Arc-Flash Incident Energy Calculator
Incident energy is the thermal energy an arc would deliver to a surface at a working distance, in calories per square centimetre. 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.
Run the numbers
The working calculator is free and needs no signup.
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 | The Lee method applies above 600 V; below that, IEEE 1584 empirical models apply |
Worked example
Doubling the clearing time from 0.1 s to 0.2 s does not double incident energy — it roughly quadruples it, because energy scales with the square of time. A breaker setting change that shaves 100 ms off clearing can move a task from one PPE category to a lower one without touching the hardware.
Which standard governs this
The theoretical Lee equation is documented in IEEE 1584, which is the governing standard for arc-flash calculation. NFPA 70E sets the workplace requirements: risk assessment, approach boundaries, PPE, and equipment labelling.
What this calculation does not account for
Lee is a theoretical open-air model and is conservative below 600 V, where IEEE 1584's empirical equations fit measured data far better. It does not model arc-in-a-box enclosure effects, electrode configuration or arc gap. This is a screening estimate only. Labelling and PPE selection require a full study by a qualified engineer.
Common mistakes
Treating a screening number as a labelling 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.