5.2 Gross vs Net Capacity & Deductions
Key Takeaways
Gross capacity is the raw number printed on the load chart; net capacity is what is actually available to hang on the hook after required deductions are subtracted
Standard deductions include the hook block, headache ball, jib or boom extension (erected or stowed), auxiliary boom head, wire rope over a stated length, and all rigging below the hook
Net capacity = gross chart capacity − all applicable deductions; if net capacity is less than the total load to be lifted, the lift is an overload
Parts of line must provide enough hoist-line capacity: required parts ≈ total hook load ÷ rated single-line pull, rounded up to a whole number
An erected jib usually must be deducted even when it is not being used because its weight still acts on the boom
Gross Capacity Is Not What You Can Hang on the Hook
The number you read from the load chart is the gross rated capacity. It is the total weight the crane is rated to support at that radius and configuration. But that total has to carry everything below the boom point, not just the freight. The amount actually available for the load itself is the net capacity.
The core relationship the exam tests:
Net capacity = Gross chart capacity − Sum of all applicable deductions
If the total load (the object plus its rigging plus weight-handling devices) exceeds net capacity, the lift is an overload and is prohibited, even if the object by itself weighs less than the gross chart number.
What Counts as a Deduction
Deductions are weights that hang on or act on the boom and reduce what is left for the load. The chart notes list which apply. Typical deductions:
| Deduction item | Why it is deducted |
|---|---|
| Hook block (lower load block) | Hangs on the main hoist line and is not in the gross rating |
| Headache ball (overhaul ball) | Used on single-line/whip; its weight reduces net capacity |
| Jib / boom extension (erected) | Adds weight and leverage to the boom even when not lifting on it |
| Jib / boom extension (stowed on boom side) | Often still a deduction; weight is carried by the boom |
| Auxiliary boom head / aux sheave | Added hardware at the boom tip |
| Wire rope beyond a stated length | Long lines add measurable weight; notes give a per-foot figure |
| Rigging (slings, shackles, spreader bar, lifting beam, links) | Everything between the hook and the load |
Key trap: an erected jib is commonly deducted from the main-boom capacity even when the jib is not the lifting attachment, because its weight and offset still load the boom. The chart note will state this.
Worked Example: Computing Net Capacity
(All values illustrative, not from any real manufacturer chart.)
A lift is planned with these facts:
- Gross chart capacity at the planned radius/boom: 24,000 lb
- Hook block: 600 lb
- Erected jib (not being used, but installed): 1,300 lb
- Slings, shackles, and spreader bar: 900 lb
- Object to be lifted: 20,500 lb
Step 1 — Total the deductions that reduce net capacity (hook block + erected jib):
600 + 1,300 = 1,900 lb
Step 2 — Net capacity = gross − deductions:
24,000 − 1,900 = 22,100 lb
Step 3 — Total load on the hook (object + rigging):
20,500 + 900 = 21,400 lb
Step 4 — Compare:
21,400 lb (total load) < 22,100 lb (net capacity) → the lift is within capacity.
Note how the rigging is treated as part of the load on the hook, while the hook block and jib are treated as capacity deductions. Either approach reaches the same conclusion as long as every item is counted exactly once. The dangerous error is counting nothing, or counting an item zero times.
Parts of Line and Line-Pull Limits
The hoist rope has a rated single-line pull (sometimes called maximum permissible line pull or rated line pull): the most one part of wire rope is permitted to carry. To lift a heavy load you reeve the rope through the block in multiple parts of line so each part carries a fraction of the total.
Required parts of line (ignoring friction for the written-exam method):
Parts of line = Total weight on the hook ÷ Rated single-line pull, then round up to the next whole number.
Worked Example: Parts of Line
- Total weight on the hook (load + block + rigging): 38,000 lb
- Rated single-line pull: 12,000 lb per part
38,000 ÷ 12,000 = 3.17 parts
You cannot reeve a fractional part, and you must not exceed the per-line pull, so you round up to 4 parts of line. Three parts would carry 38,000 ÷ 3 = 12,667 lb per part, which exceeds the 12,000 lb limit and is unsafe. Four parts carry 38,000 ÷ 4 = 9,500 lb per part, which is within the limit.
Two independent limits must both be satisfied for any heavy lift:
- Capacity limit: net chart capacity must be ≥ total load (this section's first example).
- Line-pull limit: parts of line must be enough that no single part exceeds rated single-line pull (this example).
A crane can be within its chart capacity yet still be unsafe because the hoist line is reeved with too few parts. The exam tests both checks.
Gross chart capacity is 30,000 lb. The hook block weighs 700 lb, an erected unused jib must be deducted at 1,500 lb, and the slings/shackles weigh 800 lb. What is the maximum object weight that can be lifted?
30,000 lb
29,300 lb
27,800 lb
27,000 lb
A total hook load of 38,000 lb must be lifted with a rope rated at 12,000 lb single-line pull. How many parts of line are required?
2 parts
3 parts
4 parts
5 parts
An object weighs 19,000 lb. Rigging is 800 lb, the hook block is 500 lb, and the gross chart capacity at the planned radius is 20,000 lb. Is the lift within capacity?
Yes, because the object (19,000 lb) is less than the gross capacity (20,000 lb)
Yes, because net capacity is 20,300 lb
No, because the total load of 20,300 lb exceeds the 19,500 lb net capacity
No, because gross capacity must always be doubled for safety
Sections you finish are checked off in the contents.