MCCB in Motor Control Centers (MCC)

06 Aug, 2026
Posted by: Muhammet KUL

What is the role of an MCCB in a Motor Control Center? In an MCC, a molded case circuit breaker either protects the whole section as the incomer (typically 250-1600 A, IEC 60947-2) or protects a single motor branch inside a starter bucket (typically 16-250 A). Get the sizing or coordination wrong and a locked rotor on one pump can trip the section incomer instead of the bucket feeder, or a contactor welds shut during a fault the breaker was supposed to clear cleanly. This article covers incomer vs feeder placement, Type 2 coordination with contactor and overload, withdrawable and plug-in bucket construction, busbar bracing against breaker let-through, and how MCCB choice interacts with form of separation.

MCCB as the MCC Section Incomer

The section incomer sits between the main horizontal bus and the vertical bus feeding a column of starter buckets. It is sized on the connected/diversified load of that column, not the sum of every bucket's full-load current — diversity factors of 0.6-0.8 are common on multi-motor columns, agreed with the process engineer, not assumed by the panel builder. Frame choice here favors electronic trip: ABB Ekip, Schneider Micrologic, or Siemens ETU, because LSIG settings let the incomer hold discrimination and selectivity coordination against 6-10 downstream feeder breakers without oversizing the frame.

Key takeaway: Size the incomer on diversified column load plus margin for future buckets, not the arithmetic sum of nameplate FLCs — oversizing the incomer just pushes the coordination problem downstream.

Breaking capacity at the incomer is the highest in the column, since prospective fault current is largest closest to the main bus. This is where the S or H class (Schneider) or a high-Icu Tmax XT/Sentron 3VA frame earns its keep — a section incomer undersized on Icu is the single most common MCC design error we see corrected late, usually after a fault study reveals a higher available fault current than the original one-line assumed.

MCCB as Feeder to Starter Buckets

Each bucket feeder breaker protects one motor branch circuit: breaker, contactor, thermal or electronic overload, and the motor cable. Two sizing decisions matter. First, the thermal/long-time element (In or Ir) sits above full-load current with headroom for the overload relay to do the actual thermal protection — the breaker's job is short-circuit clearing, not overload trip, so it is set loose on purpose. Second, the magnetic/instantaneous element (Ii on TM units, Isd/Ii on electronic) must sit above motor starting current with margin, or every start becomes a nuisance trip.

Formula: Motor Feeder MCCB Magnetic Setting — Source: IEC 60947-4-1 §8.2 (coordination), motor starting current per nameplate

I_i ≥ k × I_LR

Symbol	Description	Unit
I_i	Instantaneous/magnetic trip setting of the feeder MCCB	A
I_LR	Motor locked-rotor (starting) current from nameplate or manufacturer data	A
k	Margin factor, typically 1.2-1.5 depending on starting method (DOL vs soft-start)	-

See our guide on how to set MCCB trip settings for the full Ir/Isd/Ii procedure. Some panel builders set Ii to the maximum available on the frame "to be safe." Don't. An oversized magnetic setting delays the breaker's fault-clearing role and pushes more let-through energy into the contactor and overload during a genuine short circuit — the setting has to be tight enough to clear fast, loose enough to ride through a normal start.

Type 2 Coordination: MCCB, Contactor, and Overload

Type 2 coordination is a combination-motor-starter test result under IEC 60947-4-1 where, after a specified short-circuit test, the contactor and overload relay show no damage other than contact welding that can be separated with a screwdriver, and the overload relay itself is undamaged.

Type 1 only requires no danger to personnel or the installation — the contactor can be destroyed. Type 2 requires the combination survive and be put back into service after minor contact cleaning. For an MCC bucket, this means the feeder MCCB's let-through current and let-through energy (I²t) at the contactor's rated fault current must fall inside the values published in the manufacturer's coordination table for that specific breaker/contactor/overload combination — not a generic "same brand" assumption.

Key takeaway: Type 2 coordination is validated per combination (breaker model + contactor model + overload model + fault current), published in the manufacturer's coordination tables — mixing brands across bucket components voids the tested combination and the coordination claim.

What we see in the field: buckets built with an MCCB from one brand and a contactor from another because of stock availability, with no coordination table checked. It may work. It may also mean a fault welds the contactor solid on a motor that then can't be de-energized by the overload relay — a failure mode the Type 2 test exists specifically to catch. If mixing brands, get a written coordination statement from the breaker manufacturer before committing to the panel build.

Withdrawable and Plug-in Buckets for Hot-Swap

Withdrawable (drawout) construction mounts the bucket on rails with disconnecting primary/secondary contacts, so the entire unit — MCCB, contactor, overload, terminals — can be racked out for replacement without de-energizing the vertical bus, subject to interlocking that isolates the bucket first.

Plug-in buckets use a simpler stab connection to the vertical bus, still removable without disturbing neighboring buckets, but generally without the full test/disconnect/connect position sequence of true drawout gear. Fixed buckets bolt directly to the bus with no removal capability short of a shutdown — see fixed vs plug-in vs withdrawable MCCB construction for the full comparison.

The MCCB itself typically stays fixed-mounted inside the bucket in all three schemes — it is the bucket, not the individual breaker, that is withdrawable. The practical benefit is replacement time: a failed motor starter bucket in a drawout MCC can be swapped in minutes from spares inventory, versus an hour or more of de-energization, lockout, and rewiring on a fixed bucket. This matters most on continuous-process lines where an unplanned motor stop cascades into a full line stop.

Key takeaway: Drawout buckets trade higher unit cost and cabinet depth for minutes-not-hours replacement time — justify the added cost against the actual cost of unplanned downtime on that specific motor, not as a blanket standard for every column.

Interlocking is the part that gets shortcut under schedule pressure: a bucket must not be withdrawable while its breaker is closed, and the primary disconnect contacts must fully break before the bucket clears the bus stabs. Verify this on the actual mechanism during factory acceptance testing, not from the catalog drawing.

Busbar Bracing vs Breaker Icu

The vertical and horizontal busbars in an MCC carry a short-time and peak withstand rating of their own, independent of any single breaker's Icu. Because MCCBs generally carry no Icw (short-time withstand) rating — that is an ACB characteristic, distinct from the Icu vs Ics vs Icw ratings that do apply — the busbar behind an MCCB section must be braced for the full prospective fault current at that point in the system, not a reduced value assuming the MCCB will interrupt it quickly.

Current-limiting MCCBs reduce peak let-through current and let-through energy compared to a non-limiting breaker of the same Icu, which is a real benefit to downstream cable and contactor protection. It does not, by itself, reduce the busbar bracing requirement upstream and at the connection point, because the fault has to flow through the bus before the breaker's limiting action takes effect within the first half-cycle.

Key takeaway: Specify vertical and horizontal busbar bracing (kA, 1 s and peak) against the prospective fault current at that bus location from the fault study — do not substitute the incomer breaker's Icu rating as a proxy for bus withstand.

This depends on the fault study assumptions holding through the life of the installation. A transformer upgrade or utility infrastructure change upstream can raise prospective fault current at the MCC without anyone revisiting the bus bracing — worth a line item in any electrical audit of an existing MCC before adding load.

Form of Separation and MCCB Placement

IEC 61439-2 defines forms of separation (1 through 4b) describing how busbars, functional units, and outgoing terminals are separated from each other inside the assembly. An MCC built to Form 4b, for instance, separates each bucket's terminals from neighboring buckets and from the busbar compartment, so a fault or maintenance action in one bucket cannot expose or affect the next. Form 1 has no internal separation at all.

The MCCB's physical mounting inside the bucket has to respect these barriers: cable entries, arc venting, and any withdrawable mechanism all need clearances that keep the assembly within its declared form. A higher form generally means a deeper, more compartmentalized bucket, which affects which MCCB frame sizes physically fit without redesigning the enclosure.

Key takeaway: Confirm the target form of separation before selecting bucket depth and MCCB frame — retrofitting a higher form onto an existing MCC structure is rarely a breaker swap, it is an enclosure redesign.

Higher forms cost more and take more panel footprint. Some engineers argue every new MCC should default to Form 4b for maximum safety and future maintainability. In practice, the choice tracks the site's operating philosophy — a facility that never works on a live section under load may accept Form 2b, while one with routine live maintenance justifies the higher form regardless of frame or breaker choice.

Frequently Asked Questions

Should the MCCB or the contactor open first on a motor overload?

Neither, in a healthy overload condition — the overload relay trips the contactor open while the MCCB stays closed. The MCCB only operates on a short circuit or a fault the overload relay cannot clear, which is the basis of the Type 1/Type 2 coordination test.

Can I mix an ABB feeder MCCB with a Schneider contactor in the same bucket?

Only with a documented Type 2 coordination statement from the breaker or contactor manufacturer covering that exact combination and fault current. Without it, treat the combination as untested and assume Type 1 behavior only.

Does a withdrawable bucket need a withdrawable MCCB?

No. In nearly all MCC designs the breaker is fixed-mounted inside the bucket, and the bucket assembly — not the individual breaker — is what racks in and out on rails or plug-in stabs.

How is the MCC incomer sized differently from a feeder breaker?

The incomer is sized on diversified column load with margin for spare buckets and rated for the highest prospective fault current in the section, since it sits closest to the main bus. Feeder breakers are sized per motor branch, with a magnetic setting above starting current and thermal protection left to the overload relay.

Does a current-limiting MCCB reduce the busbar bracing requirement?

Not by itself. The busbar must still be braced for the prospective fault current at that point in the system, since the fault flows through the bus before the breaker's current-limiting action takes effect within the first half-cycle.

Conclusion

An MCCB in an MCC is doing two different jobs depending on where it sits: broad section protection at the incomer, or tight branch-circuit protection at the bucket feeder working alongside a contactor and overload relay. Getting the magnetic setting right on the feeder, verifying a real Type 2 coordination table rather than assuming brand compatibility, checking bucket interlocking on drawout construction, and bracing the busbar against prospective fault current rather than any single breaker's Icu — these are the checks that separate a coordinated MCC from one that fails its first real fault. For frame selection and breaking-capacity classes across brands, see the MCCB engineering guide, and browse current stock in the molded case circuit breakers collection.