NCCCO Load Charts and Capacity Calculations 1 — Questions and Answers
Question 1: When reading a mobile crane load chart, what two primary variables determine the rated lifting capacity?
- Working radius and boom length/angle (Correct answer)
- Hoist line speed and rope diameter
- Counterweight weight and outrigger pad size
- Engine horsepower and hydraulic pressure
Correct answer: Working radius and boom length/angle
Load charts are organized by working radius (horizontal distance from center of rotation to the load) and boom length or angle configuration. These two variables intersect to give the maximum rated load.
Every crane load chart uses working radius — the horizontal distance from the crane's center of rotation to the center of the load — combined with the boom configuration (length and/or angle) to determine rated capacity. As radius increases or boom length increases, capacity decreases due to increasing moment arm and reduced structural stability.
Question 2: A mobile crane load chart shows a capacity of 40,000 lb at a 30-foot radius. The rigger plans to lift a 38,000-lb load plus 800 lb of rigging hardware. What is the total suspended load?
- 38,800 lb — rigging hardware weight must be added to the load weight (Correct answer)
- 38,000 lb — rigging hardware is excluded from load chart calculations
- 40,800 lb — the chart capacity must also be included
- 37,200 lb — only net load weight without any hardware
Correct answer: 38,800 lb — rigging hardware weight must be added to the load weight
Total suspended load includes the load weight plus all below-the-hook rigging hardware (shackles, slings, spreader bars, hooks). Here: 38,000 + 800 = 38,800 lb total.
Rated capacity from a load chart represents the total suspended load: payload plus all below-the-hook devices. The rigger must total the load weight (38,000 lb) plus all rigging hardware (800 lb) = 38,800 lb and compare that total to the rated capacity. At 38,800 lb versus 40,000 lb rated (97%), this also qualifies as a critical lift per ASME P30.1 (>90% of rated capacity), requiring a written lift plan.
Question 3: What does the deduction column on some load charts represent?
- The weight of the hook block and ball that must be subtracted from the rated capacity to get net load capacity (Correct answer)
- The amount by which boom length reduces ground bearing pressure
- A safety margin already built into the published capacities
- The minimum counterweight required for each configuration
Correct answer: The weight of the hook block and ball that must be subtracted from the rated capacity to get net load capacity
The deduction column lists the weight of the crane's own hook block/ball assembly. This weight counts as part of the lifted load and must be deducted from the rated capacity to determine the actual net weight you can attach to the hook.
On many load charts, rated capacity includes the weight of all load handling devices (hook block, ball, slings). The deduction column tells the operator how much of the rated capacity is consumed by the hook block/ball at that configuration. Net usable load = rated capacity minus the deduction value. Failing to account for this can cause overloading.
Question 4: A crane is rated for 20 tons at a 40-foot radius on fully extended outriggers. The operator is setting up on a slope of 2%. What is required before using the rated capacities?
- Re-level the crane to within manufacturer tolerances — off-level operation voids the load chart (Correct answer)
- Apply a 2% derating factor to all load chart values
- Proceed normally as 2% slope is within OSHA's acceptable range
- Extend outriggers on the downhill side fully and use half capacity on the uphill side
Correct answer: Re-level the crane to within manufacturer tolerances — off-level operation voids the load chart
Load charts are developed for level operation (within manufacturer-specified tolerance, typically ±1%). Operating off-level shifts the center of gravity and reduces tipping stability — the crane must be leveled before using rated capacities.
ASME B30.5-2.1.1 and manufacturer specifications require mobile cranes to be level within the tolerance stated in the load chart (typically ±1%). Even a 1-2% out-of-level condition can reduce the tipping load capacity by 10-30% depending on the swing direction relative to the slope. The crane must be re-leveled using outriggers or blocking before proceeding.
Question 5: Which of the following correctly describes 'working radius' as used in crane load charts?
- Horizontal distance from the center of the crane's rotation to the center of the suspended load (Correct answer)
- The length of the boom from pivot pin to tip sheave
- The maximum reach of the boom measured along its centerline
- Vertical distance from ground level to the hook at maximum height
Correct answer: Horizontal distance from the center of the crane's rotation to the center of the suspended load
Working radius is the horizontal distance — not the boom length — from the center of rotation (turntable center pin) to the plumb line of the suspended load. It determines the overturning moment.
Working radius determines the moment arm of the load. Even with a fixed boom length, swinging the load outward increases the radius and decreases capacity. Operators must measure or calculate the actual horizontal distance to the load center and use that radius in the load chart, not the boom length itself.
Question 6: A crane load chart note states 'Capacities are based on structural strength and do not exceed 75% of tipping load.' What does this note indicate?
- The published capacities already include the ASME-required stability derating, so no further reduction is needed for normal lifts (Correct answer)
- The operator must manually reduce all chart values by an additional 25%
- These capacities apply only to static lifts; dynamic lifts require a 50% reduction
- The crane is rated for lattice boom only and not hydraulic boom configurations
Correct answer: The published capacities already include the ASME-required stability derating, so no further reduction is needed for normal lifts
ASME B30.5 requires mobile crane manufacturers to rate capacities at no more than 75% of the actual tipping load (on outriggers). The load chart already incorporates this derating — operators use the published values directly.
Per ASME B30.5-2.1.1.2, rated loads on outriggers shall not exceed 75% of the tipping load for that configuration. On rubber tires, the limit is 85% of tipping load. Because this derating is already factored into the load chart, operators do not apply additional reductions. The load chart is the authoritative safe working limit.
NCCCO Load Charts and Capacity Calculations 1
When reading a mobile crane load chart, what two primary variables determine the rated lifting capacity?