Challenge: Size This Cable for a Pilbara Mine — Can You Get It Right?
A 250m underground run, 45°C ambient, sandy soil, 12 other circuits in the trench. Most engineers undersize this cable. Can you get it right on the first try?
February 26, 2026
The Problem
You're designing a cable run for a Pilbara iron ore mine in Western Australia. The specifications:
- Load: 3-phase motor, 75 kW, 400V, pf 0.85, efficiency 94%
- Cable route: 250m from MCC to motor, buried in trench
- Protection: 200A MCCB at the MCC
- Ambient soil temperature: 45°C
- Soil thermal resistivity: 2.5 K·m/W (sandy)
- Other circuits in trench: 12 (all similar size)
- Cable type: XLPE/SWA/PVC, copper, 3-core + earth
- Standard: AS/NZS 3008.1.1:2017
- Maximum voltage drop: 5% (20V at 400V)
The Challenge
Without looking at a calculator, estimate:
- What cable size do you think is needed?
- What's the combined derating factor?
- Will voltage drop or current capacity be the governing factor?
Write down your answers before reading the solution.
Step-by-Step Solution
Motor Full Load Current
P = √3 × V × I × pf × η
I = P / (√3 × V × pf × η) = 75,000 / (√3 × 400 × 0.85 × 0.94) = 135.5A
Derating Factors (AS/NZS 3008.1.1:2017)
| Factor | Table | Value |
|---|---|---|
| Ambient soil 45°C (XLPE, 90°C max) | Table 23 | 0.89 |
| Soil resistivity 2.5 K·m/W | Table 28 | 0.86 |
| Grouping: 13 circuits, buried direct | Table 22 | 0.57 |
Combined derating = 0.89 × 0.86 × 0.57 = 0.436
Required Base Cable Rating
I_z ≥ I_n / derating = 200 / 0.436 = 459A
(Note: we use the MCCB rating of 200A, not the motor FLC of 135.5A, because the cable must be protected by the MCCB.)
From AS/NZS 3008.1.1:2017, Table 13, Column 16 (3-core XLPE/SWA, buried direct):
- 150mm²: 376A — too small
- 185mm²: 427A — too small
- 240mm²: 497A — passes (497A > 459A)
Voltage Drop Check
Using mV/A/m from Table 30 for 240mm² Cu at 90°C: r = 0.0975 Ω/km, x = 0.0722 Ω/km
VD = I × L × (r cosφ + x sinφ) × √3
At FLC (135.5A): VD = 135.5 × 0.250 × (0.0975 × 0.85 + 0.0722 × 0.527) × √3 = 6.0V (1.5%)
Voltage drop: 1.5% — well within 5% limit.
The Answer
240mm² XLPE/SWA copper, 3-core + earth
Current capacity governs, not voltage drop. The combined derating of 0.436 (56% capacity lost!) means you need nearly 4× the motor's FLC in base cable rating.
Common Mistakes
- Using motor FLC instead of MCCB rating: The cable must be sized for the protection device rating, not just the load current
- Missing the grouping factor: 13 circuits gives 0.57 — this single factor dominates
- Guessing 95mm²: Most engineers' first instinct for a 135A motor load. It's undersized by 250%
- Forgetting soil resistivity: Sandy Pilbara soil at 2.5 K·m/W adds another 14% derating
Check your answer: Run this exact scenario in the Cable Sizing Calculator.
Frequently Asked Questions
What standards govern cable sizing calculations?
The primary standards are AS/NZS 3008.1.1:2017 (Australia/NZ), BS 7671:2018 (UK), IEC 60364-5-52 (International), and NEC Article 310 (USA). Each has different assumptions for ambient temperature, installation methods, and derating factors.
Why do different standards give different cable ratings?
Standards differ in reference ambient temperature (AS/NZS uses 40°C, BS 7671 uses 30°C), test conditions, grouping factor calculations, and installation method classifications. A 50mm² XLPE cable can vary by 15% between standards.
How do I apply derating factors correctly?
Derating factors must be applied multiplicatively: Final Rating = Base Rating × k₁ (ambient) × k₂ (grouping) × k₃ (thermal insulation) × k₄ (ground temp). Each factor comes from specific tables in the relevant standard.
Related Articles
- Cable Sizing Calculator - Interactive calculator with standards compliance
- Voltage Drop Calculator - Interactive calculator with standards compliance
Try It Yourself
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