Cable Sizing: The 50m Office Feeder — AS/NZS vs BS 7671 vs IEC vs NEC
Same 100A three-phase load, same 50m cable run, four different standards. See exactly where AS/NZS 3008, BS 7671, IEC 60364, and NEC give different cable sizes — and why.
Every electrical engineer has faced this question: which cable size does this circuit need? The answer depends entirely on which standard you design to. This article takes one scenario and runs it through all four major standards to show exactly where the results diverge and why.
The Scenario
A straightforward three-phase distribution circuit in a commercial office building:
- Load: 100A three-phase balanced
- Cable run: 50 metres
- Installation: PVC/XLPE copper cables in surface-mounted conduit
- Ambient temperature: 30°C (standard conditions)
- Grouping: Single circuit (no grouping derating)
- Voltage: 400V three-phase
This is about as simple as cable sizing gets — no exotic installation methods, no extreme temperatures, no grouped circuits. The differences you see here are purely from how each standard defines base current ratings and derating methodologies.
Side-by-Side Results
Scenario
100A three-phase, 50m, PVC copper, surface conduit, 30°C ambient
| Parameter | AS/NZS | BS 7671 | IEC 60364 | NEC |
|---|---|---|---|---|
Installation method designation | Method B2Enclosed in conduit on wallAS/NZS 3008.1.1, Table 3 | Reference Method BEnclosed in conduit on wallBS 7671, Table 4A2 | Reference Method BEnclosed in conduit on wallIEC 60364-5-52, Table B.52.1 | Table 310.16Raceway/conduitNEC 310.16 |
Base current rating (35mm²) | 126APVC 75°C, 3-coreTable 13, Col 7 | 110APVC 70°C, 3 or 4-coreTable 4D2A, Col 6 | 110APVC 70°C, 3-coreTable B.52.4 | 110A75°C column, #2 AWG ≈ 33.6mm²Table 310.16 |
Temperature correction (30°C) | 1.0030°C is referenceTable 22 | 1.0030°C is referenceTable 4B1 | 1.0030°C is referenceTable B.52.14 | 1.0030°C is reference310.15(B)(1) |
Minimum cable size selected | 35mm²126A > 100A ✓Table 13 | 35mm²110A > 100A ✓Table 4D2A | 35mm²110A > 100A ✓Table B.52.4 | #2 AWG (33.6mm²)115A at 75°C > 100A ✓Table 310.16 |
Voltage drop (%) | 1.8%mV/A/m methodTable 30 | 1.9%mV/A/m methodTable 4Ab | 1.9%mV/A/m methodAnnex G | 1.7%Different R/X valuesNEC Ch.9, Table 9 |
Voltage drop limit | 5%Total circuitAS/NZS 3000, 3.6.2 | 5%Total circuitTable 4Ab Note | 4%Total circuitIEC 60364-5-52, 525 | 3% branch / 5% totalRecommended, not mandatoryNEC 210.19(A) Note |
Why the Numbers Differ
Conductor Temperature Rating
The single biggest difference is the assumed maximum conductor temperature:
- AS/NZS 3008 uses 75°C as the standard rating for PVC cables — this gives higher base ampacities
- BS 7671 and IEC 60364 use 70°C for PVC cables — 5°C lower means lower base ampacity
- NEC offers both 60°C and 75°C columns in Table 310.16, with the termination temperature often governing (60°C for most connections)
The Temperature Rating Trap
An engineer moving from AS/NZS to BS 7671 will find that the "same" cable has a lower current rating. This is not an error — it reflects a more conservative thermal assumption in the BS/IEC framework. A 35mm² PVC cable rated at 126A under AS/NZS is only rated at 110A under BS 7671.
Installation Method Classification
All four standards classify installation methods into reference categories, but the boundaries differ:
- AS/NZS has unique methods (e.g., Method V — cables on ventilated trays) not found in BS/IEC
- BS 7671 follows IEC closely but adds UK-specific methods (e.g., Method 100 for thermally insulating walls)
- NEC uses a fundamentally different table structure — one large table (310.16) covers most raceway and cable installations
Voltage Drop Methodology
While all four standards use similar physics (I × Z × L), the impedance values in their lookup tables differ because they are derived from different national cable manufacturing standards and testing conditions.
Voltage Drop (All Standards)
Vd = (mV/A/m) × I_b × L / 1000
The mV/A/m values differ between standards because:
- AS/NZS uses Australian cable manufacturing data (based on AS/NZS 1125)
- BS 7671 uses UK cable data (based on BS 6004/BS 5467)
- NEC Chapter 9 Table 9 uses American conductor dimensions (AWG system)
AWG vs Metric Sizing
NEC uses the American Wire Gauge system. The nearest AWG equivalent to 35mm² is #2 AWG (33.6mm²), which is slightly smaller. This means:
- At 75°C: #2 AWG is rated at 115A — adequate for 100A
- At 60°C: #2 AWG is rated at 95A — not adequate, requiring #1 AWG (42.4mm²)
The termination temperature rating (not the cable insulation rating) often controls the NEC result.
Practical Guidance
For this scenario, all four standards agree on 35mm² (or the AWG equivalent). But this agreement breaks down quickly when you add:
- Higher ambient temperature — AS/NZS becomes more permissive due to the 75°C base
- Grouped circuits — grouping factor tables differ between standards
- Longer cable runs — NEC's 3% branch circuit limit catches longer runs faster
Multi-Standard Projects
When designing for projects that must comply with multiple standards (e.g., an Australian firm designing for a Middle East project referencing IEC), always size to the most conservative standard applicable. In most cases, this means IEC 60364 for voltage drop and BS 7671 for current rating.
Key Takeaways
- Same scenario, same answer (this time) — all four standards select 35mm² for this simple case
- The margins differ — AS/NZS has 26% headroom (126A vs 100A), BS/IEC have only 10% (110A vs 100A)
- Voltage drop limits vary significantly — from 3% (NEC branch) to 5% (AS/NZS and BS 7671)
- AWG vs metric creates confusion — #2 AWG is close to but not exactly 35mm²
- Temperature rating is the root cause — 70°C vs 75°C PVC rating drives most of the ampacity difference
Related Resources
- Grenfell Tower: Fire-Resistant Cable Sizing Worked Example — Step-by-step BS 7671 cable sizing for a 24-storey vertical riser
- Piper Alpha: Emergency Generator Cable Sizing — Cable sizing under extreme conditions with full derating analysis
- Cable Derating: 12 Cables in a Tray at 40°C — How grouping and temperature derating differ across all four standards
- Voltage Drop: The 100m Workshop Cable — NEC's 3% rule vs the world on the same cable run
- The Complete Cable Sizing Comparison — Every factor, all four standards, side by side
- View all worked examples →
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Lead Electrical & Instrumentation Engineer
18+ years of experience in electrical engineering at large-scale mining operations. Specializing in power systems design, cable sizing, and protection coordination across BS 7671, IEC 60364, NEC, and AS/NZS standards.