UL 508A vs UL 845: MCC Certification Standards Explained
When specifying motor control equipment, you will encounter references to UL 508A and UL 845. These are different standards with different scopes, and understanding which applies to your equipment is important for code compliance, insurance, and safety. This guide explains both standards and when each applies.
UL 845: Motor Control Centers
What It Covers
UL 845 is the standard specifically for Motor Control Centers. It covers the complete MCC assembly including:
- The structural enclosure (sections, frames, doors)
- Horizontal and vertical bus systems
- Plug-in units (buckets)
- Internal wiring
- Overcurrent protective devices
- Motor starters and controllers
- Control circuits
Key Requirements
UL 845 requires:
Temperature Rise Testing: The complete MCC assembly must be tested to verify that bus bars, connections, and components do not exceed maximum temperature rise limits under rated load conditions. Typical limits are 65 degrees C rise over 40 degrees C ambient for bus connections.
Short-Circuit Testing: The MCC must be tested or evaluated for its short-circuit withstand rating. This ensures the bus bars, connections, and enclosure can withstand the mechanical and thermal stresses of a fault current event.
Bus Bar Sizing: Horizontal and vertical bus bars must meet specific current-carrying capacity requirements based on the cross-sectional area, material, and configuration.
Interlocking: UL 845 specifies requirements for mechanical interlocks between bucket disconnects and doors, and between bucket insertion/removal mechanisms and disconnects.
Marking: Specific nameplate requirements including voltage, bus ratings, short-circuit current rating, and manufacturer identification.
Who Certifies
UL 845 MCCs are manufactured by major MCC manufacturers (Schneider Electric/Square D, Siemens, Eaton/Cutler-Hammer, ABB/GE, Rockwell/Allen-Bradley) in UL-certified manufacturing facilities. The entire MCC lineup receives the UL 845 listing mark.
When It Applies
UL 845 applies to:
- New MCC installations designed as integrated motor control center assemblies
- Factory-assembled MCCs from major manufacturers
- Equipment that an Authority Having Jurisdiction (AHJ) requires to be listed as an MCC
UL 508A: Industrial Control Panels
What It Covers
UL 508A covers Industrial Control Panels, which are assemblies of industrial control equipment (switches, motor controllers, relays, VFDs, etc.) mounted in an enclosure. This is a broader, more general standard than UL 845.
Key Requirements
UL 508A requires:
Short-Circuit Current Rating (SCCR): The panel must be marked with its short-circuit current rating, calculated based on the lowest-rated component in the power circuit. NEC 409.110 requires that the SCCR equal or exceed the available fault current at the installation point.
Overcurrent Protection: Proper sizing and application of overcurrent protective devices per NEC and UL requirements.
Wiring: Wire sizing, routing, and termination per UL 508A requirements. Wire type and temperature ratings must be appropriate for the application.
Spacing: Minimum electrical spacing (creepage and clearance) between uninsulated live parts and between live parts and ground.
Marking: Nameplate requirements including voltage, SCCR, enclosure type, and manufacturer identification.
Component Listing: Individual components (breakers, contactors, overloads, VFDs) must be UL-listed or recognized for use in UL 508A panels.
Who Certifies
UL 508A panels can be built by:
- Major manufacturers in certified facilities
- Smaller panel shops that are UL 508A certified
- Any manufacturer with a UL 508A listing program
The certification is typically easier and less expensive to obtain than UL 845, making it accessible to more manufacturers.
When It Applies
UL 508A applies to:
- Standalone motor control panels (not plug-in MCC buckets)
- Custom control panels with motor starters
- VFD panels
- Any industrial control enclosure that is not an integrated MCC assembly
Key Differences
| Feature | UL 845 (MCC) | UL 508A (Control Panel) |
|---|---|---|
| Scope | Complete MCC assemblies | Individual control panels |
| Bus System | Horizontal and vertical bus required | No bus system (direct wiring) |
| Plug-In Feature | Yes (removable buckets) | No (fixed mounting) |
| Temperature Rise Test | Required on assembly | Not typically required |
| Short-Circuit Test | Required on assembly | Calculated (SCCR) |
| Manufacturer Scope | Major MCC manufacturers | Broad (many panel shops) |
| Cost to Certify | Higher | Lower |
| NEC Reference | NEC 430 Part VIII | NEC 409 |
How This Affects Replacement Buckets
This is where it gets practical. When you need a replacement MCC bucket:
OEM Replacement
An OEM bucket from the original MCC manufacturer maintains the UL 845 listing of the overall MCC assembly. The bucket is designed, tested, and certified as part of the UL 845 MCC system.
Aftermarket Replacement
An aftermarket replacement bucket (like those built by MCC Depot) is typically built and labeled as a UL 508A industrial control panel:
- It is a self-contained control panel that happens to plug into an MCC
- It meets all UL 508A requirements for overcurrent protection, wiring, spacing, and SCCR
- It uses UL-listed components (breakers, contactors, overloads)
- It fits the existing MCC with correct stab configuration
Does This Matter?
For most practical purposes, a properly built UL 508A replacement bucket provides equivalent safety and performance to a UL 845 OEM bucket:
- Same overcurrent protection
- Same component quality (often identical components)
- Same SCCR evaluation
- Same wiring standards
- Properly sized stab connections for the application
The AHJ (local electrical inspector) has the final authority on acceptability. In our experience, properly built and labeled UL 508A replacement buckets are accepted by inspectors in the vast majority of jurisdictions.
SCCR: A Critical Consideration
Short-Circuit Current Rating is one of the most important aspects of both UL 845 and UL 508A:
- NEC 110.10 requires equipment to have an interrupting rating not less than the available fault current
- NEC 409.110 specifically requires SCCR marking on industrial control panels
- NEC 430.98 requires SCCR marking on MCCs
When ordering a replacement bucket, the SCCR of the bucket must equal or exceed the available fault current at the MCC location. MCC Depot calculates SCCR for every bucket we build and marks it on the nameplate.
Bottom Line
UL 845 is for complete MCC assemblies. UL 508A is for individual control panels. Both ensure safe, code-compliant equipment when properly applied. For replacement MCC buckets, UL 508A certification with proper SCCR evaluation provides a practical, cost-effective, and safe alternative to OEM UL 845 buckets.
MCC Depot builds UL 508A certified replacement buckets with proper SCCR ratings for all major MCC platforms.
Call 307-442-0382 or email sales@mccdepot.com for certified replacement buckets.