Model the MCC as one box. Enter the enclosure size class the standard gives for a 480 V motor control center — about 20 in [508 mm] wide and 50 in [1270 mm] high — and run one calculation for the lineup. Every bucket door then carries the same incident energy label. The buckets are compartments in one connected air space, and IEEE 1584-2018 has no input for internal partitions. Box size does move your number, but between enclosures: the 2018 equations are normalized to a 20 x 20 x 20 in [508 mm] box, and they push more energy toward you as the box around the arc gets smaller. Here is what the size input means, which dimensions to enter, and what moved against the old 2002 model.
What the 2018 model asks you for
The 2018 edition calculates incident energy from seven inputs: voltage, bolted fault current at the equipment, arc duration from the upstream device's clearing time, conductor gap, electrode configuration, working distance and enclosure size. The method spans 208 V through 15 kV. The 2002 equations it replaces knew one world: vertical electrodes inside a box, with the box effectively fixed at the typical normalized cube. The 2018 set widened that. You pick the electrode configuration — vertical electrodes in a box (VCB), the classic panelboard and MCC assumption; vertical electrodes ending in an insulating barrier (VCC), like a stab landing on a bus; or horizontal electrodes aimed at the opening (HCB), the one that puts the most energy on you.
Enclosure size enters through a correction factor. The equations are normalized to that same reference box, and the factor adjusts the result when your box is bigger than typical: more volume spreads the arc's energy, and the number falls. Smaller boxes do the reverse. A shallow enclosure — depth of 8 in [203 mm] or less — is its own class and reads high, because the arc sits close to the opening and close to you.
Why one bucket does not get its own number
An MCC is a structure, not a row of enclosures. The buckets plug or bolt onto a vertical bus, and the wireways and bucket openings leave the interior as one connected air volume; bucket walls are functional partitions, not sealed pressure boundaries. When an arc strikes at a stab or a lug, the plasma and hot gas expand into that volume. The box that bounds the arc is the structure, and that is the box the model wants.
The electrical side points the same way. Every bucket lands on the same vertical bus behind the same incoming protection, so the bolted fault current, the arcing current and the clearing time are the same at every bucket. Same inputs, same equation, same number: model the lineup as one node and put one label on every door. Running the biggest bucket, or all six separately, gives you six ways to be wrong — the model has no partition input, so a bucket-sized box over-concentrates an arc that would actually spread into the section.
Two things do split an MCC into more nodes. A genuinely isolated section — full metal barriers, gasketed covers, its own air volume, such as an incoming or utility section — gets its own dimensions and its own calculation. So do separate lineups fed from different sources. For the smaller version of the same question, one box holding several circuits, the numbers are in arc flash with multiple circuits in one junction box.
The dimensions you enter
Take the enclosure size from the equipment you are labeling, never from the pieces inside it. The standard prints a table of typical enclosure sizes by voltage and equipment type, and your study software offers the same list on the equipment screen.
| Equipment you are labeling | Enclosure size to enter | Why |
|---|---|---|
| 480 V MCC lineup | The MCC class from the standard's table (a structure 50 in [1270 mm] high) | The buckets share the structure's connected air space |
| LV switchboard or switchgear section | The class the standard lists for that equipment | One structure, one air volume, same logic |
| Panelboard or junction box | The box's real width, height and depth | It is one enclosure, however many circuits it holds |
| Wall-mount starter or drive box | Real outside dimensions; at 8 in [203 mm] deep it is in the shallow class | One enclosure at the shallow boundary |
A six-section lineup of class-width sections is 120 in [3048 mm] of steel, and you still enter the MCC class rather than 120 in of width: the model was built on single-section boxes, and the class carries the geometry the equations expect. For single boxes, real dimensions win. Our 20 x 20 x 8 in stainless wall-mount enclosure sits exactly on the shallow-class depth, the geometry that reads high — worth knowing before you hang a drive starter in one. If you are choosing that box in the first place, the enclosure selector takes the environment and the DIN module count, and how to size an electrical enclosure covers the physical clearances. Arc flash adds its own input on top of both.
Re-labeling a 480 V MCC with the 2018 equations
The inputs: a 480 V MCC lineup with six buckets, starters through one drive bucket, fed from one incoming section. Enclosure entry: the MCC class from the standard's typical-enclosure table, as entered above. Electrode configuration: vertical electrodes in a box. The old label, from the 2002 equations, reads 8.0 cal/cm² at the working distance — exactly NFPA 70E's category 2 minimum arc rating of 8 cal/cm². Comparisons published with the 2018 guide put a typical 480 V box about 10 to 15 percent above the old model.
The rule: hold the fault current, clearing time, gap and working distance where they were, and let the model change move the number. As a check before the software run, scale the old typical-box value by 1.10 to 1.15. The software run is the number of record, because the electrode configuration you pick can move it far more — horizontal electrodes aimed at the opening have landed more than a factor of two from the old model.
The arithmetic: 8.0 cal/cm² × 1.10 = 8.8 cal/cm², and 8.0 cal/cm² × 1.15 = 9.2 cal/cm².
The decision: 9.2 cal/cm² clears the 8 cal/cm² category 2 rating, so plan category 3 clothing at 25 cal/cm², or an incident-energy label reading 9.2 cal/cm² at the stated working distance. Print one label set for the lineup and put it on all six bucket doors. If the energy moved, the boundary moved with it — read both off the same run, and match the suit to the task the way arc flash PPE for junction boxes lays out.
What to record, and what to order
Write the enclosure basis into the study, node by node: enclosure size class, electrode configuration, conductor gap, working distance, and the incident energy and boundary that came out. The door label carries the incident energy, the working distance it applies at, and the arc flash boundary. Anyone re-running the study after a bucket swap needs those five entries to reproduce your number.
A replacement box keeps the study valid only if its dimensions match. When a starter or a drive moves to its own stainless wall-mount enclosure, order the size you modeled rather than the size that happens to fit the wall: our boxes are built to your dimensions and delivered in 10 business days, counted from your approval of the drawing. If the fault can point at the worker instead of up a bus, read what an arc flash rated enclosure is and when it applies, and set the internals per arc flash compliant enclosure wiring. One caution to close: the label sizes PPE for the arcing fault only. Work de-energized where you can, lock it out, and test for absence of voltage before your hands go in.
Enclosures in this article
| Enclosure | Material | Outside H x W x D / display | Rating |
|---|---|---|---|
| 20 × 20 × 8 in | 304 stainless | 20 x 20 x 8 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
Questions
Should you run a separate arc flash calculation for each MCC bucket?
No. Every bucket lands on the same vertical bus behind the same incoming protection, so the bolted fault current, arcing current and clearing time are identical at each one — and so is the incident energy. Model the MCC as a single node using the MCC class from the standard's table, and put the same label on every door.
Does a bigger enclosure lower incident energy?
Yes, at the same current and clearing time. The 2018 equations are normalized to a 20 x 20 x 20 in [508 mm] reference box, and the enclosure size correction factor lowers the result as the box grows, because the same arc fills more volume. In the other direction, shallow boxes with depth of 8 in [203 mm] or less read high, since the arc sits close to the opening and to you.
Why did our incident energy rise after re-running the 2002 study?
Two inputs appeared: enclosure size and electrode orientation. On the standard's typical reference box at 480 V, the 2018 equations land roughly 10 to 15 percent above the 2002 set. With horizontal electrodes aimed at the opening, published comparisons differ by more than a factor of two. Budget for relabeling, and expect PPE categories to move at boundaries such as 8 cal/cm².
What if the MCC compartments really are sealed?
Then each sealed compartment is its own enclosure and gets its own node with its own dimensions. That is rare: bucket walls and wireways normally leave the structure's air connected, which is exactly why one class covers the lineup. Look for full metal barriers with gasketed covers before you split a lineup into per-section calculations.
Does the arc flash boundary move with enclosure size too?
Yes. The boundary is the distance at which incident energy falls to 1.2 cal/cm² [5.0 J/cm²], and it follows the energy outward as the box shrinks or the electrodes aim at the opening. Take it from the same software run as the energy value; it scales with a distance exponent, so it does not track energy one-to-one.
