Cable Pulling Calculations for Mining — Lessons from 18 Years Underground

In 2019, a contractor at a gold mine in Western Australia attempted to pull a single run of 630mm² XLPE copper cable down a 280-metre decline at 1:7 gradient. The conduit fill was acceptable. The cable was correctly sized for the 800A feeder. The pull crew had a 5-tonne winch and a pulling eye rated at 50kN. On paper, everything checked out.

Seventy metres into the pull, the dynamometer read 38kN — already at 85% of the cable manufacturer's maximum tension limit. They stopped, backed out, applied lubricant, and tried again. Same result at the same point. The cable jacket showed scuffing and compression marks at the first bend. Three shifts of production were lost while the engineering team recalculated. The problem was gravity. Nobody had properly accounted for the weight component of 280 metres of cable on a 1:7 slope, compounded by two tight bends where the decline turned at the first sublevel. When the numbers were finally run, the required pulling tension exceeded T_max by 22%. That cable was never going in as a single pull.

The entire failure cost the site roughly $45,000 in cable damage, winch hire, and lost production — all preventable with a calculation that takes less than ten minutes.

Why Mining Is Different

Surface cable pulls and mining cable pulls share the same physics, but mining amplifies every variable that makes cable pulling difficult.

Gravity is relentless. Most surface installations are horizontal or have minor grade changes — a cable tray rising from a basement to ground level, a duct bank crossing under a road. Mining declines run at 1:7 to 1:5 (8° to 11°) for hundreds of metres. Some ventilation shafts are steeper. Every metre of inclined cable adds a gravity component to the pulling tension that doesn't exist on flat ground.

Ambient temperature is extreme. Surface installations in temperate climates assume 25°C for cable jacket properties. Underground mines routinely operate at 40–50°C rock temperature at depth. Virgin rock temperature increases approximately 1°C per 30–40 metres of depth in most geological formations. At 45°C, XLPE jacket material is measurably softer than at 25°C, which directly affects sidewall bearing pressure limits.

Space is limited. A decline heading is typically 5.0–5.5m wide and 4.5–5.0m high. Cable trays are bolted to the walls. Bends where the decline changes direction are constrained by the rock geometry — you cannot install a long-radius sweep when the tunnel turns at a right angle. Bend radii of 600–900mm are common, compared to 1,200mm or more on surface installations.

Friction is higher. Dust, grit, and moisture from mine ventilation coat conduit interiors. A nominal friction coefficient of μ = 0.35 on clean PVC conduit can climb to μ = 0.50 or higher in a dusty underground environment. Some mines use GRP (fibreglass reinforced plastic) conduit for chemical resistance, which has inherently higher friction than PVC.

Runs are long. The distance from the portal substation to the working face can be 500m to 2km. Even with intermediate junction boxes, individual pull sections of 200–400m are routine.

The Gravity Problem

On a horizontal straight section, friction is the only force opposing the pull. The tension builds linearly with length:

On an inclined section, the cable's weight has a component along the slope direction. For an uphill pull (pulling the cable up the decline), both friction and gravity resist the pull:

Where:

• w = cable weight per unit length (N/m), including all cables being pulled simultaneously
• L = section length along the slope (m)
• α = inclination angle from horizontal (positive = uphill pull direction)
• μ = coefficient of friction

IEEE 1185-2019, Section 5.2 — Tension calculation for inclined sections

The sinα term is the critical one for mining. Consider a 300m decline at 15° (approximately 1:3.7, which is steep but exists at some operations). For a 185mm² single-core XLPE copper cable weighing approximately 21.5 N/m:

• Friction component: 21.5 × 300 × 0.35 × cos(15°) = 2,181 N
• Gravity component: 21.5 × 300 × sin(15°) = 1,669 N

Gravity adds 77% on top of the friction force for this section alone. On a horizontal surface, the total would be 2,258 N (friction only, cos(0) = 1). On the 15° decline, it is 3,850 N — a 70% increase. And this is before any bends.

For a typical mining decline at 12° and 300m length, the gravity term is still substantial: sin(12°) = 0.208, contributing about 1,341 N versus the friction component of 2,205 N. That is a 61% increase over horizontal.

Bend Physics in Confined Spaces

Mining drives often have sharp bends where the decline changes direction at sublevels, or where the access drive meets the main decline. These bends are constrained by rock geometry — the tunnel goes where the orebody dictates, not where a cable engineer would prefer.

The capstan equation governs tension at bends on horizontal planes:

But in mining declines, bends are rarely horizontal. A bend at the top of a decline (transitioning from horizontal to downhill) is a convex bend — the cable goes over a crest. A bend at the bottom (transitioning from downhill to horizontal) is a concave bend — the cable goes through a trough.

IEEE 1185-2019, Section 5.3 — Tension at vertical and inclined bends

The practical implication: a concave bend at the bottom of a decline is the worst case. It gets the full capstan multiplier AND an additive gravity component. In a mining context, the bottom of the decline — where you transition from the inclined section to a horizontal drive heading toward the orebody — is where tension peaks. This is exactly where the 630mm² pull in our opening story failed.

Convex bends at the top of the decline partially compensate, with gravity subtracting from the capstan amplification. But "partially" is the operative word — the exponential multiplier typically dominates the linear gravity correction.

SWBP in Hot Environments

Sidewall bearing pressure (SWBP) is the radial force per unit length where the cable presses against the conduit wall at bends:

Where T is the pulling tension at the bend and R is the bend radius.

AEIC CS8, Section 7 — Maximum allowable sidewall bearing pressure

The standard SWBP limits published in IEEE 1185-2019 Table 3 and AEIC CS8 Section 7 are based on testing at 25°C ambient temperature:

This temperature derating matters enormously in mining because tight bends (small R) and high accumulated tension (from long inclined sections) both drive SWBP upward. A bend that passes SWBP checks at 25°C on the surface may fail at depth.

The combination is insidious: the same mine that has long inclines driving up tension also has high rock temperatures reducing the allowable SWBP. Both variables push in the wrong direction simultaneously.

8 Practical Lessons from Underground Operations

These are lessons learned from cable pulling operations at underground mines across Australia, Indonesia, and Southern Africa. Every one of them comes from a job that went wrong.

1. Always calculate both pull directions. The optimal direction on an inclined route is not always obvious. Pulling uphill means gravity opposes you on the straights, but the direction you encounter bends changes which bends are convex and which are concave. I have seen cases where pulling downhill (with gravity assistance on the straights) produced higher maximum tension because it turned a favourable convex bend into an unfavourable concave one. Run the numbers both ways. Every time.

2. Use cable rollers in every straight section, spaced at 3m maximum. Underground conduit installations accumulate debris between installation and cable pulling — sometimes weeks or months apart. Rollers lift the cable off the conduit floor, eliminating the static friction that would otherwise require breakaway force. On a 200m straight in a dusty decline, the difference between pulling on rollers (μ ≈ 0.15) and pulling on bare conduit (μ ≈ 0.50) is the difference between a routine pull and a failed one.

3. Pre-lubricate for any pull exceeding 150m total length. Apply cable pulling lubricant to the first 50m of cable and inside the conduit at every access point. Replenish lubricant at intermediate access points during the pull. The cost of lubricant is negligible compared to the cost of a failed pull. For mining operations, use a water-based lubricant rated for the ambient temperature — some polymer-based compounds lose effectiveness above 40°C.

4. Monitor tension with a dynamometer and compare to calculated values in real time. Mount the dynamometer between the winch and the pulling grip. If measured tension exceeds 80% of your calculated value, stop and investigate. If it exceeds 90%, abort the pull. The calculation assumes ideal conditions; any deviation (debris in the conduit, misaligned joints, a partially crushed section) will produce higher actual tension than calculated.

5. Place pull boxes at strategic locations. The best placement is before a bend that follows a long straight section. The long straight builds up tension; the bend multiplies it. By breaking the pull at this point, you reset the entry tension at the bend to the reel back-tension value instead of the accumulated straight-section tension. This can reduce the final tension by 40–60% on complex routes.

6. Account for reel back-tension. A cable drum on a reel stand has inherent resistance from the drum brake and the inertia of the remaining cable mass. Typical values range from 100 N (small drums, well-maintained stand) to 500 N (large drums, heavy remaining cable). For short pulls, this is negligible. For long pulls in mining where you are fighting gravity on every metre, the reel back-tension adds directly to the initial tension and propagates through every subsequent segment.

7. Never exceed 70% of calculated T_max in practice. The maximum allowable tension per NEC 300.34 and IEEE 1185-2019 assumes perfect pulling grip alignment, uniform cable temperature, and no pre-existing jacket damage. None of these assumptions hold perfectly in a mine. A 30% safety margin accounts for the unknowns. Some mining companies mandate 60% as their internal limit.

NEC, 300.34 — Conductor bending radius and tension limits

8. Document everything. Mining regulators in every jurisdiction require evidence that cable installations comply with standards. A calculation sheet showing segment-by-segment tension buildup, SWBP at each bend, and comparison to allowable limits is not optional paperwork — it is the evidence that protects you in an incident investigation. I keep calculation records for a minimum of 7 years, matched to the cable's service life.

Worked Example: Decline Shaft Cable Pull

Scenario: A new feeder cable must be pulled from the portal substation down to the first sublevel pump station at an underground gold mine. The route follows the main decline.

Route geometry:

• 40m horizontal from substation to decline portal
• 90° convex bend (R = 0.8m) at the decline entry
• 300m decline at 12° downward
• 90° concave bend (R = 0.8m) at the bottom of the decline
• 90° horizontal bend (R = 0.8m) turning into the sublevel drive
• 60m horizontal to the pump station
• 90° horizontal bend (R = 0.8m) into the pump station switchroom

Cable: 185mm² single-core Cu XLPE, OD = 24.0mm, weight = 2.19 kg/m = 21.5 N/m

Conditions: Unlubricated GRP conduit, μ = 0.35, ambient temperature 42°C

Maximum allowable tension: T_max = 70 × 185 × 1 = 12,950 N (single conductor, per NEC 300.34)

IEEE 1185-2019, Table 3 — Maximum conductor pulling tension

Pull direction: Portal to pump station (downhill)

Starting tension T_0 = 200 N (reel back-tension, large drum)

Segment 1 — 40m horizontal:

Bend 1 — 90° convex (decline entry, cable goes over the crest):

Segment 2 — 300m decline at 12° (downhill, gravity assists):

Note the gravity term subtracts because we are pulling downhill. Without gravity, this section would add 21.5 × 300 × 0.35 = 2,258 N. Gravity saves us 1,393 N on this section.

Bend 2 — 90° concave (bottom of decline, cable goes through the trough):

Bend 3 — 90° horizontal (turning into sublevel drive):

Segment 3 — 60m horizontal:

Bend 4 — 90° horizontal (into switchroom):

Final tension: 9,762 N — 75% of T_max (12,950 N). This exceeds our 70% practical limit of 9,065 N.

SWBP check at worst bend (Bend 4):

SWBP = 9,762 / 0.8 = 12,203 N/m — far above the 4,400 N/m limit, even before temperature derating.

This pull fails on SWBP even though tension is borderline. Solutions:

1. Add lubricant (μ = 0.20): reduces T_7 to approximately 4,800 N, SWBP to 6,000 N/m — still fails SWBP
2. Lubricate + larger bend radius (R = 1.5m) at Bend 4: SWBP = 4,800 / 1.5 = 3,200 N/m — PASS
3. Split the pull with a pull box after Bend 3, resetting tension before the final straight and bend

Option 3 is the most robust for a mining operation: install a pull box at the sublevel drive junction, pull the decline section first (Segments 1–3, Bends 1–3), then pull the final 60m and Bend 4 separately with a starting tension near zero. The second pull would produce a tension of only 452 N at Bend 4, giving a SWBP of 565 N/m — well within limits.

Calculate Before You Pull

Every failed cable pull I have investigated in 18 years of mining electrical work had one thing in common: nobody ran the calculation beforehand. The pull crew assumed that if the cable fit in the conduit and the winch was big enough, the pull would succeed. Conduit fill tells you whether the cable fits. The winch rating tells you whether you have enough force. Neither tells you whether the cable will survive the pull.

Mining cable installations have too many variables working against you — gravity, heat, dust, tight bends, long runs — to rely on rules of thumb. Run the numbers for every pull. Both directions. Check SWBP at every bend. Apply temperature derating if you are below 200m depth.

Related Resources

• Cable Pulling Physics: Every Bend Multiplies Tension — Deep dive into the capstan equation and worked examples
• Mining Cable Pulling Feasibility Study — Full worked example at a copper-gold mining operation
• Cable Sizing Guide: The Complete Method — Size the cable before planning the pull
• View all worked examples →