Retrofitting Obsolete MCC Buckets: A Complete Process Guide

When your MCC is 20, 30, or 40 years old, original components become increasingly difficult to source. Breakers are discontinued, contactors are obsolete, and overload relay heater elements are unavailable. Retrofitting replaces the internal components with modern equivalents while keeping the existing bucket enclosure and stab assembly. This guide walks through the complete retrofit process.

What is a Bucket Retrofit?

A retrofit (or rebuild) involves:

Removing all internal components from an existing MCC bucket
Installing new, current-production components
Re-wiring the bucket with new wire
Testing the completed bucket
Reinstalling in the original MCC location

The bucket enclosure, stab assembly, and door are retained. Everything inside is new.

When Retrofitting Makes Sense

Good Candidates for Retrofit

Bucket enclosure is structurally sound (no severe corrosion or damage)
Stab assembly is in good condition (good spring tension, clean contacts)
Door and interlock mechanism work properly
The bucket model is rare or difficult to reproduce (legacy MCCs)
Only internal components need replacement
Budget is constrained but the bucket enclosure is serviceable

Poor Candidates for Retrofit

Enclosure is severely corroded or damaged
Stab assembly is degraded (weak springs, pitted contacts, overheating evidence)
Multiple structural issues (bent frame, broken hinges, damaged mounting)
The bucket is a common model easily available as a complete replacement
A configuration change is needed (e.g., converting FVNR to VFD)

The Retrofit Process

Step 1: Assessment and Documentation

Before removing any components, thoroughly document the existing bucket:

Photographs:

Overall bucket with door closed
Door open showing all components
Close-up of each component (breaker, contactor, overload, transformer)
All nameplates and labels
Wiring routing and connections
Stab assembly (rear of bucket)
Door-mounted devices

Measurements:

Bucket height (6", 12", 18", 24", 36", 48")
Component mounting hole patterns
Wire entry locations
Door-mounted device cutout dimensions

Electrical Data:

Breaker type and amperage
Contactor NEMA size or model number
Overload relay type and FLA range
Control transformer VA rating and voltage ratio
Motor HP, voltage, and FLA
Control voltage

Wiring Documentation:

Create a wiring diagram or sketch of all connections
Label every wire before disconnecting
Note wire sizes and types
Document terminal block configurations

Step 2: Component Selection

Select modern replacement components that fit the existing bucket:

Circuit Breaker:

Match the amperage rating and interrupting capacity
Verify the mounting pattern fits the existing bucket cutout
Cross-reference the obsolete breaker to a current-production model
Consider upgrading to a motor circuit protector (MCP) if the existing unit uses a standard thermal-magnetic breaker

Contactor:

Match the NEMA size (or equivalent IEC rating)
Verify physical dimensions fit the available space
Check coil voltage matches control power (typically 120V AC)
Select new auxiliary contacts as needed

Overload Relay:

Strongly recommend upgrading from thermal to electronic overloads
Electronic overloads offer adjustable range (covers multiple motor sizes)
No heater elements to stock or replace
Better protection features (phase loss, phase unbalance, ground fault)
Verify the mounting pattern matches the contactor

Control Transformer:

Match VA rating (or increase if adding control devices)
Match voltage ratio (typically 480V primary to 120V secondary)
Standard mounting dimensions usually accommodate new transformers

Door-Mounted Devices:

Replace pushbuttons, selector switches, and indicator lights
Match mounting hole sizes and patterns
Use new devices rated for the application

Step 3: Component Removal

With the bucket de-energized and removed from the MCC:

Disconnect all wiring from each component (label wires if not already done)
Remove the circuit breaker or fused disconnect
Remove the contactor
Remove the overload relay
Remove the control transformer
Remove terminal blocks
Remove all old wiring
Remove door-mounted devices
Clean the bucket interior (remove dust, debris, old wire clippings)
Inspect the enclosure for any damage, corrosion, or wear

Step 4: Enclosure Preparation

Prepare the bucket for new components:

Repair any minor corrosion (sand, prime, paint)
Drill new mounting holes if new components have different mounting patterns
Install any needed adaptor plates for component size differences
Clean and inspect the stab assembly
Verify door hinge and interlock mechanism
Replace any damaged hardware (screws, nuts, rivets)

Step 5: Component Installation

Install new components in order:

Circuit Breaker: Mount and secure. Verify proper orientation for stab power input.
Contactor: Mount below the breaker. Ensure adequate spacing for arc chute clearance.
Overload Relay: Mount to the contactor load side (most electronic overloads mount directly to the contactor).
Control Transformer: Mount in available space, typically on the side wall.
Terminal Blocks: Install on DIN rail at the top or bottom of the bucket.
Door-Mounted Devices: Install pushbuttons, selector switches, and lights.

Step 6: Wiring

Wire the bucket per the original wiring diagram (updated for any new component requirements):

Power Wiring:

Stab assembly to breaker input terminals
Breaker output to contactor line-side terminals
Contactor load-side through overload relay current transformers
Overload relay to load terminals (for motor cable connection)

Control Wiring:

Control transformer primary from breaker load side
Control transformer secondary through control fuse
Stop button, start button, contactor coil, overload relay auxiliary contact in series control circuit
Seal-in (holding) contact parallel with start button
Indicating lights from appropriate control points

Wire Standards:

Use new wire throughout (never reuse old wire)
Minimum 14 AWG for control circuits
Power wire sized per NEC 430 for the motor FLA
MTW (Machine Tool Wire) or THHN/THWN rated for 90 degrees C
Wire color coding: black/red/blue for power phases, white for neutral, green for ground, other colors for control circuits

Step 7: Testing

Before reinstalling in the MCC:

Visual Inspection:

Verify all connections are tight
Check wire routing (no interference with moving parts)
Confirm proper fusing
Verify overload relay settings match motor data

Electrical Testing:

Measure control transformer output voltage
Verify control circuit operation (press start, contactor closes; press stop, contactor opens)
Check overload relay operation (test button)
Megger power circuit to ground
Verify interlock mechanism operates correctly

Step 8: Installation and Commissioning

Insert the retrofitted bucket into the MCC section
Verify stab engagement (bucket seats fully)
Connect motor cables and control wiring to the bucket terminals
Close the disconnect
Test motor operation: start, run, stop
Measure motor current on all three phases
Verify overload relay is reading correctly
Check all indicating lights and controls
Perform a thermographic scan after 30 minutes of operation

Step 9: Documentation

Update all records:

Mark the retrofit date and components used on the bucket nameplate or label
Update the one-line diagram
Record new component part numbers in the maintenance system
File the wiring diagram with the retrofit documentation
Update the spare parts inventory list

MCC Depot Retrofit Services

MCC Depot offers professional retrofit services for MCC buckets of all brands. You can send us your bucket, and we will:

Assess the enclosure and stab condition
Replace all internal components with new, current-production parts
Wire and test the bucket
Return it ready to install

We also build complete replacement buckets when retrofit is not the best option.

Call 307-442-0382 or email sales@mccdepot.com to discuss your retrofit needs.