Arc-Flash Incident Energy Calculator
Estimate arc-flash incident energy and boundary with the Lee and Doughty equations, as a first pass before a full IEEE 1584 study.
Arc-Flash Incident Energy
Incident energy is the thermal energy an arc would deliver to a surface at a working distance, in calories per square centimeter. It determines the PPE category and the arc-flash boundary that must appear on the equipment label. Two inputs dominate it, and only one is obvious: available fault current, and how long the upstream device takes to clear. Halving clearing time roughly halves incident energy, which is why protection settings matter more to arc-flash exposure than almost anything else.
The formula
| Bolted fault current | From the fault-current calculation at that point |
| Clearing time | How long the upstream protective device takes to interrupt, in seconds |
| Working distance | Distance from the arc to the worker's face and chest, commonly 18 inches for LV gear |
| System voltage | Above 600 V the calculator uses the Lee equation; at 600 V and below, the Doughty equations. Both are from NFPA 70E Annex D |
Worked example
At 480 V in an enclosure, with 22.6 kA bolted fault current and an 18 in working distance, the Doughty equation gives 4.3 cal/cm² at 0.1 s clearing and 8.5 cal/cm² at 0.2 s. Double the time, double the energy: incident energy is directly proportional to arcing time. The arc-flash boundary grows less, from about 3.6 ft to 5.7 ft, because energy falls off steeply with distance. A breaker setting change that shaves 100 ms off clearing can move a task into a lower PPE category without touching the hardware.
Which standard governs this
The Lee and Doughty equations are documented in NFPA 70E Annex D. IEEE 1584 is the governing standard for a full arc-flash calculation. NFPA 70E sets the workplace requirements: risk assessment, approach boundaries, PPE, and equipment labeling.
What this calculation does not account for
The Lee equation is a theoretical open-air model and tends to be conservative. The Doughty equations used at 600 V and below are empirical, and valid only within the 16–50 kA range they were fitted to. Neither models electrode configuration, arc gap or enclosure size the way the IEEE 1584 arcing-current method does. This is a screening estimate only. Labeling and PPE selection require a full study by a qualified engineer.
Common mistakes
Treating a screening number as a labeling result. Using a clearing time from the device curve without accounting for the arcing current being lower than bolted fault current, which can put the device on a slower part of its curve. Assuming higher fault current always means higher incident energy — sometimes it clears the device faster and lowers it.
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