Requirements for Filler Plates in Distribution Boxes

Filler plates must meet thickness, placement, and load distribution requirements to ensure structural integrity or proper panel coverage.Structural Steel ApplicationsIn structural steel connections, filler plates (also called finger shims or packing plates) are used to fill gaps between connected members. According to AISC 360-22:Maximum thickness without strength reduction: Up to 0.25 in. (6 mm) can be used in slip-critical connections without reducing bolt shear strength for standard holes (Clause J3-3(c)), .Total thickness allowance: Fully inserted fillers up to 0.25 in. (6 mm) per joint are permitted without changing connection strength. For fillers greater than 0.25 in. but less than 0.75 in. (19 mm), the shear strength of bolts must be reduced by the factor [1 − 0.4(t − 0.25)], where t is the total filler thickness .Orientation: Filler orientation is independent of load direction.Extension and securing: Fillers thicker than 0.25 in. should extend beyond the joint and be secured with sufficient bolts to distribute the load uniformly over the combined cross-section of the member and filler .Welded connections: For welded fillers 0.25 in. or thicker, the filler should extend beyond the splice plate and be welded to the supporting member. Welds must transmit splice-plate stress and prevent overstress along the weld toe .Electrical Distribution BoxesFor electrical panelboards, filler plates are used to cover unused spaces and maintain proper spacing:Panel coverage: Filler plates plug or bolt into empty slots to cover unused breaker spaces, ensuring safety and compliance with panel design .Sizing: The number of filler plates depends on the number of mounting inches required by the missing breaker. Blank fillers are ordered accordingly to match the panel's configuration .Installation: Filler plates may include support brackets and hardware to secure them in place, maintaining alignment and preventing accidental contact with live components .Key ConsiderationsLoad distribution: In structural applications, ensure fillers do not compromise slip-critical or bearing connections.Thickness compliance: Always verify filler thickness against AISC or manufacturer specifications.Secure attachment: Whether bolted or welded, fillers must be properly fastened to prevent movement or stress concentration.Panel safety: In electrical boxes, fillers prevent exposure to live parts and maintain proper spacing for future breakers. By following these guidelines, filler plates can safely fill gaps in both structural and electrical distribution systems while maintaining performance and safety standards.