Arc flash incident energy
The calculation that decides what an electrician wears when they open a panel. It is an empirical regression on laboratory arcs, and it is the reason NFPA 70E exists in its current form: before it, incident energy was estimated from a table of fault current and time, and people were burned.
| Quantity | Value | Note |
|---|---|---|
| System voltage | 480 V | low voltage, 2002 model |
| Bolted fault current | 25.0 kA | from the short-circuit study |
| Electrode gap | 25 mm | IEEE 1584 Table 4 |
| Configuration | In a box | K = −0.097, K1 = −0.555 |
| Grounding | Solidly grounded | K2 = −0.113 |
| Arcing current | 14.34 kA | the regression output |
| Normalised energy | 4.07 cal/cm² | at 610 mm, 0.2 s |
| Working distance | 455 mm | |
| Distance exponent x | 1.641 | 1.641 in a box below 1 kV |
| Clearing time | 0.100 s | protection operating time |
| Incident energy | 20.67 cal/cm² | 86.5 J/cm² |
| Arc flash boundary | 2579 mm | 2.58 m |
| PPE category | 3 | Category 3 |
| Time for 8 cal/cm² | 39 ms | the protection target |
The clearing time is the variable the electrical engineer actually controls. Incident energy is linear in it, so halving the protection time halves the hazard — which is why arc-rated switchgear and faster relays matter more than warning labels.
lg Ia = K + 0.662 lg Ibf + 0.0966V + 0.000526G + 0.5588V lg Ibf − 0.00304G lg Ibf · V in kV, G in mm.
lg En = K1 + K2 + 1.081 lg Ia + 0.0011G · At 610 mm and 0.2 s.
E = 4.184 Cf En (t/0.2)(610^x/D^x) · Cf = 1.5 below 1 kV.
DB = [4.184 Cf En (t/0.2) 610^x / 1.2]^(1/x) · Where E = 1.2 cal/cm².
A clear path from inputs to results.
The calculation that decides what an electrician wears when they open a panel. It is an empirical regression on laboratory arcs, and it is the reason NFPA 70E exists in its current form: before it, incident energy was estimated from a table of fault current and time, and people were burned.
How to use
- Enter the system voltage and the bolted fault current.
- Set the electrode gap and the working distance.
- Enter the clearing time and whether the equipment is enclosed.
- Read the incident energy, the arc flash boundary and the PPE category.
The formula
Reference: IEEE 1584-2002, Guide for Performing Arc-Flash Hazard Calculations; NFPA 70E for the PPE categories.
Worked example
A 480 V panel with 25 kA of fault current
- 480 V, 25 kA bolted fault, 25 mm gap.
- 455 mm working distance, 100 ms clearing, in an enclosure, solidly grounded.
- 1
The arcing current regresses to 14.34 kA, about 57% of the bolted fault.
- 2
The normalised energy at 610 mm and 0.2 s is 43.4 cal/cm².
- 3
Scaling to the 455 mm working distance with x = 1.641 and the 100 ms clearing time gives 20.67 cal/cm².
- 4
That is 87 J/cm², which puts the task in PPE Category 3.
- 5
The arc flash boundary, where the energy falls to 1.2 cal/cm², is 2.58 m from the equipment.
14.34 kA arcing · 20.67 cal/cm² · 2.58 m boundary · Category 3.
Interpreting the result
Clearing time is the variable the electrical engineer actually controls, and incident energy is linear in it. Halving the protection time halves the hazard, which is why arc-rated switchgear and faster relays matter more than warning labels — and why the calculation is really a protection-coordination study in disguise.
Assumptions & limitations
- The IEEE 1584-2002 empirical equations, valid from 208 V to 15 kV.
- A three-phase arc in a box or in open air, with the standard electrode gap.
- The clearing time is the protection operating time, not the arcing duration.
- No current-limiting devices and no arc-rated mitigation.
Frequently asked questions
Why use the 2002 model rather than 2018?
Because it is what the NFPA 70E tables and most installed software still use, and because the two differ by 30% or more on some configurations — a live argument in the industry.
What is a Category 3 task?
PPE rated for 8 to 25 cal/cm²: an arc-rated flash suit over natural-fibre clothing. Above 40 cal/cm² no PPE is considered adequate and the work should be de-energised.
What is the arc flash boundary?
The distance at which the incident energy falls to 1.2 cal/cm², the threshold for a second-degree burn. Anyone inside it needs arc-rated PPE.
Does the arcing current always equal the bolted fault?
No, and that is the point of the regression. An arc has impedance, so the arcing current is typically 50–60% of the bolted value — which changes the energy substantially.
Methodology last reviewed 2026-10-22. Results are informational and not financial or engineering advice — see our disclaimer.