Understanding MCC Bus Ratings: 600A to 4000A Explained
The bus system is the electrical highway inside your motor control center. Every bucket draws power from it, and its rating determines how much total load the MCC can handle. Misunderstanding bus ratings leads to overloaded equipment, nuisance trips, and potential safety hazards.
What is an MCC Bus?
An MCC bus is a set of copper or aluminum bars that conduct electricity through the motor control center. There are two distinct bus systems in every MCC:
Horizontal Bus
The main power bus that runs horizontally across the top (or bottom) of the MCC lineup, connecting all vertical sections together. This bus connects to the incoming power feed through the main disconnect and distributes power to every section.
Vertical Bus
Each vertical section has its own vertical bus that taps off the horizontal bus. The vertical bus runs from top to bottom within a section and provides the connection points (stab locations) for individual MCC buckets.
Common Bus Ratings
Horizontal Bus Ratings
| Rating | Typical Application |
|---|---|
| 600A | Small MCCs, limited motor loads |
| 800A | Medium facilities, moderate motor loads |
| 1200A | Large industrial plants, multiple large motors |
| 1600A | Heavy industrial, mining, water treatment |
| 2000A | Large process facilities |
| 2500A | Heavy process industry |
| 3000A | Major industrial complexes |
| 4000A | Very large facilities, utility applications |
The horizontal bus rating is the maximum continuous current the bus can carry at rated temperature rise (typically 65 degrees C rise over 40 degrees C ambient).
Vertical Bus Ratings
Vertical bus ratings are typically lower than horizontal bus ratings:
| Rating | Application |
|---|---|
| 300A | Standard sections with smaller buckets |
| 600A | Sections with larger buckets or multiple large starters |
The vertical bus rating limits the total load you can connect in a single section. If you have a section with a 300A vertical bus, the sum of all bucket continuous currents in that section should not exceed 300A.
Bus Material: Copper vs. Aluminum
Copper bus is the industry standard for MCCs. It offers:
- Higher conductivity (lower resistance)
- Better resistance to corrosion
- Easier to make bolted connections
- Higher current density (smaller cross-section for same rating)
Aluminum bus is occasionally used to reduce cost:
- Lower material cost
- Lighter weight
- Requires larger cross-section for same current rating
- More susceptible to oxidation at connections
- Requires special hardware (Belleville washers, anti-oxidant compound)
Most MCC manufacturers use tin-plated copper bus as standard. The tin plating prevents oxidation and ensures reliable bolted connections over the life of the equipment.
Bus Bracing (Short-Circuit Rating)
Beyond the continuous current rating, MCC buses have a short-circuit withstand rating, also called the bracing rating. This is measured in kA (kiloamperes) and represents the maximum fault current the bus can withstand without mechanical damage.
Common withstand ratings:
- 22 kA - Light commercial and industrial
- 42 kA - Standard industrial
- 65 kA - Heavy industrial
- 85 kA - Very heavy industrial
- 100 kA - Utility and heavy process
The bus bracing must exceed the available fault current at the MCC location. A coordination study determines the available fault current based on the utility source, transformer impedance, and conductor lengths.
How Bus Ratings Affect Bucket Selection
The vertical bus rating limits what you can install in a section:
Example: 300A Vertical Bus Section
- Slot 1: 50 HP starter (65A FLA)
- Slot 2: 25 HP starter (34A FLA)
- Slot 3: 100A feeder
- Slot 4: 15 HP starter (21A FLA)
- Total: 220A - within 300A vertical bus rating
If you tried to add another large starter, you would exceed the vertical bus capacity and need to either use a different section or upgrade to a 600A vertical bus.
Determining Available Capacity
To determine how much capacity remains in your MCC:
Step 1: Find the Bus Rating
Check the MCC nameplate for horizontal and vertical bus ratings. The nameplate is typically on the main section or incoming breaker section.
Step 2: Calculate Current Load
Add up the full-load amperes of all connected buckets. For motor starters, use NEC Table 430.250 values or motor nameplate FLA. For feeders, use the breaker trip rating or actual measured load.
Step 3: Apply Demand Factor
Not all motors run simultaneously in most applications. NEC allows demand factors in some cases, but for MCC bus sizing, it is conservative to assume all loads may operate at the same time unless a load management system prevents simultaneous operation.
Step 4: Compare to Rating
The total calculated load should not exceed the bus rating. Industry practice is to limit loading to 80% of the bus rating for continuous loads per NEC 220.
Can You Upgrade a Bus?
Upgrading an MCC bus is generally not practical:
- Horizontal bus upgrades require disassembling the entire MCC lineup
- Vertical bus upgrades require removing the section from service
- Both require re-certification of the assembly
In most cases, it is more cost-effective to add a new MCC section or a separate MCC for additional loads.
Bus Connection Maintenance
Bus connections are critical reliability points. Loose connections cause:
- Increased resistance
- Localized heating
- Oxidation of contact surfaces
- Progressive degradation
- Potential arc flash events
Annual thermographic inspections can identify hot connections before they fail. All bus connections should be retorqued during scheduled maintenance outages per the manufacturer's specifications.
Need Help with Your MCC?
MCC Depot builds replacement buckets compatible with all major MCC bus systems. Whether you have a Square D, Siemens, GE, or Cutler-Hammer MCC, we can provide buckets that connect properly to your vertical bus.
Call 307-442-0382 or email sales@mccdepot.com for assistance with your MCC bucket needs.