Short answer: with every other input identical, IEEE 1584-2018 calculates less incident energy for a shallow enclosure — one up to 203.2 mm [8 in] deep — than for a typical one, because the enclosure's side walls have less depth to focus the arc plasma toward the opening. The box does not absorb the arc; it aims it, and a shallow box is a shorter nozzle. One sentence in the guide's test annex reports the opposite trend in measured tests, and this article shows you how to handle that before it reaches your label.
What the depth term does in the calculation
The 2018 edition of IEEE 1584 computes incident energy for an arc inside a metal box using an enclosure size factor, applied to the three enclosed electrode configurations: VCB, vertical electrodes inside a box; VCBB, vertical electrodes ending in an insulating barrier inside the box; and HCB, horizontal electrodes inside a box, pointing at the opening. Open-air configurations, VOA and HOA, have no box, so depth never enters their results. The guide sets the typical enclosure and the typical electrode gap from your equipment class and voltage, and you can also enter the box's real height, width and depth. Volume itself is not an input; the fitted coefficients work on the three dimensions.
Switch the class from typical to shallow and change nothing else — same bolted fault current, same gap, same working distance, same clearing time, same configuration — and the calculated energy drops. The equations are curve fits to the 2018 laboratory test program, so the drop comes out of the fit. Of the three enclosed configurations, HCB reads highest because it points the plasma at the opening. If you work with MCC buckets, the same size factor carries the load there.
| Enclosure depth | Class the model runs | Effect on calculated incident energy |
|---|---|---|
| Open air (VOA, HOA) | No enclosure | No size factor; depth never enters |
| Up to 203.2 mm [8 in] | Shallow | Lower than typical at identical inputs |
| Just over that | Typical | Baseline for the equipment class |
| Over 228.6 mm [9 in] | Typical; the annex's deep test group | The annex reports these tests measured lower than the shallow group — the known conflict |
Why a shallow box throws less energy at you
Drop the picture of the box as a vessel the arc fills up. A three-phase arc clears in tens of milliseconds — one 60 Hz cycle is 16.7 ms — and in that window the enclosure walls channel the arc's plasma and hot vapor out through the opening the way a nozzle collimates a jet. The more wall length runs ahead of the arc, the tighter the jet is when it leaves the box.
Two effects compete when you shrink the depth. The electrodes sit closer to the opening, which raises the energy arriving at your working distance. The walls lose focusing length, which lowers it. The 2018 model returns the net of the two, and the net is the second effect: less wall, less focused jet, lower calculated energy at the same distance. Your study software shows exactly this when you flip the depth input and nothing else.
The annex sentence that reads the other way
The guide's annex on the test program reports that measured incident energy was higher for enclosures no deeper than the shallow line above than for enclosures deeper than 228.6 mm [9 in]. That is the opposite direction from the equation, and the two figures leave a 25.4 mm [1 in] band between them that belongs to neither group — the mark of two specific test series being compared, not a continuous trend with depth.
Two explanations are live among users of the guide: the sentence is printed backwards, or the two series differed in more than depth, such as electrode gap, voltage or configuration. Neither changes what you calculate. The equations are the calculation; the annex sentence is a report of one comparison. Handle it at the label instead: for any box at or under the shallow line, run the study both ways and post the higher result.
Checking the class for a 480 V panelboard in a 200 mm deep box
Inputs, with where each comes from:
- Panelboard: 480 V, three-phase, the standard study object for this question.
- Enclosure: a stainless wall box with a 16 x 16 in face and 200 mm of depth, the dimension taken from your drawing; rated NEMA 4X / IP66 by the manufacturer, not UL listed.
- Electrode configuration: enclosed, so the size factor applies — all three enclosed configurations take it.
- Held constant between runs: voltage, bolted fault current, electrode gap, working distance, clearing time, electrode configuration.
The rule in words: the guide runs an enclosure as shallow up to that depth and as typical beyond it.
The arithmetic: 200 mm divided by 25.4 mm per inch gives 7.87 in, which sits 0.13 in [3.2 mm] under the line. A 205 mm box works out to 8.07 in and crosses into typical, so 5 mm of depth flips the class.
The result: the study returns two numbers from one model, the shallow run lower and the typical run higher, with the enclosure size factor the only difference between them. Both are legitimate outputs; the label takes one.
The decision: post the typical-run number, the higher of the two, while the annex conflict stands, and keep the shallow run in the report as the model's direction. Working rule — at or under the shallow depth, run both classes; deeper than that, typical alone.
What depth still has to do in the panel
Depth buys wiring room, and the wiring rules push back hardest. Wire bending space at terminals is set by conductor size under NEC 312.6, and how to size an electrical enclosure works both the bending and the fill numbers for a real layout. NEC 312.8 caps how full a box holding switches or overcurrent devices may be.
Heat pushes the same way. A stainless wall sheds about 3.7 W per square metre per kelvin of temperature rise, so a shallower box with less surface runs hotter at the same losses. And if the box also lands several energized circuits, each adds to the incident-energy question in its own right.
A display panel sits exactly on this class line, and custom enclosures built to your depth settle the mechanical side of it.
What to put on the drawing and in the study
Give the study the box you will actually hang: inside depth, width and height, the electrode configuration you assumed, and, for a shallow box, both class runs with the higher one flagged for the label. The working space in front is set by voltage and the equipment, and the enclosure mounting guide covers how NEC 110.26(A) asks for room to open the door at least 90 degrees whatever depth you choose.
On the order, set the depth to the layout and let the study follow the box. Custom enclosures are built to your depth from one piece, delivered in 10 business days, counted for custom work from your approval of the drawing. If you are still choosing the box itself, the enclosure selector narrows the candidates by inside dimensions and mounting clearance. And the label governs live work: match the task to the label and your site's energized-work rules before anyone opens the door on a running panel.
Enclosures for this job
Catalog sizes that suit the job this article answers, picked from its subject; open one for its full specifications.
| Enclosure | Material | Outside H x W x D / display | Rating |
|---|---|---|---|
| 12 × 10 × 6 in | 304 stainless | 12 x 10 x 6 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 20 × 16 × 8 in | 304 stainless | 20 x 16 x 8 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 24 × 20 × 8 in | 304 stainless | 24 x 20 x 8 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
Questions
Which electrode configurations does the enclosure depth term apply to?
The three enclosed ones: VCB, vertical electrodes in a box; VCBB, vertical electrodes ending in an insulating barrier in the box; and HCB, horizontal electrodes pointing at the opening. Open-air configurations, VOA and HOA, carry no enclosure size factor, so depth changes nothing there. HCB calculates the highest energy of the three because it aims the plasma straight at the opening.
Is the lower shallow-enclosure number safe to put on an arc-flash label?
Post the higher run. If your box sits at or under the shallow line, calculate it once as shallow and once as typical, and label with the typical result while the guide's annex reports the opposite test direction. Deeper than that, typical is the only class and the single run stands.
Does the shallow class change the arc-flash boundary too?
Yes, in the same direction. The boundary sits where incident energy falls to 1.2 cal/cm2 [5 J/cm2], so the lower shallow-class energy pulls the boundary closer to the box. Labeling with the higher typical-run result keeps the larger boundary as well, which is the safer pair to hand the electrician.
Should I order a shallower box to cut the arc-flash number?
Choose depth for the wiring and the heat, then let the study follow the box. Wire bending space at terminals is set by conductor size under NEC 312.6, and a shallower stainless box has less wall area to shed heat, about 3.7 W per square metre per kelvin for stainless. A label trimmed by a class change saves nothing if the panel then needs a deeper redesign.
