MCC Vertical Section Design: Bus Configuration and Layout

Understanding MCC vertical section design helps you make informed decisions about bucket placement, load distribution, and future expansion. This technical guide covers the internal architecture of MCC sections and how they affect your motor control installations.

Anatomy of a Vertical Section

A standard MCC vertical section is a steel enclosure measuring approximately 20 inches wide, 90 inches tall, and 20 inches deep (or 24 inches deep in modern designs). Each section contains:

Structural Frame

The frame provides mechanical support for all internal components:

• Welded or bolted steel construction
• Mounting rails for bucket guides
• Bolt holes for section-to-section joining
• Lifting provisions for installation

Vertical Bus Bars

Three vertical copper or aluminum bus bars (one per phase) run the height of the section:

• Connected to the horizontal bus at the top (or bottom) of the section
• Provide power connection points for each bucket compartment
• Rated 300A or 600A continuous
• Bus bars are typically tin-plated copper for corrosion resistance

Bucket Compartments

The vertical space is divided into compartments for individual buckets:

• Separated by horizontal barriers (steel or insulating material)
• Each compartment has stab contact points aligned with the vertical bus
• Guide rails ensure proper bucket alignment during insertion
• Standard compartment heights: 6", 12", 18", 24", 36", 48"

Wire Way

A vertical wire way on one or both sides provides space for:

• Field wiring (motor cables, control cables)
• Cable entry from the top or bottom
• Wire routing between compartments
• Wire way covers for safety and organization

Rear Access

The rear of the section provides access to:

• Vertical bus connections
• Wiring terminations
• Bus bar splice connections to adjacent sections
• Individual rear covers (modern designs) or full-section rear panel (older designs)

Bus Configuration Types

Back-to-Back Configuration

Two MCC lineups sharing a common back:

• Buckets face outward in both directions
• Common horizontal bus serves both sides
• Saves floor space compared to two separate lineups
• Requires rear access from the center (or accessed from the ends)
• Less common due to reduced accessibility

Front-Only Configuration

Standard single-row MCC:

• All buckets face the same direction
• Rear access from behind the MCC
• NEC 110.26 requires working space in front (36" minimum)
• Rear access space recommended (not always required by code)
• Most common configuration

Free-Standing Configuration

Individual sections not part of a multi-section lineup:

• Self-supporting structure
• Used for isolated motor control requirements
• Requires access from all sides
• Less common in large installations

Horizontal Bus Connection

The horizontal bus connects all vertical sections together:

Top Bus

Horizontal bus runs across the top of all sections:

• Most common configuration
• Bus connections are above the top-most bucket compartment
• Easier access for bus maintenance
• Vertical bus taps off the horizontal bus downward

Bottom Bus

Horizontal bus runs across the bottom:

• Less common
• Used when top access is restricted
• Vertical bus taps off the horizontal bus upward

Split Bus

Some MCC designs use a split horizontal bus:

• Main bus can be divided into two or more sections
• Each section has its own main disconnect
• Provides redundancy and isolation capability
• Allows maintenance on one section while the other remains energized

Bucket Arrangement Strategies

Load Balancing

Distribute motor loads across all three phases evenly:

• Each bucket draws power from all three phases (3-phase motors)
• But single-phase feeders and control circuits create imbalance
• Monitor phase currents at the horizontal bus to identify imbalance
• Relocate single-phase loads if imbalance exceeds 10%

Thermal Management

Arrange buckets to manage heat:

• Place high-heat buckets (VFDs, large starters) in lower positions where cooler air enters
• Avoid stacking multiple VFD buckets without ventilation space
• Leave empty spaces above high-heat buckets for convection
• Consider the total heat load per section versus the section's ventilation capacity

Accessibility

Arrange frequently accessed buckets at convenient heights:

• Place buckets requiring frequent operation (start/stop buttons) at mid-height (36"-60" from floor)
• Put feeder buckets with minimal interaction at the top or bottom
• Keep critical/emergency equipment at easily accessible heights
• Consider ADA accessibility if required

Future Expansion

Plan for growth:

• Leave empty bucket spaces in each section for future motors
• Size the vertical bus for the anticipated future load, not just the initial load
• Use a horizontal bus rating that accommodates planned expansion
• Document all empty spaces with their available stab ratings

Section Types

Standard Motor Control Section

The most common section type:

• Multiple bucket compartments
• Vertical bus with stab connections
• Can hold any combination of starter and feeder buckets
• Typically accommodates 3-7 buckets depending on heights

Main Incoming Section

Contains the main disconnect for the MCC:

• Main circuit breaker or main fusible disconnect
• Incoming cable termination
• Metering (CT/PT compartment for power monitoring)
• Ground fault relay (if required)
• May include surge protection

Transition Section

Used when the MCC changes configuration:

• Back-to-back to front-only transition
• Horizontal bus turns or changes direction
• Usually a full-height empty section with bus routing only

Auxiliary Section

Houses non-motor-control equipment:

• PLC panels
• Power monitoring equipment
• Communication equipment
• Building automation controllers
• May not have a vertical bus

Specifying a New Section

When adding a section to an existing MCC:

1. Match the manufacturer and model: Section dimensions and bus connections must match
2. Determine vertical bus rating: 300A or 600A based on anticipated bucket loads
3. Plan bucket layout: Define which buckets go where
4. Verify horizontal bus capacity: Ensure the existing horizontal bus can handle the additional load
5. Arrange installation: New sections bolt to existing sections with bus splice connections

MCC Depot can assist with planning and providing replacement buckets for new or existing MCC sections. We work with all major MCC brands.

Call 307-442-0382 or email sales@mccdepot.com for technical assistance.