Mobile Crane Operator 339A Red Seal: Hoisting Calculations
If you are preparing for the Mobile Crane Operator (339A) Red Seal exam, the calculation block is where a lot of candidates lose marks they did not need to lose. “Performs hoisting calculations” is 20 of the 110 questions, and it is the only block that is almost pure arithmetic: you either apply the right formula or you do not. This guide walks through the three calculation tasks the Red Seal blueprint names — determines load weights, calculates crane capacity and performs rigging calculations — with worked examples you can redo on paper.
What the exam is made of
The exam is weighted by Major Work Activity (MWA). Calculations (B) carry 18% and operating the crane (G) carries 23%, so together they decide roughly four questions in ten.
| Block | Major Work Activity | Questions | Share |
|---|---|---|---|
| MWA A | Performs common occupational skills | 7 | 6% |
| MWA B | Performs hoisting calculations | 20 | 18% |
| MWA C | Inspects and maintains crane | 14 | 13% |
| MWA D | Performs rigging | 13 | 12% |
| MWA E | Plans lift, prepares site and sets up crane | 16 | 15% |
| MWA F | Assembles, disassembles and transports crane | 15 | 14% |
| MWA G | Operates crane | 25 | 23% |
Source: Red Seal Exam Breakdown — red-seal.ca. Total 110 questions.
1. Determining load weight
When the weight is not on a tag or a drawing, you work it out: weight = volume × density. Example: a steel plate 6 m × 2.4 m × 25 mm thick. Volume = 6 × 2.4 × 0.025 = 0.36 m³. Steel is about 7,850 kg/m³, so the plate weighs about 2,826 kg. That is the load only — the rigging goes on top of it (see step 3).
2. Rigging calculations: sling angle
A sling gets more heavily loaded as it gets flatter. With the angle measured from the horizontal, the force in each leg of a symmetrical multi-leg sling is:
tension per leg = load ÷ (number of legs × sin θ)
For a 10,000 kg load on a two-leg sling:
| Angle from horizontal | Tension per leg | Compared with a straight lift |
|---|---|---|
| 90° (vertical legs) | 5,000 kg | ×1.00 |
| 60° | 5,774 kg | ×1.15 |
| 45° | 7,071 kg | ×1.41 |
| 30° | 10,000 kg | ×2.00 |
At 30° each leg carries as much as the whole load. That is why sling rating tags and employer rules set a minimum working angle — always follow the tag and the site procedure rather than a number from memory. Exam questions also test the trap: the angle in the formula is from the horizontal; if a question gives the angle between the legs or from the vertical, convert first.
3. Calculating crane capacity
The load chart gives gross capacity for one configuration at one radius. What you may lift is the net capacity: gross capacity − everything hanging from the boom tip that is not the load — hook block or ball, slings, shackles and any other rigging, plus any deductions the manufacturer’s chart notes require for stowed or erected attachments.
Illustrative numbers — not from any real chart: gross capacity 12,000 kg at the working radius, hook block 350 kg, slings and shackles 180 kg. Net capacity = 12,000 − 350 − 180 = 11,470 kg. A 10,000 kg load is 87.2% of net capacity, so it is within the chart — and the planning question becomes whether that margin is acceptable under your employer’s lift-planning rules.
One more trap: the chart is indexed by radius, not boom length. Radius is the horizontal distance from the centre of rotation to the load. As a first estimate, radius ≈ boom length × cos(boom angle) — a 30 m boom at 60° gives 15 m — but boom deflection under load and the offset of the boom foot make the real radius larger, which is why operators use the measured radius and the chart, not the estimate.
Try three questions
A symmetrical two-leg sling lifts an 8,000 kg load with each leg at 60° from the horizontal. What is the tension in each leg?
- A) 4,000 kg
- B) 4,619 kg
- C) 6,928 kg
- D) 8,000 kg
Answer: B. 8,000 ÷ (2 × sin 60°) = 8,000 ÷ 1.732 = 4,619 kg. A is the vertical-leg answer (ignores the angle).
The load chart gives 9,500 kg at the working radius. The hook block weighs 300 kg and the rigging 150 kg. What is the maximum load that can be lifted?
- A) 9,500 kg
- B) 9,350 kg
- C) 9,200 kg
- D) 9,050 kg
Answer: D. Net capacity = 9,500 − 300 − 150 = 9,050 kg. Deduct both items; the chart figure is gross.
A 24 m boom is at a 70° boom angle. Estimate the radius (ignore deflection and boom-foot offset).
- A) 8.2 m
- B) 12.0 m
- C) 22.6 m
- D) 24.0 m
Answer: A. radius ≈ 24 × cos 70° = 24 × 0.342 = 8.2 m. C is 24 × sin 70° — the height, not the radius.
How to study this block
- Redo the three formulas above until you can do them with a calculator and no notes — they are worth many exam questions.
- Practise with your own crane’s chart. Reading a chart (configuration, radius, boom length, deductions) is a skill, and the exam tests the reading as much as the arithmetic.
- Do the unit conversions on paper: kg / lb, metres / feet, degrees / radians on your calculator.
- After the maths, move to blocks E and G (planning the lift and operating), the biggest non-calculation blocks.
Is certification required where you work?
- British Columbia: not on the compulsory list (certification is voluntary)
- Ontario: compulsory, listed as “Hoisting Engineer — Mobile Crane Operator 1 (339A) / 2 (339C)”
- Alberta: compulsory, listed as “Crane and Hoisting Equipment Operator — Mobile Crane”
BC note: SkilledTradesBC lists Mobile Crane Operator with an effective date of July 5, 2027.
Checked against the provincial compulsory-trade lists on 2026-09-14 (SkilledTradesBC, Skilled Trades Ontario, Alberta Apprenticeship and Industry Training). Rules change — confirm with your province before you rely on this.
Worked examples use round illustrative numbers. Real lifts must be planned from the manufacturer’s load chart for the exact crane and configuration, the sling tags, and your employer’s and province’s procedures.
🎧 Watch & Listen: 30 Mobile Crane Operator (339A) Practice Questions & Worked Answers
Weighted to the official Red Seal Mobile Crane Operator (339A) exam breakdown, read aloud with a 5-second pause before each worked answer and code explanation.
Take the Mobile Crane Operator (339A) Exam Simulation
Access full-length timed exam simulations, independently audited Red Seal question banks, comprehensive Canadian code references, and instant AI Journeyperson explanations.
Is Mobile Crane Operator (339A) certification required where you work? Check the rules for British Columbia, Ontario or Alberta.