Short answer: category 2 means 8 cal/cm² or less at the working distance, and putting a box around open gear can push the calculated number up, not down. Below 1 kV, the 2002 edition of IEEE 1584 calculated an arc in an enclosure at three times the open-air figure. Cladding an open starter bank can therefore move its label from category 2 to category 3 before anyone touches a wire. Model both cases before you order a single box, cut clearing time where the enclosed result tops 8, and put fuse access behind a door-interlocked disconnect. Here is the whole retrofit job, the arithmetic and the checks you run at the end.
What a category 2 room costs in calories
NFPA 70E picks the PPE category from incident energy at a stated working distance, and category 2 tops out at 8 cal/cm². Category 1 covers up to 4 cal/cm², category 2 runs from just over 4 to 8, and the clothing must be arc-rated at least 8 cal/cm². A room you "enter at category 2" really means no equipment label in it exceeds 8 at its own working distance, because the standard labels equipment, not rooms. The second number on each label is the arc flash boundary: OSHA's arc-flash guidance, following NFPA 70E, sets that line where incident energy falls to 1.2 cal/cm², about the onset of a second-degree burn. Keep aisles outside those boundaries until each bank is boxed.
Working distance moves the answer as much as the calories do. IEEE 1584 assumes 18 in [455 mm] at panelboards and motor-control buckets, and 24 in [610 mm] at low-voltage switchgear. Energy falls with the square of distance, so doubling your working distance cuts the figure to a quarter.
Why the box can raise the number
IEEE 1584 calculates an open build as an arc in open air and a boxed build as an arc in a box, with the walls aiming the plasma at the person at the door. The 2002 edition applied a flat multiplier of 3 below 1 kV. The 2018 edition split that into two inputs you control — how enclosure size enters the IEEE 1584 calculation works the detail.
Electrode configuration comes first. Vertical electrodes in a box (VCB) calculate lowest; electrodes terminating on a barrier (VCBB) come next; horizontal electrodes aimed at the opening (HCB) calculate highest. Your as-built sketch decides which one the model uses. Box size comes second: the 2018 model derives an equivalent box from its height and width, banded at 20 in [508 mm], 26 in [660 mm] and 49 in [1245 mm]. The smaller the band, the more the arc concentrates at the opening, so a bigger footprint means a quieter calculation. Depth pulls the other way, and why shallow enclosures calculate lower arc flash incident energy explains it.
A standard box does all this while staying a standard box. Arc containment is a different product class — switchgear type-tested to IEEE C37.20.7 — and what an arc-flash-rated enclosure is and when it applies draws that line. Your retrofit lowers the calculated exposure; it is not arc containment.
Survey the room before you price one box
Six inputs per bank, and you can plan the whole job from one walk-down. Lock out each bank before you open anything to sketch the bus — under NFPA 70E, taking an energized cover off is energized work.
| What to collect | Where it comes from | What it decides |
|---|---|---|
| Incident energy and working distance | the arc flash study | which banks already sit at or under 8 cal/cm² |
| Bolted fault current at the line side | short-circuit study | the starting point of both models |
| Clearing device and clearing time | study and trip settings | how much time you can cut |
| Electrode geometry | the locked-out sketch | VCB, VCBB or HCB in the model |
| Tasks needing an open cover | operations list | fuseways, resets, tuning ports |
| Wall space and aisle width | room walk-down | the biggest box you can hang |
Expect a surprise in the data: several open banks will already calculate under 8 cal/cm², because few plants have checked. Those boxes buy contact protection, and the model only has to keep the number where it already sits.
The retrofit, step by step
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List every open bank. Walk the room and record each wall-mounted drive, starter, disconnect and fuse board, with its feed, its upstream clearing device and its label. Mark anything unlabeled, or labeled before the last change to the fault duty.
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Pull the numbers for each. From the study: incident energy at the working distance, bolted fault current at the line side, clearing time and the device that delivers it. Items with no results get modeled in the next step, not deferred.
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Model both cases per bank. Run the as-built open-air case, then the proposed arc-in-a-box case with the electrode configuration and real box dimensions you intend to build. If you cannot prove the electrodes end free of a barrier, model VCBB and bank the margin. Merging banks into one bigger box changes the source the arc sees, so read how multiple circuits in one box change the calculation before you consolidate.
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Size the box from the model and the code. Take the largest footprint the wall and aisle allow, because the larger band calculates lower. Hold the NEC 110.26(A) working space in front — at least 30 in [760 mm] wide or the equipment width, with room for the door to swing 90 degrees. Give large conductors the bending space NEC 312.6 sets at terminals, and let the enclosure sizing guide work the rest of the geometry.
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Cut clearing time where the enclosed result tops 8. Energy scales almost linearly with time, so halving the clearing time halves the calories. Three levers, cheapest first: a faster fuse class in the upstream device, a tighter instantaneous setting, and an arc-flash maintenance setting that drops trip time only while work proceeds downstream.
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Put energized access behind an interlock. Fuses, reset handles and tuning ports belong in a compartment whose door is interlocked with a disconnect: the door holds until the switch is off, so a fuse change stops being energized work. Keep a door-mounted handle below 6 ft 7 in [2.0 m] at the top of its travel, the limit NEC 240.24(A) sets.
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Re-run the study on the as-built boxes. Final dimensions, final electrode configuration, final trip settings — then read the labels. Pass is at or under 8 cal/cm² at 18 in [455 mm]; aim for 6 or less, because a lower arcing current can stretch clearing time and creep a label upward.
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Label and document. New labels on every box, arc flash boundaries marked on the room plan, and the trip-setting sheet filed in each door pocket. NFPA 70E asks for the arc flash risk assessment to be reviewed at least every five years; book that date now.
Worked example: a starter bank that tripled, then came back
One bank from the survey: a 480 V starter bank, open on the wall, fed from a breaker that clears a 22 kA bolted fault in 0.40 s.
- As-built, open air: the study says 5.0 cal/cm² at 18 in. The bank already meets category 2, and the box is for contact protection.
- Proposed box, 20 x 20 in equivalent, arc in a box at the 2002 factor: 5.0 × 3 = 15 cal/cm². The retrofit triples the number and pushes the label into category 3.
- Fix the time: set the breaker to clear the same fault in 0.15 s. 15 × (0.15 ÷ 0.40) = 5.6 cal/cm², back inside category 2 with 2.4 cal/cm² of margin.
- Fix the size too: a 36 x 30 in box sits in a larger size band than a 20 in one, and the 2018 correction moves in your favor, padding the margin.
The arithmetic scales fast. An open figure of 10 cal/cm² becomes 30 in the box, and the clearing time must drop from 0.40 s to 0.10 s to land at 7.5 cal/cm². That is arc-flash-relay or zone-selective-interlock territory, a hardware change rather than a setting.
Checks once the boxes are hung
- With each disconnect closed, its door holds, and defeating the interlock takes a tool rather than a tie wrap.
- Every label reads at or under 8 cal/cm² at its stated working distance; anything over goes back to step 5.
- Trip settings in the gear match the sheet in the door pocket.
- Clear space in front of 480 Y/277 V gear runs 3.5 ft [1.1 m] deep — the 151 to 750 V-to-ground band under NEC 110.26(A) — and the door swings its 90 degrees.
- The study revision is filed and the five-year review date is on the schedule.
What to order
Order a custom wall-mounted box, built from one piece, in the exact size the model used. Indoors and dry, powder-coated carbon steel does the job; rooms near washdown or food production take 304 stainless, and chloride air takes 316L. The custom enclosure builder takes your width, height, depth and every cutout on one drawing, and the factory cutout process puts door holes for handles and glands in the same build. Use the enclosure selector to check usable size and DIN space before you commit.
On our boxes, a Type 12 dust rating is a manufacturer rating for the specified design — rated by the manufacturer, not UL listed — and NEMA does not test or endorse enclosures. For what goes inside, arc flash compliant enclosure wiring: what to specify covers the layout. Custom work is delivered in 10 business days, counted from your approval of the drawing.
Mistakes that undo the target
| Mistake | What it costs | Do this instead |
|---|---|---|
| Cladding the open front and keeping the old label | open air becomes arc in a box; 5 cal/cm² becomes 15 behind the same label | model the enclosed case first, then re-label |
| Hanging the smallest box that clears the wall | the 20 in band concentrates the arc at the opening | take the largest footprint the aisle allows |
| Fuses behind a bolted cover | cover removal is energized work at the full label value | feed the fuse compartment through a door-interlocked disconnect |
| Reading a NEMA type or IP code as arc protection | types and codes say nothing about calories | treat the enclosure type and the arc flash label as separate documents |
| Merging banks to save boxes | one arc sees a larger source | model the merged case before ordering |
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 PPE categories sit above 8 cal/cm²?
Category 3 runs from just over 8 to 25 cal/cm², and category 4 runs from 25 to 40. Above 40 cal/cm² NFPA 70E lists no PPE category, so energized work is off the table and the exposure has to be engineered down. Clearing time is the lever: halve it and you halve the calories, which is how most retrofits bring a label back inside category 2.
How much does working distance change the calories?
Energy falls with the square of distance. IEEE 1584 assumes 18 in [455 mm] at panelboards and motor-control buckets and 24 in [610 mm] at low-voltage switchgear, so moving a task from 18 to 24 in cuts the incident energy to about 56 percent: 18 divided by 24, squared, is 0.56. A deeper door and recessed parts buy some of that distance for free.
Is a fuse change behind the new door still energized work?
Not when the fuse compartment is fed through a door-interlocked disconnect: the interlock holds the door shut until the switch is off, so the fuses are dead when exposed. A bolted cover is different. With parts live behind it, NFPA 70E treats removing the cover as energized work, needing full PPE at the label's calories and an energized-work permit.
Does the retrofit box need a NEMA 4X rating indoors?
A dry control room needs Type 12, which covers dust and dripping liquids, and nothing more; move to 4X where the room is hosed down or the air carries chlorides. Enclosure types are manufacturer ratings for the specified design — ours are not UL listed — and no type implies a calorie level.
