Square D Model 4 MCC: Identifying and Replacing Obsolete Buckets

The Square D Model 4 MCC is the predecessor to the Model 5 and Model 6 platforms. Produced from the 1960s through the early 1980s, Model 4 MCCs are among the oldest motor control centers still in service. If you are maintaining a Model 4, understanding its unique characteristics is essential for keeping it running.

Identifying a Model 4

Nameplate

The MCC nameplate should identify it as a "Model 4" or "Mod 4" MCC manufactured by Square D Company. The nameplate is typically on the main incoming section.

Age

If your Square D MCC was installed before 1980, there is a good chance it is a Model 4. MCCs installed between 1978 and 1985 could be either Model 4 or early Model 5, so check the nameplate.

Physical Characteristics

Model 4 MCCs have several distinctive features:

Older construction style: Heavier gauge steel, simpler fabrication
Door design: Different handle and latch mechanism from Model 5/6
Stab design: Unique to Model 4, not compatible with Model 5 or 6
Bus configuration: Earlier bus bar design with different connection methods
Paint color: Often original ANSI 61 gray, though many have been repainted

Stab Differences

This is the critical identification point. Model 4 stabs are physically different from Model 5 and Model 6 stabs:

Different contact finger geometry
Different mounting arrangement on the bucket
Different engagement pattern with the vertical bus

Model 4 buckets will NOT fit in Model 5 or Model 6 MCCs, and vice versa.

Obsolescence Challenges

Component Availability

Model 4 MCCs used components that were current production in the 1960s and 1970s. Many are long discontinued:

Circuit Breakers: Original Square D FA, FH, and KA series breakers used in Model 4 buckets are discontinued. Some can be cross-referenced to newer Square D breakers, but physical mounting may require adaptation.

Contactors: Original Square D Class 8536 NEMA starters were robust and long-lived. Many are still functional after 40+ years. Replacements are available through Schneider Electric (current Square D parent company).

Overload Relays: Original thermal overload relays with specific heater elements. Heater element tables for Model 4-era overloads may be difficult to find.

Control Transformers: Standard VA ratings are replaceable with current-production transformers.

Structural Concerns

After 40-60 years of service, the MCC structure itself may show wear:

Corrosion, especially in humid or chemical environments
Bus bar joint deterioration
Structural fatigue from repeated bucket insertion/removal
Worn stab contact points on the vertical bus

Replacement Options

Individual Bucket Replacement

The most practical approach for functional MCCs:

Build new buckets with correct Model 4 stab configuration
Use current-production components (breakers, contactors, overloads)
Maintain the existing MCC structure

MCC Depot builds Model 4 compatible replacement buckets with new components and correct stab assemblies.

Bucket Retrofit

Convert existing Model 4 buckets to use current-production components:

Keep the original bucket enclosure and stab assembly
Replace internal components with modern equivalents
Upgrade wiring and control devices
Add electronic overload relays for better protection

This approach works well when the bucket enclosure and stabs are in good condition.

Full MCC Replacement

When the Model 4 MCC structure is deteriorated or when modernization is needed:

Replace with a new Square D Model 6 or equivalent MCC
Upgrade to current safety standards (arc flash, dead-front construction)
Add network communication capability
Higher cost but eliminates obsolescence concerns

Model 4 vs. Model 5 vs. Model 6

Feature	Model 4	Model 5	Model 6
Era	1960s-early 1980s	Late 1970s-mid 1990s	Mid 1990s-present
Status	Obsolete	Discontinued	Current production
Stab Design	Model 4 specific	Model 5 specific	Model 6 specific
Arc Resistance	No	No	Available
Dead Front	No	Limited	Yes
Smart MCC	No	No	Available

Maintenance Priorities for Model 4 MCCs

If you are keeping your Model 4 in service, prioritize these maintenance activities:

Annual

Thermographic survey of all bus connections and stab points
Visual inspection of all bucket interiors
Test all disconnect mechanisms
Verify all overload relay settings

Every 3-5 Years

Retorque all bus connections
Clean and inspect all stab assemblies
Test all circuit breakers (time-current characteristic testing)
Replace any deteriorated wiring

As Needed

Replace buckets with failing or obsolete components
Address any corrosion or structural issues
Upgrade overload relays from thermal to electronic

Documentation

For Model 4 MCCs, maintaining good documentation is critical:

Photograph every bucket (exterior and interior)
Record all nameplate data
Document stab configuration details
Keep a log of all replacements and modifications
Maintain one-line diagrams showing all bucket positions and ratings

This documentation makes ordering replacement buckets much easier and faster.

Get Help with Your Model 4

MCC Depot has extensive experience with legacy Square D MCCs. We can:

Help identify your Model 4 bucket specifications
Build exact replacement buckets with modern components
Provide retrofit solutions for your aging equipment
Offer guidance on replacement vs. repair decisions

Call 307-442-0382 or email sales@mccdepot.com with photos of your Model 4 MCC and buckets.