The Definition of Rated Capacity

Definition of Rated Capacity

Rated capacity is the maximum load that a piece of lifting or load-handling equipment is designed to support safely under a specific set of conditions. It is set by the manufacturer, or in some cases by a qualified person, and it applies to cranes, hoists, slings, rigging hardware, forklifts, scaffolds and aerial work platforms. Depending on the equipment, the same idea may be called rated load, safe working load or working load limit (WLL).

Rated capacity is never a single fixed number for all situations. A mobile crane's capacity changes with boom length, boom angle and load radius. A sling's capacity depends on the type of hitch and the angle of the legs. A forklift's capacity assumes the load's center of gravity sits at the load center shown on the nameplate. Change the conditions and the capacity changes with them.

OSHA rules repeatedly prohibit exceeding rated capacity. For example, 29 CFR 1926.1417(o)(1) states that crane equipment "must not be operated in excess of its rated capacity," and 29 CFR 1910.184(c)(4) states that slings "shall not be loaded in excess of their rated capacities." Overloading is a leading path to structural failure, tip-over and dropped loads.

Where Rated Capacity Applies

Mobile and tower cranes

For construction cranes, rated capacity comes from the manufacturer's load chart, which lists allowable loads for each configuration. Under 29 CFR 1926.1417(o)(3), the operator must verify that the load is within rated capacity, either by getting the load weight from a reliable source or calculation before the lift, or by using a load weighing or load limiting device. If the load is more than 75 percent of the maximum rated capacity at the longest radius used in the lift, the weight must be verified before proceeding. Section 1926.1417(o)(2) also states that the operator must not be required to exceed rated capacity.

Overhead cranes and hoists

Under 29 CFR 1910.179(b)(5), the rated load of an overhead or gantry crane must be plainly marked on each side of the crane, and each hoist must have its rated load marked on it or its load block, legible from the ground or floor. See overhead crane safety.

Slings and rigging

OSHA's sling standard defines rated capacity, or working load limit, as "the maximum working load permitted by the provisions of this section." Alloy steel chain slings must carry permanently affixed identification showing size, grade, rated capacity and reach, and synthetic web slings must be marked with the rated capacity for each type of hitch.

Forklifts

A forklift's nameplate lists its capacity at a stated load center. OSHA's powered industrial truck guidance notes that capacity assumes the load's center of gravity is at that load center, and that attachments generally reduce capacity and must be listed on the nameplate. See forklift safety.

Scaffolds

Under 29 CFR 1926.451(a)(1), each scaffold and component must support its own weight and at least four times the maximum intended load. Section 1926.451(f)(1) prohibits loading scaffolds beyond their maximum intended loads or rated capacities, whichever is less.

What Changes Rated Capacity

• Radius and boom angle: as a crane's load moves farther from the center of rotation, capacity drops.
• Configuration: outrigger extension, counterweight, jib use and the area of operation all affect the chart value.
• Rigging weight: hook blocks, slings, spreader bars and other lifting gear generally count toward the load on crane charts.
• Hitch type and sling angle: a choker hitch has a lower capacity than a vertical hitch, and lower sling angles increase the tension in each leg.
• Load center: long or off-center loads on a forklift reduce the weight it can safely carry.
• Condition: wear, corrosion, heat damage and modifications can make the marked capacity meaningless until the equipment is inspected.

Practical Steps for Staying Within Rated Capacity

• Get the load weight from a reliable source, such as shipping documents or the manufacturer, rather than estimating.
• Add the weight of all rigging and lifting gear to the load.
• Check the chart for the actual radius, boom length and configuration of the lift, including the worst-case position.
• Confirm every component in the load path, including hooks, shackles and slings, is rated for its share of the load.
• Inspect equipment before use and remove items with missing or unreadable capacity markings.
• For critical or complex lifts, write a lift plan and use a qualified rigger.
• Never treat a load moment indicator or overload alarm as a substitute for planning.

Example of Rated Capacity

A mechanical contractor planned to set a rooftop air handling unit with a mobile crane. The shipping documents listed the unit's weight, and the rigging crew added the weight of the hook block, spreader bar and slings. Because the crane could not get close to the building, the lift radius was longer than the crew first expected.

The operator checked the load chart for the actual radius and outrigger setup and found that the total load was close to the limit at that position. The contractor moved to a larger crane with more margin, documented the calculation in a lift plan and confirmed each shackle and sling was tagged with a rated capacity above its share of the load.

Frequently Asked Questions: Rated Capacity

Is rated capacity the same as breaking strength?

No. Breaking strength is the load at which a component fails. Rated capacity is set well below that, using a design factor, so that normal use, wear and dynamic loading do not cause failure.

Is rated capacity the same as working load limit?

For slings, yes. OSHA's sling standard, 29 CFR 1910.184, uses rated capacity and working load limit as the same term. Other equipment may use rated load or safe working load to mean the same thing.

Can a crane ever lift more than its rated capacity?

Only during a load test performed under the manufacturer's instructions and the applicable standard. For overhead cranes, 29 CFR 1910.179(k) states that test loads must not be more than 125 percent of the rated load unless the manufacturer recommends otherwise. Normal operations must stay within rated capacity.

What should happen if a sling's capacity tag is missing?

The sling should be taken out of service until it is properly identified. Without the tag, workers cannot confirm the sling is suitable for the load or the hitch being used.

Who is responsible for staying within rated capacity?

The employer is responsible for planning lifts and providing suitable equipment, and the operator and riggers are responsible for checking the load, configuration and rigging before each lift. Any worker can use stop work authority if a lift looks overloaded.