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Arc Flash Calculator

Incident energy and arc flash boundary for 208 to 600 V equipment, IEEE 1584-2018

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System

V

Three-phase line-to-line, 208 to 600 V

kA

Three-phase prospective fault current at the equipment, 0.5 to 106 kA

Arc Duration

ms

The upstream device's clearing time at the arcing current shown in the results, not at the bolted current

ms

Clearing time at the reduced arcing current shown in the results. It is usually longer than the time above, because the lower current sits on a slower part of the curve, and it often governs.

Equipment

Fills the typical gap, enclosure and working distance (IEEE 1584-2018 Tables 8 and 10)

VCB suits most switchboards; HCB (horizontal busbars pointing at you) gives the highest energy

mm

Between phase conductors, 6.35 to 76.2 mm

mm

Arc to face and chest, at least 305 mm: 609.6 mm for LV switchgear, 457.2 mm for MCCs, panelboards and junction boxes (Table 10)

mm
mm
mm

203 mm or less with height and width under 508 mm counts as shallow

Arc Flash

Enter the bolted fault current and both arc durations to see the incident energy.

How It's Calculated

IEEE 1584-2018 fits its equations to arc tests in five electrode configurations. Below 600 V the method takes four steps:

  1. Arcing current at 600 V, Eq. (1), with the bolted fault current in kA and the gap in mm:
  1. Arcing current at the system voltage in kV, Eq. (25), and the reduced arcing current from the variation factor , Eq. (2):
  1. Enclosure correction, Eqs. (11) to (15): the enclosure's width and height give an equivalent size in inches, and a quadratic in it gives (1 for open air). Bigger boxes let the arc spread and lower the energy; shallow boxes concentrate it.
  2. Incident energy and boundary, Eqs. (6) and (10), with in ms and the working distance in mm. The arc flash boundary is the distance at which falls to 1.2 cal/cm²:

Both arcing current cases are run with the clearing time at that current, and the higher incident energy is the result. The 2700 V and 14.3 kV equations of the standard are for medium voltage and are not used below 600 V.

Assumptions: a three-phase arc in a three-phase, 50 or 60 Hz AC system from 208 to 600 V (IEEE 1584-2018 4.2); bolted fault current from 0.5 to 106 kA; gaps from 6.35 to 76.2 mm; working distance at least 305 mm; enclosure width at least four times the gap, with heights and widths above 1244.6 mm taken as 1244.6 mm (4.8.1). The PPE category is a guide from the NFPA 70E thresholds (1.2, 4, 8, 25 and 40 cal/cm²); select clothing with an arc rating at least equal to the incident energy.

Worked Example

IEEE 1584-2018 Annex D.2: a 480 V switchboard with 45 kA bolted fault current, VCB conductors with a 32 mm gap in a 610 × 610 × 254 mm enclosure, a working distance of 609.6 mm, clearing in 61.3 ms at the full arcing current and 319 ms at the reduced one:

  • , so at 480 V
  • Equivalent enclosure size 24.016 in (typical),
  • Full case: (2.77 cal/cm²), boundary 1,029 mm
  • , so
  • Reduced case: (12.70 cal/cm²), boundary 2,669 mm

The reduced arcing current takes 319 ms to clear instead of 61.3 ms, so it gives over four times the energy and governs. The calculator reproduces every figure of Annex D.2. As a guide that is PPE category 3 (arc rating of at least 25 cal/cm²).

Incident Energy by Clearing Time

Incident energy in cal/cm² for a 415 V switchboard: VCB, 32 mm gap, 508 mm enclosure, 609.6 mm working distance, full arcing current. The clearing time matters more than the fault level.

Clearing time (ms)10 kA bolted25 kA bolted50 kA bolted
200.20.61.0
500.51.42.4
1001.12.84.8
2002.15.69.6
5005.313.924.0

Frequently Asked Questions

Arc Flash Questions Answered

How is arc flash incident energy calculated?

IEEE 1584-2018 first works out the arcing current from the bolted fault current, the conductor gap and the electrode configuration, then the incident energy at the working distance from the arcing current, the arc duration and an enclosure size correction. Energy grows linearly with time, so the protective device's clearing time is usually the biggest lever.

Why are there two arcing currents?

The arc current varies from one arc to the next. The standard runs a second case at a reduced arcing current, Iarc × (1 − 0.5 VarCf), because a lower current can sit on a slower part of the protective device's curve and clear much later. You report whichever case gives the higher incident energy.

What arc duration should I enter?

The total clearing time of the upstream protective device at the arcing current, read from its time-current curve, including any intentional delay. Do not use the clearing time at the bolted fault current: the arcing current is lower, often 50 to 90% of it at low voltage, and may not reach the instantaneous trip.

What is the arc flash boundary?

The distance from the arc at which the incident energy falls to 1.2 cal/cm² (5.0 J/cm²), the onset of a second-degree burn on bare skin. Anyone inside it needs arc-rated clothing rated for the incident energy at their working distance.

Which electrode configuration should I choose?

VCB (vertical conductors in a box) fits most switchboards and panelboards. VCBB applies where vertical conductors end on an insulating barrier, such as some breaker line-side terminals. HCB, horizontal conductors pointing at the worker, drives the arc plasma outward and gives the highest energy. VOA and HOA are open-air conductors with no enclosure.

Does IEEE 1584 apply in Australia and New Zealand?

There is no AS/NZS standard for calculating incident energy, so IEEE 1584 is the method used in practice; AS/NZS 4836 and the ENA NENS 09 guideline on arc flash PPE expect the arc hazard to be assessed before live work. NFPA 70E PPE categories are an American scheme; many Australian sites set their own PPE levels against the calculated incident energy.

Is the calculator valid for 690 V, single-phase or DC systems?

No. This calculator covers three-phase AC from 208 to 600 V, bolted fault currents from 0.5 to 106 kA and gaps from 6.35 to 76.2 mm, the low-voltage range of IEEE 1584-2018. Single-phase and DC arcs need other methods, and 690 V systems need the standard's medium-voltage equations, which interpolate between 600 V, 2700 V and 14.3 kV.

Reviewed by Suheib Sherif, electrical engineer at CableHero. The calculator is a design aid: the designer remains responsible for checking the installation against the standards.

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