In this article9 sections
A motor control center (MCC) is a floor-standing assembly of vertical sections sharing a horizontal power bus, with each section holding a stack of plug-in or fixed units, called buckets, that contain the starter, short-circuit protection and control for one motor or small feeder. MCCs are low-voltage equipment, usually 480 V or 600 V in North America and 400 V or 690 V on IEC projects, and they group the starters for a process area in one electrical room. North American MCCs are built to UL 845 and NEMA ICS 18, with CSA C22.2 No. 254 in Canada. IEC projects treat the motor control centre as a low-voltage switchgear and controlgear assembly under IEC 61439-2.
For the asset register a CMMS is loaded from, an MCC is a nested record of lineup, sections, bucket positions, the starters or drives installed in them and the motors they feed. The single line diagram shows where the MCC sits in the power system, and the MCC schedule and elevation drawing show what is inside it.
Motor control center, switchgear, switchboard and distribution board compared
Older sites often file the four assemblies under one discipline code and call them by each other's names. The units each one holds tell them apart.
| Assembly | What its units hold | Typical loads | Standards |
|---|---|---|---|
| Motor control center (MCC) | Combination starters, drives and small feeders in plug-in or fixed buckets | Low-voltage motors and small feeders in one process area | UL 845, NEMA ICS 18, CSA C22.2 No. 254, IEC 61439-2 |
| Low-voltage switchgear | Power circuit breakers, usually drawout, each in its own compartment | Incomers, bus ties, MCCs and large loads | ANSI C37.20.1, UL 1558, IEC 61439-2 |
| Switchboard | Group-mounted molded-case or insulated-case breakers, or fused switches | Service entrance and building distribution | UL 891, NEMA PB 2 |
| Distribution board (panelboard) | Branch circuit breakers in a wall-mounted cabinet | Lighting, small power, heat tracing, instrument supplies | UL 67, NEMA PB 1, IEC 61439-3 |
Medium-voltage switchgear and controllers are outside this note. On the register, switchgear and switchboards are parents of breakers and an MCC is the parent of buckets. An MCC feeder unit can also supply a lighting panelboard, so a fed-from chain can pass through an MCC to a branch circuit.
How a motor control center is built
Vertical sections. A lineup is a row of vertical sections bolted side by side, in North America commonly about 20 inches wide and 90 inches tall, with the usable height divided into unit spaces of fixed increments. Sections are numbered from one end.
Bus. The horizontal main bus runs the length of the lineup with a continuous current rating and a short-circuit rating in kA, beside a ground bus. Each section's vertical bus taps the main bus, plug-in units stab onto it, and its rating limits what the section can hold.
Wireways. Horizontal wireways carry cables between sections, and a vertical wireway beside each unit stack takes field cables to the buckets.
Buckets. A bucket, or unit, is the compartment for one starter or feeder, with its own door, nameplate and a disconnect handle interlocked with the door. Plug-in buckets are designed to be withdrawn with the disconnect open, and large units are fixed and bolted to the bus. IEC withdrawable units add connected, test and disconnected positions.
Incoming section. Power enters through a main breaker, a main fused switch or main lugs only. A main breaker gets a register row of its own.
North American MCCs carry a wiring class and type from NEMA ICS 18 and UL 845. Class I units are independent, and Class II adds factory wiring and interlocks between units. Type A has no terminal blocks. Type B adds terminal blocks in each unit, as B-d with power wiring direct to the starter or B-t with power terminals on smaller starters. Type C wires each unit to master terminal blocks in the section's horizontal wireway, so field cables land in one place per section.
IEC projects specify a form of internal separation under IEC 61439-2 instead. Form 1 has none, Form 2 separates the busbars from the functional units, Form 3 also separates the units from each other, and Form 4 also separates each unit's outgoing terminals. The class and type or the form, the enclosure or IP rating, and any arc-resistant construction belong on the MCC row.
Starter and feeder units in MCC buckets
The unit type decides the spare parts and inspection tasks a planner attaches.
| Unit type | Contents of the bucket | What the record adds |
|---|---|---|
| Full-voltage non-reversing (FVNR), also direct-on-line (DOL) | Disconnect and short-circuit device, one contactor, overload relay, control power transformer | Size or contactor rating, overload, short-circuit device |
| Full-voltage reversing (FVR) | Two interlocked contactors that swap two phases, one overload relay | Both contactors and the interlock |
| Star-delta (wye-delta) | Main, star and delta contactors with a transition timer, for a six-lead motor | Contactor ratings, open or closed transition |
| Reduced-voltage autotransformer (RVAT) | Autotransformer with taps, start and run contactors | Tap in use, contactor ratings |
| Soft starter | Thyristor controller, usually with a bypass contactor | Controller model and current rating, bypass |
| Variable frequency drive (VFD) | Drive, fuses or breaker, line reactor or filter, optional bypass contactor | Drive model, rating, firmware, bypass arrangement |
| Feeder | Circuit breaker or fused switch, no contactor | Frame and trip or fuse rating, downstream load tag |
Intelligent MCCs add a communication module to each unit, and the record carries its network address.
What each MCC bucket carries
A starter bucket records these values.
- Position. Section number and unit position, such as 3B for the second unit space in section 3, plus the unit height. Conventions differ by builder, so keep the position as the nameplate prints it.
- Starter size. NEMA sizes run from 00 to 9, with a horsepower rating per size at each voltage. IEC starters carry a contactor current rating in utilization category AC-3 instead.
- Overload relay. Thermal relays often use heater elements, and the record carries their catalog number. Electronic relays carry a current range, a dial setting and a trip class, commonly 10, 20 or 30.
- Short-circuit protection. A motor circuit protector (an instantaneous-only breaker), a thermal-magnetic breaker or a fused disconnect, with its rating and the unit's short-circuit current rating. IEC units add a Type 1 or Type 2 coordination rating under IEC 60947-4-1.
- Control power transformer. Voltages, VA rating and fuses, or a note that control power comes from outside the bucket.
- Auxiliary contacts and control devices. Run, fault and remote contacts to the control system, pilot lights, pushbuttons, a hand-off-auto switch and interposing relays.
- The load. The motor tag, driven equipment, feeder cable tag, and the motor's rating and full-load current.
Overload settings, trip classes and short-circuit device ratings come from the motor circuit design and the protection studies. The register records them as found, with the document and revision they came from, and changing them is work for the engineer responsible for the motor circuit.
The MCC on the single line diagram and the MCC schedule
Four documents describe one MCC, and the register draws on each.
- Single line diagram. The MCC appears as a bus labelled with its tag, voltage, bus rating and short-circuit rating, fed from an upstream breaker. Each bucket is drawn as a combination starter (breaker or fuse, contactor, overload) on a line to a motor circle with its tag and rating. Crowded sheets show the MCC as one box that refers to the schedule.
- Elevation drawing. A front view of the lineup, section by section, with each unit's position label and height, the spaces, the incoming section and the cable entry.
- MCC schedule. The bucket list, one row per position, carrying the values the SLD has no room for.
- Unit schematics. One typical control circuit per starter type, with terminal numbers and the contacts wired out to the control system.
If the SLD and the schedule disagree, the schedule is usually the later document, and the nameplates in the electrical room settle the question.
An MCC schedule opens with a header block giving the lineup's tag, ratings, enclosure and wiring class and type. The unit rows carry bucket position, unit height, load tag, driven equipment, kW or hp, voltage, full-load current, starter type and size, short-circuit device, overload type and setting, control transformer, control method, auxiliary contacts and feeder cable. Some owners add a lockout point reference so an isolation list can name the bucket.
From the MCC to the CMMS register
The hierarchy follows the lineup, from MCC to section to bucket position, with each motor tied back to its bucket. SAP PM functional locations and Maximo locations both hold that structure, and the P&ID to CMMS article covers the location and equipment split it rests on.
Two choices shape the register, how to model a bucket and where to file the motor. A plug-in unit can be moved from position 3B to a spare position, so the position behaves like a location and the unit like equipment with its own history. Many owners make the position a functional location and install the unit in it as an equipment record. The motor's maintenance is planned with the machine it drives, so it usually sits in the process hierarchy, M-301 beside pump P-301, with the bucket named in a fed-from field. Filing it under the bucket puts the electrical path first and splits the pump train across two branches.
| MCC item | Register treatment | Fields the MCC documents supply | Fields from elsewhere |
|---|---|---|---|
| MCC lineup | Parent location | Tag, voltage, bus ratings, short-circuit rating, class and type or form, enclosure, fed-from breaker | Manufacturer, serial, room, arc-flash label date |
| Vertical section | Child location, or an attribute of the MCC | Section number, vertical bus rating | Usually none |
| Incoming unit | Child row under the MCC | Main breaker or switch rating, or main lugs only | Trip unit model, settings record |
| Bucket position | Child location under its section | Position, unit height, spare or space status, lockout reference | Usually none |
| Starter, drive or feeder unit | Equipment row installed at the position | Unit type, size, short-circuit device, overload type and setting, control transformer, auxiliary contacts | Serial, contactor and relay models, drive firmware |
| Motor | Equipment row with its driven machine | Tag, kW or hp, voltage, full-load current, fed-from bucket | Frame, speed, bearings, enclosure, serial |
| Feeder cable | Attribute of the bucket, or its own record | Cable tag | Size, cores, route and length from the cable schedule |
Tags follow the owner's numbering. Motors are often numbered after their driven machine, as covered in equipment tag numbering, and a bucket position is usually written as the MCC tag and position together, such as MCC-1-3B. Equipment class comes from the owner's taxonomy, often based on ISO 14224.
Spare buckets and spaces
The register treats spares and spaces differently. A spare is a complete, equipped bucket with no load connected. It can be inspected and exercised, so it gets an equipment row marked spare. A space is an unequipped position, bare or prepared with bus stabs and a door for a future bucket. It gets a location row marked space, with nothing installed.
With both recorded, the free positions on each MCC become a query instead of a trip to the elevation drawing.
Where MCC records drift
A plug-in bucket can be replaced, moved or rewired during a short outage, so MCC contents change faster than MCC drawings. Treat the schedule and the elevation as as-built claims to check against the room. Four patterns recur.
Bucket swaps. A failed unit is replaced by the spare from another position, or two units trade places to return a motor to service, and the door labels stay where they were. The schedule then names the wrong unit at both positions. Compare door labels and nameplates with the schedule, and hold any mismatch open for site confirmation.
Uprated motors. A motor replaced by a larger one may get a new starter, overload elements and cable, or only some of them. The motor list then shows the new rating against the old starter size on the schedule. Record both values with their sources and leave the row open for the engineer responsible.
Changed overloads. Heater elements get replaced with a different size and electronic dials get turned, often without a record. The register keeps the schedule value and the last recorded as-found value side by side, each with its date.
Retrofits and extensions. A drive replacing a full-voltage starter often needs more unit height, so the units below it move, and sections added to the lineup bring positions the SLD may not show. The same work can change the fault level at the MCC, leaving its short-circuit rating and arc-flash label describing an older study. The register notes the label date, and the review belongs to the engineer responsible for the study.
Checks before the register is loaded
Run these on the draft MCC register before it goes into the CMMS.
- Each position on the elevation has one row, and each row's position appears on the elevation.
- Each motor on the schedule appears on the motor list and the load list with the same rating, voltage and fed-from bucket.
- Each spare and space row carries its status, and no load tag sits on a space.
The MCC schedule template carries twelve columns, from bucket position, motor tag and full-load current to starter type, breaker rating, overload setting, control method and lockout reference, with a frozen header, an auto-filter and 48 blank rows.