VCB Electrode Configuration: Vertical Conductors in a Metal Box

What VCB means physically

VCB stands for vertical conductors in a metal box: the bus bars or cable terminations run vertically inside an enclosure. This is the construction of most low-voltage panelboards, switchboards, and metal-clad switchgear lineups. IEEE 1584-2018 defines VCB in Clause 4.3 (Table 9) as one of five electrode configurations, and each configuration carries its own row of equation coefficients — the configuration is not a label, it is a different model of how the arc plasma behaves.

How VCB shapes the incident energy

With vertical conductors, the arc plasma is driven upward along the bus run, and the enclosure walls focus the energy out through the opening — toward the worker standing in front. Two consequences follow. First, the box matters as much as the bars: the 2018 method applies an enclosure-size correction factor for the boxed configurations (VCB, VCBB, HCB), because a smaller enclosure concentrates the exposure at the working distance. Second, VCB sits mid-pack among the five configurations at typical low-voltage inputs — neither the conservative worst case nor the forgiving one.

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, normalization enclosure (correction factor 1) — our calculator's engine computes for VCB: incident energy 9.53 cal/cm² and arc-flash boundary 65.5 in. The incident-energy method therefore requires an arc rating of at least 9.53 cal/cm²; it does not assign a PPE category. At the same inputs the other configurations compute: VCBB 13.25, HCB 19.51, VOA 4.49, and HOA 9.46 cal/cm². These are single-point results, not universal rankings.

When to select VCB in the calculator

Select VCB when the equipment is an enclosed assembly with conductors running vertically and no insulating barrier between phases at the termination — most panelboards, switchboards, and vertical bus switchgear fit here. If the termination sits behind an insulating barrier, that is VCBB. If the bus runs horizontally in the box, that is HCB. When the construction is unclear, get it from the one-line, the equipment drawings, or a photo of the interior taken from a safe distance during a de-energized inspection — the configuration choice can move the result by a factor of two, so it deserves more than a guess.

Good to know: VCB is the configuration behind the worked example on our arc flash boundary page, so the numbers there and here are directly comparable. The engine always evaluates both the full and the reduced arcing current, because the reduced current can trip the protective device more slowly and produce the worse case.

Diagram

Diagram placeholder: cross-section of a vertical bus run inside a panelboard enclosure, plasma jet direction marked, worker position at the 455 mm working distance. Ships with the site diagram set; this page is text-complete without it.

The other four configurations

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