A Practical Guide to Identifying the Right MCC Replacement Parts

MCC replacement parts are the pieces that connect and hold a plug-in motor control center unit, or bucket, in its section: the stab assembly that carries power from the vertical bus into the bucket, and the door and rail hardware around it. To pick the right stab assembly you need three facts: the MCC make and line, the amperage of the bucket, and how many cable connectors it has per phase. BRAH Electric lists 14 stab assemblies and 3 hardware parts for Cutler-Hammer, General Electric, Westinghouse, Siemens, Square D and Nelson motor control centers, all shown in the table below.

MCC replacement parts at a glance

• Stab assembly: the spring contacts and leads on the back of a bucket that plug onto the vertical bus. Identify it by MCC line, amperage and connectors per phase.
• Door mounting hardware kit: hinges, quarter-turn latches and screws for a unit door.
• Top rail assembly: a replacement top rail for Cutler-Hammer Type W.
• All 17 parts are in the MCC parts listing. Every stab assembly there is described as rated for 600VAC service.
• A stab assembly from one MCC line does not fit another, even at the same amperage.

What an MCC stab assembly does

A motor control center is a row of vertical sections fed by a common horizontal bus. Each section has a vertical bus running behind the units. A plug-in unit, usually called a bucket, holds the disconnect or breaker, the starter and the control devices for one motor or feeder. When the bucket is pushed into the section, a set of spring-loaded contacts on its back grips the vertical bus bars. Those contacts, their insulating housing and the leads that run to the line side of the disconnect make up the stab assembly.

The stab assembly is the only power connection between the bus and everything in the bucket, so all of the unit load passes through it. A stab that has lost contact pressure, or has pitted or discoloured contact surfaces, runs hot. Heat then damages the housing and the lead insulation. Replacing the assembly restores the connection without replacing the bucket.

These parts are not the same as bus plug stabs, which connect a bus plug to overhead busway. If that is what you are working on, see the guide to bus plug stabs.

MCC stab assemblies and hardware in the BRAH catalogue

The table lists every part in the MCC parts sub-category. Amperage is the rating given in the catalogue description; where the description gives a range, the range is shown. The OEM number is the manufacturer part number the catalogue record lists for that part. Part numbers link to the product pages.

For anything not in the table, call (855) 355-2724 with the part number.

How to identify the right stab assembly

Work through these in order. Each one removes part of the table until one row is left.

1. MCC make and line

Read the nameplate on the MCC section, not only the label on the bucket. Buckets get moved between line-ups, and the stab has to match the bus it plugs onto. The catalogue groups stab assemblies by line: Cutler-Hammer F2100 (the 150A, 250A and 400A assemblies are also described as suitable for Five Star, 2100 and Advantage series buckets), General Electric 8000 Line, Westinghouse Type W, Siemens MARQ21, and Nelson.

2. Bucket amperage

Order the amperage of the assembly you are removing. It follows the size of the disconnect or breaker in the bucket, so the unit nameplate and the device rating are the places to confirm it. The catalogue covers 60A, 150A, 250A and 400A for Cutler-Hammer F2100; 100A, 225A and 250A for GE 8000 Line; up to 150A for Westinghouse Type W; 60A and 150A to 250A for Siemens MARQ21; and 200A for Nelson. Do not fit a lower-rated assembly because it looks the same.

3. Cable connectors per phase

On the GE 8000 Line parts, the description states how many cable connectors each phase has. MCCSTABS-GE-8000LINE-100A and MCCSTABS-GE-8000LINE-250A-1 have one connector per phase. MCCSTABS-GE-8000LINE-225A and MCCSTABS-GE-8000LINE-250A-2 have two. Count the conductors leaving each phase of the old assembly before you order: at 250A both versions are listed, and they are different parts.

4. Lead type and length

Several descriptions give the leads. The Westinghouse Type W assemblies are described with four-wire flexible cable connectors and 30 inch leads, the GE 100A assembly with 54 inch leads, and the Nelson 200A assembly with 2/0 flexible cable and 17 inch leads. Measure the run from the stab to the line terminals of the disconnect in your bucket and compare.

5. OEM number, if you can read it

If the old assembly still carries a manufacturer number, use it. The catalogue lists 4700A97G21, 4700A97G69 and 4700A97G03 against the Cutler-Hammer 150A, 250A and 400A assemblies, and 302A3771G1, 302A3771G2 and 117B6219G1 against GE 8000 Line assemblies. Type the number into site search and it returns the BRAH part whose record carries it.

Door and rail hardware for MCC units

Three parts in the listing are mechanical. MCC-CH-DOORMNT is a door mounting hardware kit for Cutler-Hammer Type W, listed against OEM number 4719A93G22; the kit includes two hinges, two quarter-turn latches, two quarter-turn screws and mounting hardware. MCC-CH-W-TOPRAILASS is a top rail assembly listed against 4719A94G32. MCC-SQD-M6-HINGE is a door mounting hardware kit for Square D Model 6.

A unit door that will not latch is more than a nuisance. The door is part of the enclosure, and it has to close and stay closed for the unit to be operated as intended. Replace broken hinges and latches instead of leaving a door tied or taped shut.

Other parts inside an MCC bucket

Most bucket repairs involve the starter, not the stab. BRAH Electric lists aftermarket replacements for the devices found in starter units under Motor Controls: contactors, overload relays, magnetic coils, contact kits and auxiliary contacts. For these, the device nameplate matters more than the MCC nameplate: copy the full part number from the contactor or relay and search for it. The guide to troubleshooting overload relays covers the relay side.

What to check before you order

• MCC manufacturer and line, from the section nameplate.
• Amperage of the existing stab assembly and of the disconnect or breaker in the bucket.
• Cable connectors per phase: one or two.
• Lead length needed from the stab to the line terminals.
• OEM number on the old assembly, if legible.
• Condition of the vertical bus where the stab lands. A new stab on damaged bus will overheat again.
• For hardware: the MCC line (Cutler-Hammer Type W or Square D Model 6) and which piece is broken.

BRAH Electric is an aftermarket manufacturer and is not an authorized distributor of Eaton, GE, Siemens, Square D or the other OEM brands named here. Parts are designed in-house and tested at its Carlsbad, California facility, and carry a 2-year warranty from the date of invoice. In-stock orders placed before 3:00 PM Pacific ship the same business day. Returns need an RMA within 30 days of shipment and carry a 25 to 50 percent restocking charge, so settle the five identification points first. If you are unsure, call (855) 355-2724, Monday to Friday, 6AM to 5PM Pacific, with photos of the old assembly and the section nameplate.

Safety when working on MCC stabs

The vertical bus behind a bucket stays energized when the bucket disconnect is open. Withdrawing a bucket or working on its stabs exposes that bus. De-energize the MCC or the section, apply lockout/tagout, and verify the absence of voltage before removing a unit. Follow the MCC manufacturer instructions and your site electrical safety program. This is work for qualified personnel only.

MCC replacement parts FAQs

What does MCC stand for?

Motor control center. It is a floor-mounted assembly of vertical sections that houses starters, feeders and other control units for many motors in one place, fed from a common bus.

What is a stab assembly in an MCC bucket?

It is the set of plug-on contacts and leads on the back of a plug-in unit. The contacts grip the vertical bus when the bucket is inserted, and the leads carry power to the line side of the bucket disconnect.

Can I replace the stab assembly without replacing the whole bucket?

Yes. The stab assembly is a separate part. Replace it with one made for the same MCC line and amperage, with the same number of connectors per phase, and inspect the bus and the line terminals at the same time.

Will a stab assembly from one MCC brand fit another?

No. Bus spacing, bus bar shape and unit mounting differ between MCC lines, so stab assemblies are specific to the line. The catalogue lists each assembly against one manufacturer and line for that reason.

What if there is no part number on the old stab assembly?

Use the section nameplate for the MCC line, the bucket disconnect for the amperage, and count the connectors per phase. Take photos of the back of the bucket and the nameplate. Those details are enough to pick a row in the table or to confirm by phone.

References

• UL 845, Motor Control Centers: the UL standard that covers motor control centers and their units.
• OSHA 29 CFR 1910.147: control of hazardous energy (lockout/tagout).
• OSHA 29 CFR 1910.333: selection and use of electrical work practices, including de-energizing and verification.
• NFPA 70E: standard for electrical safety in the workplace.