MCC Bucket Sizing Guide: Heights, Amperages, and NEMA Standards
Selecting the right MCC bucket size is fundamental to a reliable motor control installation. An undersized bucket leads to overheating and premature failure. An oversized bucket wastes space and money. This guide covers the standards and practical considerations for getting it right.
Standard Bucket Heights
MCC buckets use standardized heights measured in inches. These heights correspond to the vertical space they occupy in an MCC section:
| Height | Common Applications |
|---|---|
| 6" | Small feeders, pilot devices, dual-feeder configurations |
| 12" | Small starters (NEMA 0, 1), feeders up to 100A |
| 18" | Medium starters (NEMA 1, 2), feeders up to 250A |
| 24" | Larger starters (NEMA 2, 3), small VFDs |
| 36" | Large starters (NEMA 3, 4), medium VFDs, complex controls |
| 48" | Very large starters (NEMA 4, 5), large VFDs |
A standard 90-inch tall MCC section can hold multiple buckets stacked vertically. For example, you could fit five 18-inch buckets, or a mix of sizes totaling the available vertical space (accounting for top and bottom structural space).
NEMA Starter Sizes
The National Electrical Manufacturers Association (NEMA) defines motor starter sizes based on the motor horsepower and voltage they can control. Understanding NEMA sizes is essential for specifying starter buckets.
NEMA Size vs. Horsepower (3-Phase, 480V)
| NEMA Size | Max HP (480V) | Max FLA | Typical Bucket Height |
|---|---|---|---|
| 00 | 1.5 HP | 3.4A | 12" |
| 0 | 3 HP | 5.5A | 12" |
| 1 | 7.5 HP | 14A | 12"-18" |
| 2 | 25 HP | 40A | 18" |
| 3 | 50 HP | 78A | 24"-36" |
| 4 | 100 HP | 143A | 36"-48" |
| 5 | 200 HP | 254A | 48"+ |
| 6 | 400 HP | 480A | Full section |
These are maximum horsepower ratings. The actual motor FLA determines whether a starter can handle a particular motor. Always verify the motor nameplate FLA against the starter continuous current rating.
Voltage Considerations
Motor voltage affects starter sizing because higher voltage means lower current for the same horsepower:
- 208V motors draw approximately 2.4x the current of 480V motors at the same HP
- 240V motors draw approximately 2x the current of 480V motors
- 480V motors are the standard for industrial applications
- 575V/600V motors draw approximately 0.83x the current of 480V motors
This means a motor that requires a NEMA Size 2 starter at 480V might need a NEMA Size 3 at 208V. Always size based on actual voltage and current, not just horsepower.
Feeder Bucket Sizing
Feeder buckets are sized by their amperage rating, which is determined by the circuit breaker or fused disconnect inside:
| Amperage | Typical Bucket Height | Breaker Frame |
|---|---|---|
| 15-60A | 12" | 100A frame |
| 70-100A | 12"-18" | 100A frame |
| 125-250A | 18"-24" | 250A frame |
| 300-400A | 24"-36" | 400A frame |
| 450-600A | 36"-48" | 600A frame |
| 600-1200A | 48"+ or full section | 800A+ frame |
The breaker frame size determines the physical size of the breaker, which in turn determines the minimum bucket height. You can always use a taller bucket than required, but never a shorter one.
Wire Bending Space
NEC 430 and NEC 312.6 require minimum wire bending space in enclosures. This often dictates bucket height for larger amperage applications. Larger conductors need more room to make bends without damaging the insulation.
For example, a 400A feeder with 500 kcmil conductors requires significantly more bending space than a 100A feeder with #1 AWG. The bucket must be tall enough to accommodate the wire bending radius.
VFD Bucket Sizing
Variable frequency drive buckets require extra height for several reasons:
- VFD physical size increases with horsepower rating
- Input line reactor or filter adds components
- Output reactor may be required for long motor leads
- Ventilation space is needed above and below the VFD for airflow
- Bypass contactor (if included) adds a second contactor
A VFD application that would use a NEMA 2 starter (18" bucket) typically requires a 24" or 36" bucket to accommodate the drive, reactors, and ventilation requirements.
Practical Sizing Tips
1. Start with the Motor
Determine the motor HP, voltage, and FLA. Use NEC Table 430.250 for standard FLA values, then verify against the motor nameplate.
2. Select the Starter Size
Match the motor FLA to the appropriate NEMA starter size. If the motor FLA is close to the maximum rating for a size, consider going up one size for margin.
3. Account for Components
List everything that needs to fit in the bucket:
- Disconnect (breaker or fused switch)
- Starter or VFD
- Control transformer
- Overload relay (if separate from contactor)
- Terminal blocks
- Any additional devices (relays, timers, transient suppressors)
4. Check Manufacturer Specifications
Each MCC manufacturer has specific bucket configurations. A NEMA 2 starter fits in an 18" bucket in most Square D Model 6 configurations, but verify with the manufacturer catalog.
5. Consider Future Needs
If you might upgrade the motor in the future, consider oversizing the bucket now. It is much easier to put a smaller starter in a larger bucket than to replace the bucket entirely.
Common Sizing Mistakes
Mistake 1: Sizing by HP alone. Always verify the actual motor FLA. Special motors (high-efficiency, definite purpose, etc.) may have different current draws than standard motors.
Mistake 2: Forgetting the disconnect. The bucket must accommodate both the disconnect and the starter. A 100A breaker plus a NEMA 2 starter won't fit in a 12" bucket.
Mistake 3: Ignoring ambient temperature. If the MCC is in a hot environment (above 40 degrees C), contactors and breakers must be derated. This might push you to a larger starter size.
Mistake 4: Not accounting for VFD heat. VFDs generate significant heat. Putting a VFD in a bucket that's barely large enough leads to overheating and drive faults.
Need Help Sizing?
MCC Depot can help you determine the correct bucket size for your application. We build custom MCC buckets for all major MCC brands and can match your existing equipment exactly.
Call 307-442-0382 or email sales@mccdepot.com with your motor nameplate data and MCC information.