MCC Buckets for HVAC Systems: Sizing and Selection

HVAC systems in large commercial and industrial buildings rely on MCCs to control compressors, fans, pumps, and cooling towers. Selecting the right MCC bucket configuration for HVAC applications requires understanding the unique characteristics of HVAC motors and control requirements.

HVAC Motor Applications

Chiller Compressor Motors

The largest motors in most HVAC systems:

Centrifugal chillers: 100-2000+ HP
Screw chillers: 50-500 HP
Reciprocating chillers: 10-100 HP
Usually controlled by the chiller's integrated controller, not the MCC bucket
MCC bucket typically serves as a feeder or provides disconnect only

Air Handling Unit (AHU) Fan Motors

Supply and return fans for air distribution:

Large AHUs: 25-200 HP
Small AHUs: 1-15 HP
VFDs increasingly common for energy savings (variable air volume systems)
MCC bucket provides disconnect, motor protection, and VFD (if applicable)

Cooling Tower Fan Motors

Outdoor cooling tower fans:

Induced draft fans: 10-75 HP per cell
Forced draft fans: 5-40 HP per cell
Often VFD-controlled for capacity staging
Must account for cold-weather starting (oil viscosity, ice)

Chilled Water and Condenser Water Pumps

Circulating pumps for chilled and condenser water loops:

Primary pumps: 10-100 HP
Secondary pumps: 10-200 HP (variable flow)
VFDs on secondary pumps for energy efficiency
Constant-speed primary pumps use standard FVNR starters

Boiler System Motors

Heating system components:

Boiler feedwater pumps: 1-25 HP
Combustion air fans: 3-15 HP
Hot water circulating pumps: 1-50 HP
Usually simple FVNR starters

Sizing Considerations

Motor Service Factor

HVAC motors commonly have a 1.15 service factor, meaning they can operate continuously at 115% of nameplate HP. However:

Overload relays should still be set based on motor nameplate FLA
The service factor provides margin for voltage variation and minor overloads
Do not intentionally load the motor to the service factor rating

Starting Method Selection

Full Voltage (FVNR): Suitable for most HVAC motors under 50 HP where the utility can handle the inrush current. Simple, reliable, lowest cost.

VFD: Preferred for variable-flow applications (VAV fans, variable-speed pumps). Benefits include:

Energy savings of 30-60% on variable-load applications
Soft starting eliminates mechanical stress
Precise speed control for comfort optimization
Requires larger bucket (24"-36" typical for 25+ HP)

Soft Starter: Good compromise for large motors that do not need speed control:

Reduces starting current by 40-60%
Eliminates mechanical shock on belts and couplings
Less expensive than VFDs
No energy savings at running speed

Bucket Height for VFD Applications

VFD buckets for HVAC applications are typically larger than equivalent FVNR starter buckets:

Motor HP (480V)	FVNR Bucket Height	VFD Bucket Height
5-10 HP	12"	18"
15-25 HP	18"	24"
30-50 HP	18"-24"	24"-36"
75-100 HP	24"-36"	36"-48"

Additional height is needed for the VFD, line reactor, and ventilation space.

Building Automation System (BAS) Integration

Modern HVAC systems are controlled by building automation systems (BAS). MCC buckets must interface with the BAS for:

Control Signals

Start/Stop: BAS sends digital output to start or stop the motor
Speed Command: For VFD applications, BAS sends analog (4-20mA or 0-10V) or network command for speed setpoint
Hand-Off-Auto Switch: Allows local manual override of BAS control

Status Feedback

Run Status: Digital input to BAS confirming motor is running
Fault Status: Digital input indicating overload trip or breaker trip
Current: Analog signal (4-20mA) from CT for motor current monitoring
VFD Feedback: Speed, current, power, and fault data via network

Communication Protocols

BACnet: The dominant HVAC protocol. Many VFDs have BACnet MS/TP or BACnet/IP capability.
Modbus: Common for VFDs and power monitoring devices.
LonWorks: Used in some older BAS installations.
EtherNet/IP: For facilities with industrial automation systems.

Typical Bucket Wiring for BAS Integration

For a VFD bucket with BAS integration:

3-wire control (Start, Stop, Common) from BAS to VFD
Analog speed reference (4-20mA) from BAS to VFD
Run status relay output from VFD to BAS
Fault status relay output from VFD to BAS
Hand-Off-Auto selector switch on bucket door
Network connection (if using BACnet or Modbus over network)

Energy Code Compliance

Modern energy codes (ASHRAE 90.1, IECC) require:

VFDs on certain fan and pump motors above specific HP thresholds
Demand-controlled ventilation with variable-speed fans
Variable-flow pumping systems
Energy monitoring capability

These requirements drive MCC bucket specifications toward VFD-equipped configurations.

Common HVAC MCC Issues

VFD Overheating

HVAC VFD buckets in rooftop or penthouse MCC installations may overheat in summer:

Ensure adequate ventilation in the MCC room
Consider supplemental cooling for VFD sections
Verify VFD ambient temperature rating

Harmonic Issues

Multiple VFDs on the same MCC bus can create harmonic distortion:

Specify line reactors or harmonic filters on VFD buckets
Monitor total harmonic distortion (THD) at the MCC bus
Consider active front-end VFDs for large installations

Seasonal Starting

HVAC equipment may sit idle for months (e.g., cooling tower fans in winter):

Exercise idle motors monthly to prevent bearing damage
Verify contactors operate freely before seasonal startup
Meggering motor windings before restart after extended idle periods

MCC Depot HVAC Solutions

MCC Depot provides replacement buckets and custom builds for HVAC MCC applications. We can match your existing MCC configuration and integrate with your building automation system.

Call 307-442-0382 or email sales@mccdepot.com for HVAC MCC bucket quotes.