Incident Energy Calculation Example: IEEE 1584-2018, Step by Step

The best way to trust an incident-energy number is to watch it being built. This page walks one panel through the IEEE 1584-2018 process with the actual intermediate values — computed by our own engine, which implements the 2018 model including the reduced-arcing-current branch. The inputs are realistic for a small industrial 480 V panel; every figure shown is an engine output, not a rounded illustration. Last reviewed: July 18, 2026.

The inputs

Input	Value	Where it comes from
Nominal voltage	480 V	Equipment nameplate
Bolted (available) fault current	25 kA	Utility letter, nameplate estimate, or point-to-point calc — see the fault-current guide
Electrode configuration	VCB — vertical conductors in a metal box	Panel construction (configuration guide)
Gap between conductors	32 mm	Equipment class typical / measurement
Working distance	455 mm (18 in)	Standard low-voltage panel assumption
Clearing time	200 ms	Upstream protective device at the arcing current
Enclosure	Normalization size (correction factor 1)	Dimensions not supplied

Step 1: bolted fault current to arcing current

An arc is not a bolted fault. The arcing current is lower than the 25 kA bolted value, because arc impedance throttles the current. At 480 V with a 32 mm gap in the VCB configuration, the 1584-2018 arcing-current equation gives:

I_arc = 18.3 kA (from 25 kA bolted)

Step 2: the reduced-current branch

This is the step cheap calculators skip, and it changes real answers. The 2018 model also computes a reduced arcing current — because fault current varies, and a lower current can trip the protective device more slowly. The engine computes:

I_arc,min = 16.1 kA (about 88% of the full arcing current at these inputs)

Both branches are then evaluated: incident energy at I_arc with its clearing time, and at I_arc,min with its clearing time. The worse case governs the result. Here, with the same 200 ms clearing entered for both, the full-current branch governs — but if your device's trip at 16.1 kA is slower than at 18.3 kA (it usually is), that slower time belongs in the second input, and the reduced branch can take over. This is why one clearing-time field is not enough.

Step 3: incident energy at the working distance

With the arcing current, 200 ms clearing, VCB coefficients, and the 455 mm working distance, the incident-energy equation gives:

Incident energy = 9.5 cal/cm² at 455 mm

Under the incident-energy method, the result is not converted to a PPE category. The selected arc-rated clothing system must have a system rating of at least 9.5 cal/cm², and a qualified person must complete the task-specific PPE selection. A tested 12 cal/cm² system may satisfy the arc-rating requirement, subject to the rest of that assessment.

Step 4: the arc-flash boundary

Solving the same equation for the distance at which energy falls to 1.2 cal/cm² (the second-degree-burn threshold):

Arc-flash boundary = 65.5 in (about 5 ft 5 in)

Anyone inside that line during the task belongs inside the arc-flash risk assessment. At the stated working distance, the selected arc-rated system must be rated at least 9.5 cal/cm²; this calculated result is not Category 3. The boundary math is covered in depth on the arc flash boundary page.

The full picture, one table

Quantity	Value
Bolted fault current (input)	25 kA
Arcing current	18.3 kA
Reduced arcing current	16.1 kA
Clearing time (both branches)	200 ms
Incident energy at 455 mm	9.5 cal/cm²
Arc-flash boundary	65.5 in
PPE selection basis	Incident-energy method; arc rating ≥ 9.5 cal/cm²

What to notice

• Clearing time is the biggest lever. Re-run at 100 ms and the energy roughly halves. Settings and device choice move the answer more than any other input.
• The reduced branch is not optional. It exists because optimistic current assumptions understate hazard — the NEMA trap covered in the fault-current guide.
• Every value is inspectable. Run this exact panel in the free arc flash calculator and you will see the same intermediates — that is the point of showing them.

For how these numbers land on the physical label, see how to read an arc flash label.