Spare and space units in a UL 845 motor control center are factory-built empty compartments, usually 6 in., 12 in., 18 in., or full-section height, reserved for future starters, VFDs, soft starters, or feeder devices [S3]. Specifying 10 to 20 percent of total buckets as spares on a new installation is the standard practice for keeping headroom for process additions without rebuilding the lineup [S3].
An MCC cabinet is engineered as a complete listed assembly under UL 845, which is why power-connected buckets must come from the original MCC manufacturer, with serial-number traceability, to preserve the listing [S2]. This article lays out the spec rules, unit-height math, UL 845 listing boundaries, and field pitfalls that drive a clean spare-unit strategy.
Spare vs Space: The Two Reserved-Compartment Types
An MCC schedule distinguishes between a "spare" bucket, which is a fully wired, tested, and ready-to-energize unit, and a "spare space" or "space unit," which is a blank compartment with a blank door, a closure plate, and the bus stabs covered or shuttered [S3]. Spare spaces dominate new installations because they cost less up front while still defining a future footprint in the column.
Unit height is the physical currency: starter buckets for small FVNR motors occupy a single 6 in. modular space, while reversing starters, larger NEMA Size 3 and 4 contactor combinations, and VFDs typically consume 12 in. to 18 in. or more, depending on the drive horsepower and harmonic filter requirements [S3]. Planning spare spaces at 6 in. increments lets future loads be matched to the right bucket size without reworking the column.
What the UL 845 Listing Actually Allows in a Spare
"You (or a UL508A panel shop) cannot build an MCC bucket with any power devices that would connect to the bus bars on your own and maintain the UL 845 listing of the MCC. There is no viable option for field assembled units, they must come from the factory," is how one long-time forum respondent summarized the rule for power-connected gear [S2]. A spare space, with an appropriate solid back pan and door, can house anything that does not directly connect to the vertical bus, fed instead from a separately sourced 120V control circuit breaker [S2].
Panel shops operating under UL508A can modify factory-built control circuits inside a listed bucket, but cannot fabricate a new bus-stabbed unit from scratch and call it UL 845 [S2]. For obsolete MCCs where the OEM is gone, UL2727 covers pre-qualified switchboard modification shops, but the documentation and possible retesting make it expensive enough that owners use it only when the gear is too critical to abandon [S2].
Common Future Loads Dropped into a Spare Space
"A starter bucket is only one of many choices available for the space," a senior practitioner noted, adding that the spare can hold "a softstart, vfd, power supply, ethernet switch, feeder breaker, fused disconnect, an empty bucket, the sky is the limit almost" [S2]. That latitude holds for the cabinet's body; the bus-stabbed device inside still must be OEM-supplied to keep the UL 845 listing intact.
For sizing a future combination starter, the bucket must accommodate the contactor NEMA size, the disconnect or breaker height, the control transformer if 120V control is used, and the overload relay space, with the schedule typically recording all four for procurement [S3]. VFDs and soft starters usually demand more vertical space than a comparable-horsepower FVNR bucket because of the heat sink, line reactor, or bypass contactor arrangement, and that growth should be reserved at the time of the initial column layout, not after the fact [S3].
Counting Spares: The 10 to 20 Percent Rule and Its Caveats
Reserving 10 to 20 percent of the total bucket count as spares is the rule of thumb for new MCCs, covering modest process growth without forcing a column addition [S3]. The number is a starting point; continuous-process plants with future VFD conversions, water and wastewater facilities with phased pump additions, and packaging lines with frequent SKU changes often sit at the upper end of that range, while a dedicated utility MCC feeding a few fixed pumps may need fewer.
Standard bus ratings step from 600A up through 800A, 1200A, 1600A, and 2000A, and the bus should be selected one standard size above the calculated demand with margin for the spare-bucket load growth, since rebuilding a bus is far more disruptive than oversizing it at the factory [S3]. For very large motors that share an MCC with many smaller ones, the largest-motor starting current added to the running load of the others, not the steady-state demand, often governs the bus and main breaker selection, which directly affects how aggressive a spare strategy can be [S3].
Documenting Spare Buckets in the MCC Schedule and Elevation View
An MCC schedule should record, per bucket, the compartment number, the load tag, the feeder type, the starter size and NEMA or IEC contactor rating, the overload relay range, the short-circuit protective device, the cable size, the bucket unit height, the control voltage, and the spare or spare-space designation [S3]. Without a unit-height column, the future starter won't fit; without a designation column, the spare tends to be quietly consumed by a control-only device and the process growth has nowhere to land.
Elevation-view tools used in power-system software, such as the column-number and starter-space fields, let engineers model the bucket layout to match field conditions and identify spare starters available for a new motor [S4]. Split buckets, where two half-height devices share a compartment left and right, are common in the field and typically modeled as one stacked unit with a left/right note, a workaround that reflects a gap in current software handling [S4].
Manufacturer Guidance on Unit Density and Arc-Resistant Spares
Modern MCC builders publish selection guides that walk through the structure, power bus, and unit-design steps in order, then present a list of unit types from lugs, breakers, and feeders through non-motor loads, starters, space-saving NEMA starters, metering, and soft starters [S5]. The unit-type catalog is the place to confirm which future devices can drop into a given bucket size without forcing a non-standard enclosure height.
Arc-resistant MCCs, identified by an arc-duration rating, an arc-resistant baffle option, and a device-limited rating, extend up to 600V with bus ratings to 3000A on the latest product lines, which matters for spare planning because adding future buckets inside an arc-rated lineup should be done with factory-matched units to keep the rating intact [S5]. Selecting an MCC whose arc rating covers the future spare count, not just the initial load list, prevents the common failure mode of owners discovering that a future VFD bucket had to be added as a non-arc-rated section beside an arc-rated lineup.
Selection Criteria for Deciding What Goes in a Spare
Pick a spare's future device on four criteria: the bus-stab current draw of the largest expected future load versus the column bus rating, the unit height the device will need, whether the device will be power-connected to the vertical bus (which forces OEM procurement) or control-only (which a UL508A shop can wire into a listed bucket), and whether the spare must support an arc-resistant rating carried by the lineup [S1][S2][S5]. A simple feeder breaker in a 6 in. space is a cheap future option; a 100 hp VFD with line reactor needs 18 in. or more and should be specified that way from day one.
Across a typical industrial spec, the practical comparison reads: FVNR combination starter fits a 6 in. space for most NEMA Size 1 and 2 motors and is OEM-only for the bus-stabbed half; soft starter and VFD buckets run 12 in. to 18 in. and need thermal headroom for heat sinks; feeder-only buckets at 6 in. to 12 in. support lighting panels, sub-MCCs, and non-motor loads and are the most flexible future reserve [S3][S5]. The trade-off is flexibility versus readiness: an empty spare space is the cheapest reserve, while a wired spare FVNR bucket costs more up front but eliminates future outage time when the process changes.
Track the column bus ampacity against the calculated connected load plus diversity, normally 0.7 to 0.9 for process areas and closer to 1.0 for continuous-duty utility MCCs, and reserve the next standard bus size up to absorb the spare-bucket load growth, since post-installation bus upgrades mean a new lineup [S3]. EasyPower MCC schedules, used in recent paper-mill audits covering more than 200 MCC descriptions and elevation views, centralize the bucket status (on/off), load type (motor, VFD, sub-panel, sub-MCC), and starter size in a single database, which is exactly the data structure needed to plan future-bucket deployments without re-walking the plant [S4]. The MCC elevation vs one-line vs unit wiring diagram reference is the natural next read for engineers mapping schedule rows to the physical column and the schematic set.
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