MCC Schedule Fundamentals — Structure, Feeder Sizing, and Coordination

What an MCC Schedule Is

A Motor Control Center (MCC) schedule is the document that defines every feeder, starter, and protective device inside an MCC — bucket by bucket. It translates the motor list into a physical and electrical layout: which motor is fed from which compartment, what starting method and protection each bucket uses, and how the MCC's incoming feeder and bus are rated to carry the combined load.

Where the motor list captures what each load needs, the MCC schedule captures how the MCC itself is built to deliver it — bucket sizes, breaker/fuse ratings, contactor sizes, and busbar rating all trace back to this document.

Core Fields in an MCC Schedule

• Bucket / compartment number — physical location within the MCC vertical section, used for drawings, labeling, and field installation
• Load tag / motor tag — cross-reference to the motor list (e.g. P-101, F-203)
• Feeder type — combination starter (FVNR, FVR, two-speed), soft starter, VFD, or feeder-only (non-motor loads such as heaters or lighting panels)
• Starter size / NEMA or IEC contactor rating — sized to the motor's full load current with appropriate service factor
• Overload relay range — set per NEC Art. 430.32 (115–125% of FLA) or matched to the motor thermal curve under IEC 60947-4-1
• Short-circuit protective device (SCPD) — fuse or breaker rating per NEC Table 430.52, or coordinated per IEC Type 1/Type 2 combination testing
• Cable size and conductor count — cross-referenced to the cable schedule
• Bucket size / unit height — physical space allocation (typically 6", 12", 18", or full-height units), driven by starter size and accessory requirements
• Control voltage — 120VAC, 24VDC, or other control power source for the bucket's control circuit
• Spare / spare-space designation — reserved buckets for future loads, typically 10–20% of total buckets on new installations

Sizing the Incoming Feeder and Main Bus

The MCC's main incoming feeder and busbar must be rated for the sum of connected loads, adjusted by diversity — the same logic used in a facility-level demand load calculation, applied at MCC scale.

• Connected load — sum of full load current for every motor and feeder-only load in the MCC
• Diversity factor — not all motors run simultaneously at full load; process areas typically apply 0.7–0.9 diversity depending on the process, while continuous-duty utility MCCs (pumping stations, HVAC) often run closer to 1.0
• Largest motor starting addition — the bus and main breaker must also tolerate the starting current of the largest motor added to the running load of all others, momentarily — this is a separate check from steady-state demand and often governs when one very large motor shares an MCC with many smaller ones
• Standard bus ratings — MCC busbars are typically rated 600A, 800A, 1200A, 1600A, or 2000A; select the next standard size above calculated demand with margin for spare buckets
• Short-circuit withstand rating — the bus must be rated to withstand available fault current at the MCC location; verify against a short circuit calculation at that point in the system

Starter Type and Bucket Sizing

Bucket size scales with starter type and motor size — the schedule should record both so procurement and layout stay aligned:

• Full Voltage Non-Reversing (FVNR) — smallest footprint; suitable for DOL starting per the motor's entry in motor starting method selection
• Full Voltage Reversing (FVR) — adds a second contactor for reverse rotation; roughly one size larger than FVNR for the same motor
• Soft starter bucket — larger than FVNR/FVR due to heat sink and bypass contactor; often requires the next unit height up
• VFD bucket — largest footprint; may require a dedicated full-height unit for drives above roughly 30–37 kW, plus ventilation or a dedicated cooling arrangement for higher ratings
• Feeder-only (non-motor) — sized to the breaker/fuse frame only, no contactor or overload — typically the smallest bucket in the schedule

Coordination and Selectivity

Every bucket's protective device must coordinate with both the motor's overload/short-circuit protection and the upstream main breaker feeding the MCC:

• Type 1 coordination (IEC 60947-4-1) — permits damage to the starter under fault conditions provided no hazard to personnel; the bucket may require replacement after a fault but the fault is cleared safely
• Type 2 coordination (IEC 60947-4-1) — no damage permitted to the contactor or overload relay under fault, other than possible contact welding that is easily separable; this is the higher-cost, higher-availability standard increasingly specified for critical process areas
• NEC combination starter listing — UL 508 combination motor controllers are tested and listed as complete assemblies (contactor + overload + SCPD); NEC compliance is verified through this listing rather than a separate coordination study
• Upstream selectivity — the MCC main breaker and any upstream feeder breaker should be selective with individual bucket devices so a single motor fault trips only that bucket, not the entire MCC

Building the Schedule

• Start from the completed motor list — every motor, its FLA, starting method, and duty cycle should already be finalized
• Assign bucket numbers in a logical physical sequence matching the MCC general arrangement drawing
• Size each starter, overload, and SCPD per the motor's FLA and selected starting method
• Total the connected load, apply diversity, and check against standard bus ratings — round up, not down
• Verify short-circuit withstand rating of the bus against available fault current at the MCC
• Cross-check cable sizes against the cable schedule for every feeder leaving the MCC
• Reserve 10–20% spare buckets sized to the largest anticipated future load

Where Standards Diverge

• Listing vs. coordination approach — NEC relies on UL 508A/508 listed combination starters as tested assemblies; IEC relies on published Type 1/Type 2 coordination tables from the manufacturer for each contactor–overload–SCPD combination. A schedule built for one standard cannot simply be relabeled for the other without re-verifying the coordination basis.
• Bus rating conventions — NEC-based MCCs are commonly specified in amps at 480V three-phase; IEC-based MCCs are typically rated at 400V or 690V three-phase, with busbar ratings expressed identically in amps but at different system voltages — always confirm system voltage before comparing bus ratings across projects.
• Enclosure and form designations — NEMA enclosure types (NEMA 1, 12) describe environmental protection in North American practice; IEC uses IP ratings (IP20, IP54) for the same purpose, plus separate "Form" designations (Form 1 through Form 4b) describing internal segregation between busbars, functional units, and terminals — these Form designations have no direct NEMA equivalent.

Summary

• The MCC schedule translates the motor list into physical bucket assignments, starter sizing, and protective device ratings
• Main bus and incoming feeder are sized from connected load with diversity, plus a check for the largest motor's starting current added to running load of the rest
• Bucket size scales with starter type — FVNR smallest, VFD largest — and should be recorded alongside starting method from the motor list
• Type 1 vs. Type 2 coordination (IEC) and UL 508 listing (NEC) are different compliance paths — confirm which governs before finalizing device selection
• Reserve 10–20% spare buckets sized to the largest anticipated future load