GuideSwitchgear and motor control10 min read
Motor control center buckets
A motor control center (MCC) is a lineup of steel sections sharing a common bus, and a bucket is the plug-in unit that slides into a section to control one motor or feed one load. Each bucket holds a disconnect, a starter or a drive, control power and the terminals, behind its own door, and plugs onto the vertical bus through stabs at the back. Buckets come in heights measured in 6-inch space factors, hold starters from NEMA size 1 (10 HP at 480 V) to size 5 (200 HP), and fit only the maker and series of MCC they were built for, which is why used buckets are a market of their own.
The short version
- An MCC section is 20 inches wide and 90 inches tall (UL 845), with 72 inches of unit space; buckets take 6-inch multiples of it, from half a space factor to the whole section.
- Inside a bucket: a circuit breaker or fused disconnect with a door-interlocked handle, a contactor and overload relay (or a soft starter or drive), a control transformer, pilot devices on the door, terminal blocks, and stabs.
- NEMA starter sizes at 460 V: size 1 for 10 HP (27 A), size 2 for 25 HP (45 A), size 3 for 50 HP (90 A), size 4 for 100 HP (135 A), size 5 for 200 HP (270 A).
- Buckets interchange within a maker’s series (Allen-Bradley Centerline 2100, Square D Model 6, Eaton Freedom 2100, GE 8000-line, Siemens tiastar) across decades, and not between makers.
- Used buckets sell for a few hundred to a few thousand dollars; a used six-section 480 V MCC for tens of thousands; new lineups for several times that, with months of lead time.
- Check the stabs, the overload relays, the bus bracing against your fault current, and that the series and vintage match your lineup.
What an MCC is, and what a bucket is
A plant with thirty motors could wire each one to its own starter on the wall. A motor control center puts them all in one place: a row of steel sections, each 20 inches wide and 90 inches tall, bolted together and sharing a horizontal power bus along the top (600 to 3,000 A) that feeds a vertical bus down each section (300 or 600 A). The starters live in buckets that plug onto that vertical bus. One feeder from the switchgear serves the whole lineup, the wiring to every motor leaves from one wall, and a failed starter is swapped by pulling its bucket.
The bucket is the unit of the system. It’s built to UL 845 as part of the MCC, carries its own door with the disconnect handle through it, interlocked so the door can’t open with the disconnect on, and connects to the vertical bus through spring-loaded stabs at its back when it’s pushed home. The front of a section is a stack of bucket doors, and the lineup’s capacity is the number of space factors free.
What’s inside a bucket
- Disconnect
- A molded-case circuit breaker (thermal-magnetic or motor circuit protector, magnetic-only) or a fused switch, with an operating handle through the door and an interlock that stops the door opening while it’s closed. It isolates the bucket and provides short-circuit protection for the motor branch circuit (NEC 430).
- Starter
- A contactor sized to the motor (NEMA size), and an overload relay: bimetallic or eutectic-alloy heaters on older buckets, an electronic overload with adjustable current and trip class on newer ones. Together they’re the motor’s switch and its running protection.
- Control power transformer
- A small transformer (50 to 250 VA) with fused secondary that supplies 120 V for the coil and the pilot circuit from the bucket’s own line voltage, so each bucket is self-contained.
- Pilot devices
- On the door: a hand-off-auto selector, start and stop buttons, running and fault lights, sometimes an ammeter or an elapsed-time meter.
- Terminals and wiring
- Terminal blocks for the motor leads and the control wiring (NEMA class I or II, type A, B or C wiring, which sets how much is pre-wired to terminals at the bucket or at a master terminal board).
- Network module
- On an intelligent MCC, a communications module (EtherNet/IP, DeviceNet, Modbus, PROFIBUS) that reports current, status and faults to the plant’s control system.
- Stabs
- Three spring-finger jaws on the back that grip the vertical bus; the first thing to look at on a used bucket, since an overheated stab discolors and loses its spring.
NEMA sizes and bucket sizes
| NEMA size | Continuous amps | Max HP at 460 V | Max HP at 230 V | Typical bucket height |
|---|---|---|---|---|
| Size 0 | 18 A | 5 HP | 3 HP | 6 in |
| Size 1 | 27 A | 10 HP | 7.5 HP | 6 to 12 in |
| Size 2 | 45 A | 25 HP | 15 HP | 12 in |
| Size 3 | 90 A | 50 HP | 30 HP | 18 in |
| Size 4 | 135 A | 100 HP | 50 HP | 24 to 30 in |
| Size 5 | 270 A | 200 HP | 100 HP | 36 to 48 in |
| Size 6 | 540 A | 400 HP | 200 HP | A full section, usually in switchgear instead |
Bucket heights vary by maker and by disconnect type; a fused switch takes more room than a breaker. Reversing, two-speed and reduced-voltage buckets are taller than the full-voltage bucket of the same size.
Bucket height is reckoned in space factors: one space factor is 6 inches on Allen-Bradley Centerline, Square D Model 6 and Eaton Freedom lineups, and a section’s 72 inches is 12 of them. A lineup drawing lists each section’s units top to bottom by space factor, and the free space is what a new bucket can use. Drive (VFD) buckets are the exception to everything: a 50 HP drive with its bypass and cooling takes 36 inches or a full section, and a lineup with many drives is sized by their heat as much as their height.
Kinds of bucket
| Type | What it does | Typical use |
|---|---|---|
| FVNR (full-voltage non-reversing) | One contactor; starts the motor across the line | Pumps, fans, compressors, conveyors: most buckets |
| FVR (reversing) | Two interlocked contactors | Cranes, conveyors, gates |
| Two-speed | Contactors for two windings or a consequent-pole motor | Fans and pumps with two duty points |
| RVSS (soft starter) | A solid-state starter that ramps the voltage; often with a bypass contactor | Large pumps and compressors where inrush or water hammer matters |
| VFD (drive) | A variable-frequency drive, with bypass, line reactor and fans | Anything whose speed is controlled: pumps, fans, process lines |
| Feeder | A breaker or fused switch alone | Feeding a panel, a transformer or a remote starter |
| Main | The incoming breaker or switch for the lineup | At one end of the MCC |
| Lighting and distribution | A transformer and panelboard, or a PLC and I/O | Control power and local distribution |
Makers, series and what interchanges
| Maker | Series | Years | Notes |
|---|---|---|---|
| Allen-Bradley (Rockwell) | Centerline 2100; Centerline 2500 (IEC) | 1980s to now | The most common in North American plants; buckets interchange across the series, with changes to stabs and vertical bus covers along the way; IntelliCENTER for networked units |
| Square D (Schneider) | Model 6; Model 5 before it; Model 4 earlier | 1990s to now (Model 6) | Model 6 buckets interchange within the series; Model 5 and Model 4 buckets are a separate market |
| Eaton (Cutler-Hammer) | Freedom 2100 (F2100); Unitrol and Series 2100 before it | 1990s to now | Freedom-series starters; buckets interchange within F2100 |
| GE | 8000-line; Evolution; Spectra | 1960s to now (8000-line) | The 8000-line has had the longest run; buckets from different decades fit with limits at design changes; Evolution is the current line |
| Siemens | tiastar; System 89 (Furnas) before it | 1990s to now | tiastar buckets interchange; System 89 separate |
| Others | Westinghouse Five-Star and Type W; Allis-Chalmers; Federal Pacific | Vintage | Lineups still running; buckets scarce, and roll-in rebuilt units are made for some |
The series on the lineup’s nameplate is what a used bucket has to match. Within a series, a bucket built for a different disconnect, a different wiring class or a different vertical bus rating may still fit; between series it won’t, and a bucket pressed in anyway can arc at the stabs.
What a motor control center costs
Planning figures for 480 V lineups in 2026, equipment only, new and used; the spread is wide because networking, arc-resistant construction, drives and the maker move the price a lot.
| Item | New | Used or reconditioned |
|---|---|---|
| Section, with bus, no buckets | $6,000 to $15,000 | $2,000 to $6,000 |
| FVNR bucket, NEMA size 1 or 2 | $3,000 to $6,000 | $500 to $2,500 |
| FVNR bucket, NEMA size 3 | $4,000 to $8,000 | $1,200 to $3,500 |
| FVNR bucket, NEMA size 4 or 5 | $6,000 to $12,000 | $2,000 to $5,000 |
| Soft-starter bucket, 50 to 100 HP | $8,000 to $15,000 | $3,000 to $7,000 |
| VFD bucket, 25 to 100 HP | $8,000 to $25,000 | $3,000 to $10,000 |
| Six-section lineup with 12 starters | $60,000 to $150,000 | $15,000 to $40,000 |
| Lead time | Months; longer for networked or arc-resistant lineups | Days to weeks |
Order-of-magnitude figures from distributors’ and dealers’ quotes seen through 2026; your quote depends on the series, the options and the market that month.
Two things change the arithmetic. A plant that already has a lineup needs buckets of that series, and the used price of a size 2 bucket against a new one is the whole case for the used market. And a used lineup from a closed plant, with its buckets, is often the cheapest way to equip a new one, if its bus rating and fault bracing suit the new site.
Buying used buckets and lineups
- Series and vintage. From the lineup nameplate and the bucket label. Ask for the catalog number of the bucket; it encodes the size, disconnect and options.
- Stabs and bus. Bright, springy stabs; vertical bus with no pitting or discoloration; the bus bracing (42, 65 or 100 kA) against the fault current at your feeder, which the impedance guide works out.
- Overload relays. Eutectic and bimetallic overloads need the right heaters for your motor; electronic overloads are set on site. Many buyers replace old overloads as a matter of course.
- Disconnect and interlock. The breaker or switch operates, its rating suits the motor, and the door interlock works; a motor circuit protector must match the motor’s full-load current range.
- Control transformer and wiring. The CPT’s secondary voltage, its fuses, and whether the control wiring is class I or II and type B or C, which sets how much rewiring the installation needs.
- Drives and soft starters. Age and support: a 2005 drive in a bucket may have no parts; a new drive in an old bucket is a common and sensible refit.
- Tests. Insulation resistance of the bus, contactor and starter operation, overload and protection tests, and a control wiring check, which the motor control units we sell get before shipping.
Questions people ask
What is an MCC bucket?
The removable, plug-in unit in a motor control center that controls one motor or one feeder. It’s a steel drawer with its own door, holding a disconnect (breaker or fused switch), a starter (contactor and overload relay) or a soft starter or drive, a control transformer and the terminals, and it connects to the MCC’s vertical bus through spring stabs on its back. It can be withdrawn and replaced without de-energizing the lineup, which is the point of the design.
What sizes do MCC buckets come in?
Heights in multiples of a space factor, which is 6 inches on most makers’ lineups; a section has 72 inches of usable height, or 12 space factors. A NEMA size 1 full-voltage starter bucket is 6 to 12 inches depending on the maker and the disconnect; size 2 about 12; size 3 about 18; size 4 about 24 to 30; size 5 about 36 to 48; a drive bucket takes more, sometimes a whole section. Some makers’ compact designs fit two size 1 starters in one 12-inch unit.
Are MCC buckets interchangeable between manufacturers?
No. The stab design, the vertical bus spacing, the unit supports and the door dimensions differ between makers, so an Allen-Bradley bucket won’t plug into a Square D lineup. Within a maker’s series they usually interchange across many years: a Centerline 2100 bucket from 1990 fits a Centerline 2100 section from 2020, and 8000-line GE buckets have fit 8000-line sections since the 1960s, with limits at certain design changes.
How much does a motor control center cost?
A new 480 V lineup runs on the order of $6,000 to $15,000 per section before the buckets, with full-voltage starter buckets at $3,000 to $10,000 each depending on size and whether the lineup is networked, and drive buckets at $8,000 to $25,000; a six-section plant MCC with a dozen starters is commonly $60,000 to $150,000 and takes months to deliver. Used, a section is $2,000 to $6,000, a size 1 or 2 bucket $500 to $2,500, a size 4 or 5 bucket $2,000 to $5,000, and a complete six-section lineup $15,000 to $40,000. Planning figures, not quotes.
What is the difference between an MCC and switchgear?
Purpose. Switchgear and switchboards distribute power: a main breaker and feeder breakers. A motor control center controls motors: a lineup of starters, each with its own disconnect, overload protection and control circuit, fed from a switchboard or switchgear feeder. An MCC is built to UL 845, switchgear to UL 1558 and switchboards to UL 891, and an MCC’s buckets plug in where switchboard breakers bolt on.
Can you add a bucket to an existing MCC?
Yes, if there is space in a section and the new bucket is the right series for the lineup. The space must be free of the vertical bus covers or blank plates, the bucket’s stabs must match the bus, and the lineup’s bus rating and bracing must cover the added load and the available fault current. Used buckets of the right series are the usual way to do it, since a new bucket for an older series may no longer be made.
Sources: UL 845 (motor control centers); NEMA ICS 18 (motor control centers) and ICS 2 (starter sizes); NEC Article 430 (motors) and 409; makers’ catalogs (Rockwell Centerline 2100, Schneider Model 6, Eaton Freedom 2100, GE Evolution and 8000-line, Siemens tiastar).