B-Line Eaton series imperial cable tray and ladder, 4" Height, 11.5" Length, 30" Width, 30" and 24" Tray widths, Series 2-5 left reducer, Aluminum, 24

It's the transition piece that steps a 30 inch wide aluminum cable tray down to a 24 inch wide tray, with the reduction taken on the left side of the run. The right side rail stays in line, so the run shifts its usable width without moving the whole centerline. Rail height is 4 inches and the fitting is 11.5 inches long through the transition.

This is the left reducer, model 4A-30-LR24. Stand in the tray run looking in the direction of the reduction (from the 30 inch side toward the 24 inch side) and see which rail steps in. If the left rail is the one that moves inboard, this is your part. If the right rail moves, you need the right-hand version instead. Getting this backwards puts your tray edge off the hanger centerline.

Order Eaton B-Line 4A-30-LR24. That breaks down as 4 inch aluminum rail, 30 inch nominal width, left reducer, 24 inch radius. Quoting the full designation avoids getting a right-hand fitting or the wrong radius shipped.

Allow 11.5 inches along the run for the transition itself, 30 inches across at the wide end, and 4 inches of rail depth plus whatever clearance your cable fill and hanger hardware need above and below. The wide end sets the tightest point when you are routing around duct or pipe.

The fitting itself weighs 3.445 lb. Aluminum keeps the dead load low, which is part of why it gets specified over steel on long overhead runs where hanger spacing and rod sizing are driven by tray weight plus cable fill.