HOA Electrode Configuration: Horizontal Conductors in Open Air

What HOA means physically

HOA stands for horizontal conductors in open air: the conductors run horizontally with no enclosure — overhead bus runs, horizontal cable in open trays, outdoor horizontal terminations. IEEE 1584-2018 defines HOA in Clause 4.3 (Table 9) as the fifth of its electrode configurations. It combines the two properties that each drive exposure on their own: the horizontal orientation that aims the plasma outward, and the open air that leaves the worker with no enclosure between them and the arc.

How HOA shapes the incident energy

Horizontal electrodes drive the arc plasma outward along the conductor axis — toward a worker standing in front of or below the run. Without enclosure walls there is no focusing effect, but there is also nothing between the worker and the plasma jet. That combination makes HOA the conservative open-air selection: at typical low-voltage inputs it computes roughly double the VOA result. Like VOA, HOA skips the enclosure-size correction factor. The arcing current is the lowest of the five at our common inputs (13.24 kA), which can slow clearing — the reduced-current branch deserves attention on HOA runs.

Engine-verified numbers at a common input set

At a common input set — 480 V, 25 kA bolted fault current, 32 mm gap, 455 mm (18 in) working distance, 200 ms clearing time — our calculator's engine computes for HOA: incident energy 9.46 cal/cm² and arc-flash boundary 50.5 in. The incident-energy selection basis is arc rating ≥9.46 cal/cm², with no category conversion. Comparison at identical inputs: VCB 9.53, VCBB 13.25, HCB 19.51, and VOA 4.49 cal/cm².

When to select HOA in the calculator

Select HOA when the conductors at the work point run horizontally and are genuinely in open air: overhead horizontal bus, open-air horizontal terminations, exposed horizontal cable runs. If the work point is inside any enclosure — even an open-door cabinet — the boxed configurations are the right starting point instead. When an installation mixes orientations, compute per work point; the overhead feed into a building and the panel it lands in are different configurations with different results.

Good to know: if you can only run one open-air case for planning purposes, HOA is the defensible choice — it is the conservative one of the two. For a label, run the configuration that matches the actual construction; conservative planning numbers and label numbers answer different questions.

Diagram

Diagram placeholder: horizontal open-air bus run, plasma directed toward the ground-level worker position at 455 mm. Ships with the site diagram set; this page is text-complete without it.

The other four configurations

• VCB — vertical conductors in a metal box
• VCBB — vertical conductors terminated in an insulating barrier
• HCB — horizontal conductors in a metal box
• VOA — vertical conductors in open air

Run the numbers for your own panel in the free arc flash calculator — electrode configuration is one of the inputs, and every intermediate value is shown for review. Related reading: arc flash boundary explained, PPE selection methods, and arc flash label requirements.