:

• Low-voltage panelboards and MCCs: 18 inches
• Low-voltage switchgear ear: 24 inches
• Medium-voltage switchgear: 36 inches

Document the assumed working distance on every label. A worker reaching deeper into a cabinet than the assumed distance is no longer protected at the labeled level.

4. Electrode Configuration

This is the input most often overlooked in legacy 2002-era studies. IEEE 1584-2018 defines five electrode geometries that produce different arc behavior:

• VCB: Vertical conductors inside a metal box
• VCBB: Vertical conductors terminated in an insulating barrier inside a box
• HCB: Horizontal conductors inside a metal box
• VOA: Vertical conductors in open air
• HOA: Horizontal conductors in open air

HCB and HOA configurations tend to direct plasma toward the worker, which is why the 2018 model predicts higher energies for them than the 2002 model assumed.

5. Enclosure Size and Conductor Gap

For boxed configurations, the enclosure dimensions affect plasma containment and reflection. The 2018 model applies an enclosure size correction factor based on height, width, and depth, with separate treatment for low-voltage and medium-voltage equipment. Conductor gap (the distance between phases) is also an input and typically falls between 13 mm and 152 mm depending on equipment class.

How the IEEE 1584-2018 Calculation Works

The 2018 procedure is a multi-step interpolation across three reference voltages: 600 V, 2,700 V, and 14,300 V. The simplified flow:

1. Calculate the intermediate arcing currents at 600 V, 2,700 V, and 14,300 V using the bolted fault current, conductor gap, and electrode configuration.
2. Interpolate the final arcing current at the actual system operating voltage.
3. Apply the enclosure size correction factor for the relevant electrode configuration and enclosure dimensions.
4. Calculate intermediate incident energies at the three reference voltages for the calculated arcing time and working distance.
5. Interpolate the final incident energy at the actual operating voltage.
6. Repeat steps 1 through 5 using the reduced arcing current (95% of nominal) and take the higher of the two incident energy values.
7. Calculate the arc flash boundary as the distance at which incident energy equals 1.2 cal/cm².

Most engineers run these calculations through software such as ETAP, EasyPower, or SKM PowerTools rather than by hand. The value of understanding the steps is in being able to audit the inputs and sanity-check the outputs.

Mapping Incident Energy to NFPA 70E PPE Categories

When using the incident energy analysis method, PPE is selected based on the calculated cal/cm² value at the working distance, not from the PPE category tables. The arc rating of the chosen clothing system must equal or exceed the calculated incident energy.

That said, engineers and safety teams still use category-aligned “levels” since much of industry and PPE manufacturers have become familiar with these thresholds. The use of the term “levels” is a very important differentiator that is permitted by the NFPA 70E as a “site specific” designation of the required PPE. Users will immediately notice the correlation:

Since the 4040 cal/cm² is perhaps the most commonly available flash suit, many engineers cap the PPE levels at 40cal/cm2. This level can easily be increased to match the maximum available flash suit rating own by the client/plant.

There was an old myth that work above 40cal/cm2 should not be performed energized because PPE protection becomes uncertain and because the pressure wave and blast effects of a high-energy arc can cause injury independent of thermal exposure. This claim was unsubstantaited and removed from NFPA 70E. Above the maximum PPE (flash suit or daily workwear levels available to the worker) remote operations or the hiearchy of controls are required in lieu of local equipment operation. 

A second important distinction: the incident energy analysis method allows you to select PPE rated exactly for the hazard. If your calculation produces 5.2 cal/cm² at a panel, you can specify a 6 or 8 cal/cm² system rather than forcing workers into a 25 cal/cm² suit because the equipment “feels like” Category 3.

Arc Flash Boundary Versus Incident Energy

These two outputs are often confused. They answer different questions.

• Incident energy at working distance tells you what PPE the worker inside the boundary must wear.
• Arc flash boundary tells you how far away an unprotected person must stand.

The boundary is calculated by solving the same incident energy equation for the distance at which energy equals 1.2 cal/cm². For low-energy faults, the boundary can be inches; for high-energy faults on medium-voltage gear, it can extend many feet. Both values belong on the equipment label.

Common Mistakes Engineers Make

A few recurring errors degrade study accuracy:

• Using 2002 results without recalculating under 2018. Equipment reclassified as HCB or VCBB often shows substantially higher energies.
• Assuming the worst-case fault produces the worst-case incident energy. The maximum-current scenario often clears faster. Always run the reduced-current case.
• Failing to document working distance assumptions. A label with no working distance is incomplete.
• Ignoring upstream device aging. Time-current curves assume properly maintained breakers. A 25-year-old molded case breaker with no maintenance record may not clear in the time the curve predicts.
• Mixing the table method and the incident energy method on the same equipment. NFPA 70E does not permit this.

FAQs

How often should arc flash studies be updated? NFPA 70E requires review at intervals not exceeding five years, or whenever a major modification or renovation affects the system. Adding loads, swapping protective devices, or changing utility service impedance can all shift results.

Can I use IEEE 1584 for DC systems? No. IEEE 1584-2018 covers AC systems only. DC arc flash calculations follow different methods, typically based on Dan Doan’s papers or the Stokes-Oppenlander model.

What if my equipment is below 240 V or below 2.0 kA? The 2018 standard states that a sustained arc is unlikely. Document the exemption rather than assigning a default PPE level.

Does software automatically pick the electrode configuration? No. The engineer running the study must select it based on the actual equipment construction. Verify by inspecting drawings or the equipment itself.

What’s the difference between ATPV and Ebt on a PPE label? Both are arc ratings. ATPV (Arc Thermal Performance Value) is the energy that produces a 50% probability of a second-degree burn. Ebt (Energy Breakopen Threshold) is the energy that causes the fabric to break open. Per ASTM F1506, the lower of the two values is what appears on the garment label.