Standard low-voltage motor control centers ship in vertical sections roughly 20 in wide by 90 in tall, and bucket heights of 6 in, 12 in, 18 in, 24 in, 36 in, and 48 in set how many units stack into one section [S4][S7].
Bucket count is dictated by the largest unit in the lineup: a 90 in section filled with 6 in small-feeder buckets holds 15 units, while 18 in starter buckets (the most common size) cap the section at 5 units, and 24 in VFD or larger-motor units drop it to roughly 3 with overhead space [S4][S7].
Standard section envelope and bucket height modules
Industry-standard low-voltage MCC sections are self-supporting welded steel frames, typically 90 in tall, 20 in wide, and either 15 in or 20 in deep, bolted end-to-end to form the lineup [S6]. The 90 in internal height is divided into discrete bucket modules that work in any combination: 6 in (small feeders), 12 in (light loads and small motors), 18 in (very common size), 24 in (larger motors or VFDs), 36 in (large starters or complex controls), and 48 in (high-horsepower or multi-component units) [S4]. Two adjacent sections are typically joined into a shipping split, but every section is laid out independently with its own column number in the MCC elevation view [S3].
Because bucket heights are modular but not fractional, the real question is not "how many buckets fit" but "what is the tallest single unit in the lineup?" That single value sets the bucket count. For an all-feeder MCC using 6 in units, a 90 in section nominally holds 15 buckets. A lineup dominated by NEMA Size 1 or Size 2 FVNR starters in 18 in housings holds 5 buckets per section, with the remaining height consumed by the top horizontal wireway, the bottom horizontal wireway, and the stab assembly [S4][S7]. The relationship is purely arithmetic: bucket count per section equals 90 divided by the dominant unit height, less a small fixed overhead for bus stabs and wireways [S7].
Starter bucket height by NEMA size and load type
NEMA starter sizes 0 through 6 map to specific horsepower envelopes, and larger NEMA sizes physically demand taller buckets [S4]. A NEMA Size 1 FVNR starter for a 7.5 hp motor at 460 V typically fits in a 12 in bucket, while the same starter in a Size 3 frame for a 50 hp motor needs an 18 in or 24 in housing depending on the disconnect type and the control transformer package [S4][S7]. Reversing starters, VFD buckets, soft starter buckets, and combination starters each consume additional vertical space relative to a basic FVNR unit because of the extra contactor, the drive chassis, or the solid-state stack [S4].
The practical rule: count the tallest single starter in the lineup, then back-calculate the maximum bucket count for the rest of the section. A single 36 in reduced-voltage soft starter for a 100 hp pump motor in a section otherwise filled with 12 in feeders leaves that section at 2 starter buckets plus a few feeder buckets, not 5 [S4][S7]. When the tallest unit approaches 48 in (NEMA Size 5 or 6 contactors, or large VFDs), the section effectively becomes a single-bucket or two-bucket section, and many designers will specify a free-standing enclosure rather than an MCC section at that point [S4].
Width direction: 20 in sections and plug-in stab limits
MCC section width is fixed at roughly 20 in across the major low-voltage lines, and that width is not a design variable: it is set by the bus bar geometry, the stab finger spacing, and the finger-safe shrouds required by UL 845 [S6]. What does vary is how that 20 in wide column is split: a single full-width bucket occupies the whole column, while a 20 in section can also be subdivided into two half-buckets side-by-side for small feeders where vertical space is the binding constraint [S3]. The EasyPower elevation view and similar schematic tools let the engineer model column number and starter space, but the underlying 20 in width per section is a hardware constant, not a layout choice [S1][S3].
For a multi-section lineup, total starter bucket capacity scales linearly: a four-section lineup of 18 in starter buckets holds roughly 20 starter units plus the main incoming bucket at the top or bottom, while a four-section lineup of 24 in units holds around 12 to 14 starter units depending on how the horizontal wireways are configured [S4][S7]. The horizontal bus rating (commonly 600 A, 800 A, 1200 A, or 1600 A) and the short-circuit withstand rating (typically 42 kA or 65 kA AIC at 480 V) are independent of bucket count and are instead set by the main lug or main breaker compartment at one end of the lineup [S6].
Comparison: bucket module vs starter count per 90 in section
Selecting a bucket module is a trade between unit count, component density, and accessibility. The table below summarizes the trade for a 90 in tall, 20 in wide section, with the dominant unit height as the independent variable and starter count as the dependent variable: [S7]
6 in (small feeders, no starter): 15 buckets per section, no NEMA starter, suited to lighting panels and remote loads, fully front-accessible [S4].
12 in (NEMA Size 0 and Size 1 FVNR, up to roughly 10 hp at 460 V): 7 buckets per section nominal, accommodates a control transformer and standard HOA switch [S4][S7].
18 in (NEMA Size 2 FVNR, very common size, up to roughly 25 hp at 460 V): 5 buckets per section, allows reversing contactors and most standard VFDs up to 10 hp [S4][S7].
24 in (NEMA Size 3 and Size 4 starters, larger motors or VFDs): 3 buckets per section, accommodates combination starters with fused disconnects [S4].
36 in (large starters, complex controls, NEMA Size 5): 2 buckets per section, typically used for soft starters above 50 hp or where branch metering is required [S4].
48 in (NEMA Size 5 and Size 6 contactors, large VFDs, multi-component units): 1 bucket per section, often better served by a free-standing drive cabinet at this size [S4].
Layout pitfalls and field numbering conventions
Splitting a bucket across two sections is a common field condition that trips up one-line and elevation modeling software: many CAD and power-system tools only support a single bucket per row and force the engineer to model left/right sub-units as two stacked units, which then misrepresents the actual vertical wiring [S3]. A second pitfall is assuming the bus stab rating matches the bucket's branch rating: 250 A and 400 A plug-in units share the same 20 in column width, but the larger stab assembly may force a reduction in adjacent bucket count in the same vertical wireway [S6].
Bucket labeling is a plant-specific convention, not a national standard. The most common scheme numbers the top starter in the first vertical section as 1A, the next as 1B, and so on, with the letter I skipped; a two-unit-high starter in column 1 occupies both 1A and 1C, leaving 1B as a virtual slot [S5]. Some plants reverse this and number from the bottom of the cubicle, which breaks communication with outside technicians and replacement MCC suppliers [S5]. A documented plant standard on bucket numbering, applied consistently across all MCCs in the facility, is more important than which convention is chosen, and any change should be rolled out together with new bucket labels and updated documentation to avoid lockout/tagout errors during maintenance [S5]. For a side-by-side look at how the elevation view differs from a one-line and a unit wiring diagram, see MCC elevation vs one-line vs unit wiring diagram.
Spec workflow: pick the tallest unit first, then size the lineup
The disciplined workflow is to enumerate every motor and feeder, assign each to a NEMA size and starter type, and identify the tallest bucket height required across the entire project. That maximum height, plus the horizontal wireway overhead (typically 6 in at the top and 6 in at the bottom for a 90 in section) and the main breaker or main lug compartment, drives the section count [S4][S6]. For an all-small-feeder project, a two-section lineup at 15 buckets per section covers 28 to 30 feeders with room for the main; for a process plant dominated by 25 hp and 50 hp motors, the same two sections hold only 6 to 10 starter buckets and the lineup will need to grow to four or more sections [S4][S7].
For the broader context of how the MCC fits into a plant's power distribution alongside the soft starters and branch protection that feed it, the soft starter encyclopedia entry covers the reduced-voltage options that often justify a 36 in bucket. When the section count starts to push the limits of the electrical room, the MCC cabinet encyclopedia entry covers the enclosure depth and aisle clearance requirements that govern room layout. Trackable signals to watch: UL 845 short-circuit withstand label on the nameplate (commonly 42 kA or 65 kA at 480 V), the bus rating per section (600 A to 1600 A typical), and the NEMA starter size of the largest unit in the lineup, since that single value fixes the bucket count for the entire section [S4][S6][S7].
For the relevant spec sheets and selection criteria, see steel section.