Overload Relays in Motor Control Centers (MCC) Guide

What does an overload relay do inside a Motor Control Center (MCC)? Inside an MCC bucket it protects one motor branch against sustained overload above the motor's full-load current (FLC), tripping the bucket's contactor through its 95-96 NC contact per IEC 60947-4-1 or UL 508, while the fuse or MPCB above it clears short circuits. Get the relay wrong in a twenty-bucket lineup and the fault stays contained to one motor, or a nuisance trip takes a process motor offline mid-batch instead. This article covers bucket layout, bimetallic versus electronic selection for MCC space and communication needs, setting each relay to its own motor's nameplate FLC, trip class across mixed loads, Type 2 coordination as typically specified for MCC lineups, and wiring the relay's contacts into plant control and SCADA.

The MCC Bucket: Where the Overload Relay Sits

A Motor Control Center groups multiple motor starters into one enclosure lineup, each vertical section split into individually removable buckets. A standard combination starter bucket holds three devices in series with the motor: a short-circuit protective device (SCPD) — fuse or motor protection circuit breaker — a contactor, and the overload relay. The relay clips or bolts to the base of its contactor in most bucket designs, keeping the current path short and the bucket depth, typically 300-600 mm, workable. Drawout buckets let a technician withdraw the entire starter, overload relay included, without de-energizing the vertical bus behind it; fixed buckets require the section main or an upstream disconnect to be opened first. For the full device stack and how the three parts interact, see our thermal overload relay engineering guide.

Bimetallic or Electronic: Which Fits the Bucket

Bucket depth and the plant's appetite for data drive the bimetal-versus-electronic choice inside an MCC. A bimetallic relay from the thermal overload relays range (Schneider TeSys LRD, ABB TA25DU, Siemens SIRIUS 3RU2) covers a roughly 1:1.5 setting span and needs no external CTs, so it fits the footprint the contactor manufacturer designed the bucket around. An electronic relay reads current through internal or clip-on CTs, covers a wider 1:3 to 1:4 span on one part number, and adds phase-loss, ground-fault, and thermal-memory functions — useful when a lineup mixes motor sizes and the buyer wants fewer catalog numbers on the shelf. The trade is bucket depth and, on some frames, a taller relay body that forces a wider bucket than the bimetal equivalent. See thermal vs electronic overload relays for the trade-off outside the MCC-specific constraints covered here.

Setting the Relay to Each Motor in the Lineup

Each bucket in an MCC lineup protects a different motor, so each overload relay carries its own dial setting — there is no lineup-wide default. Set the dial to the motor's nameplate full-load current, not the branch conductor ampacity or the bucket's frame rating, and re-check the setting whenever a motor is swapped for a different frame or duty. On a delta-fed star-delta starter bucket, the relay usually sits in the delta leg and sees line FLC divided by the square root of three, not the full line current.

Duty cycle and service factor shift the margin, not the base rule. A motor with a 1.15 service factor still gets the relay dialed to nameplate FLC first; the service factor is a run-time allowance the relay's thermal curve already tolerates, not a reason to dial the relay higher. For the full setting procedure including service factor and ambient corrections, see how to select and set an overload relay.

Trip Class Selection Across Mixed Loads in One MCC

One MCC lineup often feeds pumps, fans, and at least one high-inertia load — a large centrifugal fan, a crusher, a mixer — off the same bus, and each gets its own trip class, not a uniform one. Class 10 or 10A covers standard pumps and fans whose motors reach full speed in a few seconds. A motor that takes 15-20 seconds to run up needs Class 20, and some large fan or crusher motors need Class 30; a Class 10 relay on that bucket produces a nuisance trip on every start, not better protection. What we see in the field: an MCC replacement that reuses the old bucket schedule without re-checking trip class against the new motor's actual start time is a common source of nuisance trips in the first week after cutover. Cross-check each bucket's trip class against the motor's run-up time before energizing, not after the first trip call.

Coordination Type 2 and MCC Specifications

Most industrial MCC specifications call for Type 2 coordination between the SCPD, contactor, and overload relay in every bucket: after a short circuit, the starter must be free of damage beyond light, separable contact welding, and the bucket has to return to service without the contactor or relay being replaced. Type 1 only guarantees no hazard to personnel — the starter itself can be damaged and may need parts replaced before restart. Meeting Type 2 across a mixed lineup means following the manufacturer's published coordination tables for that exact SCPD-contactor-overload relay combination, not assuming any fuse or MPCB rated above the motor's locked-rotor current will do. Read the full breakdown in Type 1 vs Type 2 coordination and the underlying IEC 60947-4-1 standard for contactors and motor starters.

Wiring the Overload Relay into MCC Control and SCADA

The relay's 95-96 NC contact wires in series with the contactor coil in every bucket — trip the relay and the coil drops, whatever the PLC is telling it to do. A second contact set, 97-98 NO or a dedicated auxiliary block, typically routes to the MCC's intelligence: a PLC input card, a motor control module, or an electronic relay's own communication port for remote fault annunciation. On buckets with electronic relays and a fieldbus module (Profibus, DeviceNet, Modbus), the same run also carries running current, percentage thermal capacity used, and trip cause back to the control room, cutting the number of separate signal wires a drawout bucket has to break when it comes out. Phase-loss protection matters more in an MCC than in a standalone panel because a lost incoming phase upstream can affect every bucket on that section at once; see phase-loss and single-phasing protection for how the differential trip mechanism catches it.

Maintenance: Drawout, Reset, and Spares Commonality

Hand (manual) reset is the default for MCC buckets feeding process motors — an operator has to walk to the lineup and press the button, which stops an unattended auto-restart into whatever fault tripped the relay in the first place. Some plants wire a subset of buckets, usually smaller pumps in non-critical service, for automatic reset through the electronic relay's own contact; check the site's operating philosophy before repeating that on a bucket running a critical process motor. See manual vs automatic reset for the criteria. Standardizing on one overload relay family across the whole MCC, same bimetal line or the same electronic platform, keeps spares to a handful of part numbers instead of one per bucket, and a drawout unit pulled from one section usually fits the base and terminal layout on another.

Conclusion

An overload relay in an MCC bucket does the same job it does in a standalone starter — protect one motor against sustained overload — but the lineup context adds constraints a single-panel installation does not: bucket depth limits, mixed trip classes across dissimilar loads on one bus, a Type 2 coordination requirement written into most specifications, and wiring that often needs to reach a plant SCADA system rather than just the local contactor coil. Set each relay to its own motor's FLC, match trip class to actual run-up time bucket by bucket, and standardize the relay family across the lineup so spares and drawout units stay interchangeable.